diff --git a/packages/typescript/src/enums/checkFlags.enum.ts b/packages/typescript/src/enums/checkFlags.enum.ts
index 7f27dc201db7b..7aab3deb385bf 100644
--- a/packages/typescript/src/enums/checkFlags.enum.ts
+++ b/packages/typescript/src/enums/checkFlags.enum.ts
@@ -13,19 +13,22 @@ export enum CheckFlags {
     ContainsPublic = 1 << 8,
     ContainsProtected = 1 << 9,
     ContainsPrivate = 1 << 10,
-    ContainsStatic = 1 << 11,
-    Late = 1 << 12,
-    ReverseMapped = 1 << 13,
-    OptionalParameter = 1 << 14,
-    RestParameter = 1 << 15,
-    DeferredType = 1 << 16,
-    HasNeverType = 1 << 17,
-    Mapped = 1 << 18,
-    StripOptional = 1 << 19,
-    Unresolved = 1 << 20,
-    IsDiscriminantComputed = 1 << 21,
-    IsDiscriminant = 1 << 22,
-    IndexSymbol = 1 << 23,
+    ContainsWritePublic = 1 << 11,
+    ContainsWriteProtected = 1 << 12,
+    ContainsWritePrivate = 1 << 13,
+    ContainsStatic = 1 << 14,
+    Late = 1 << 15,
+    ReverseMapped = 1 << 16,
+    OptionalParameter = 1 << 17,
+    RestParameter = 1 << 18,
+    DeferredType = 1 << 19,
+    HasNeverType = 1 << 20,
+    Mapped = 1 << 21,
+    StripOptional = 1 << 22,
+    Unresolved = 1 << 23,
+    IsDiscriminantComputed = 1 << 24,
+    IsDiscriminant = 1 << 25,
+    IndexSymbol = 1 << 26,
     Synthetic = SyntheticProperty | SyntheticMethod,
     NonUniformAndLiteral = HasNonUniformType | HasLiteralType,
     Partial = ReadPartial | WritePartial,
diff --git a/packages/typescript/src/enums/checkFlags.ts b/packages/typescript/src/enums/checkFlags.ts
index be3165fc706d5..8dfcbdb384974 100644
--- a/packages/typescript/src/enums/checkFlags.ts
+++ b/packages/typescript/src/enums/checkFlags.ts
@@ -13,19 +13,22 @@ export var CheckFlags: any;
     CheckFlags[CheckFlags["ContainsPublic"] = 256] = "ContainsPublic";
     CheckFlags[CheckFlags["ContainsProtected"] = 512] = "ContainsProtected";
     CheckFlags[CheckFlags["ContainsPrivate"] = 1024] = "ContainsPrivate";
-    CheckFlags[CheckFlags["ContainsStatic"] = 2048] = "ContainsStatic";
-    CheckFlags[CheckFlags["Late"] = 4096] = "Late";
-    CheckFlags[CheckFlags["ReverseMapped"] = 8192] = "ReverseMapped";
-    CheckFlags[CheckFlags["OptionalParameter"] = 16384] = "OptionalParameter";
-    CheckFlags[CheckFlags["RestParameter"] = 32768] = "RestParameter";
-    CheckFlags[CheckFlags["DeferredType"] = 65536] = "DeferredType";
-    CheckFlags[CheckFlags["HasNeverType"] = 131072] = "HasNeverType";
-    CheckFlags[CheckFlags["Mapped"] = 262144] = "Mapped";
-    CheckFlags[CheckFlags["StripOptional"] = 524288] = "StripOptional";
-    CheckFlags[CheckFlags["Unresolved"] = 1048576] = "Unresolved";
-    CheckFlags[CheckFlags["IsDiscriminantComputed"] = 2097152] = "IsDiscriminantComputed";
-    CheckFlags[CheckFlags["IsDiscriminant"] = 4194304] = "IsDiscriminant";
-    CheckFlags[CheckFlags["IndexSymbol"] = 8388608] = "IndexSymbol";
+    CheckFlags[CheckFlags["ContainsWritePublic"] = 2048] = "ContainsWritePublic";
+    CheckFlags[CheckFlags["ContainsWriteProtected"] = 4096] = "ContainsWriteProtected";
+    CheckFlags[CheckFlags["ContainsWritePrivate"] = 8192] = "ContainsWritePrivate";
+    CheckFlags[CheckFlags["ContainsStatic"] = 16384] = "ContainsStatic";
+    CheckFlags[CheckFlags["Late"] = 32768] = "Late";
+    CheckFlags[CheckFlags["ReverseMapped"] = 65536] = "ReverseMapped";
+    CheckFlags[CheckFlags["OptionalParameter"] = 131072] = "OptionalParameter";
+    CheckFlags[CheckFlags["RestParameter"] = 262144] = "RestParameter";
+    CheckFlags[CheckFlags["DeferredType"] = 524288] = "DeferredType";
+    CheckFlags[CheckFlags["HasNeverType"] = 1048576] = "HasNeverType";
+    CheckFlags[CheckFlags["Mapped"] = 2097152] = "Mapped";
+    CheckFlags[CheckFlags["StripOptional"] = 4194304] = "StripOptional";
+    CheckFlags[CheckFlags["Unresolved"] = 8388608] = "Unresolved";
+    CheckFlags[CheckFlags["IsDiscriminantComputed"] = 16777216] = "IsDiscriminantComputed";
+    CheckFlags[CheckFlags["IsDiscriminant"] = 33554432] = "IsDiscriminant";
+    CheckFlags[CheckFlags["IndexSymbol"] = 67108864] = "IndexSymbol";
     CheckFlags[CheckFlags["Synthetic"] = 6] = "Synthetic";
     CheckFlags[CheckFlags["NonUniformAndLiteral"] = 192] = "NonUniformAndLiteral";
     CheckFlags[CheckFlags["Partial"] = 48] = "Partial";
diff --git a/tsc/internal/ast/checkflags.go b/tsc/internal/ast/checkflags.go
index b928d3878508e..7800a585d6355 100644
--- a/tsc/internal/ast/checkflags.go
+++ b/tsc/internal/ast/checkflags.go
@@ -17,19 +17,22 @@ const (
 	CheckFlagsContainsPublic         CheckFlags = 1 << 8  // Synthetic property with public constituent(s)
 	CheckFlagsContainsProtected      CheckFlags = 1 << 9  // Synthetic property with protected constituent(s)
 	CheckFlagsContainsPrivate        CheckFlags = 1 << 10 // Synthetic property with private constituent(s)
-	CheckFlagsContainsStatic         CheckFlags = 1 << 11 // Synthetic property with static constituent(s)
-	CheckFlagsLate                   CheckFlags = 1 << 12 // Late-bound symbol for a computed property with a dynamic name
-	CheckFlagsReverseMapped          CheckFlags = 1 << 13 // Property of reverse-inferred homomorphic mapped type
-	CheckFlagsOptionalParameter      CheckFlags = 1 << 14 // Optional parameter
-	CheckFlagsRestParameter          CheckFlags = 1 << 15 // Rest parameter
-	CheckFlagsDeferredType           CheckFlags = 1 << 16 // Calculation of the type of this symbol is deferred due to processing costs, should be fetched with `getTypeOfSymbolWithDeferredType`
-	CheckFlagsHasNeverType           CheckFlags = 1 << 17 // Synthetic property with at least one never type in constituents
-	CheckFlagsMapped                 CheckFlags = 1 << 18 // Property of mapped type
-	CheckFlagsStripOptional          CheckFlags = 1 << 19 // Strip optionality in mapped property
-	CheckFlagsUnresolved             CheckFlags = 1 << 20 // Unresolved type alias symbol
-	CheckFlagsIsDiscriminantComputed CheckFlags = 1 << 21 // IsDiscriminant flags has been computed
-	CheckFlagsIsDiscriminant         CheckFlags = 1 << 22 // Discriminant property
-	CheckFlagsIndexSymbol            CheckFlags = 1 << 23 // Synthetic property created from index signature
+	CheckFlagsContainsWritePublic    CheckFlags = 1 << 11 // Synthetic property with public set accessors(s)
+	CheckFlagsContainsWriteProtected CheckFlags = 1 << 12 // Synthetic property with protected set accessors(s)
+	CheckFlagsContainsWritePrivate   CheckFlags = 1 << 13 // Synthetic property with private set accessors(s)
+	CheckFlagsContainsStatic         CheckFlags = 1 << 14 // Synthetic property with static constituent(s)
+	CheckFlagsLate                   CheckFlags = 1 << 15 // Late-bound symbol for a computed property with a dynamic name
+	CheckFlagsReverseMapped          CheckFlags = 1 << 16 // Property of reverse-inferred homomorphic mapped type
+	CheckFlagsOptionalParameter      CheckFlags = 1 << 17 // Optional parameter
+	CheckFlagsRestParameter          CheckFlags = 1 << 18 // Rest parameter
+	CheckFlagsDeferredType           CheckFlags = 1 << 19 // Calculation of the type of this symbol is deferred due to processing costs, should be fetched with `getTypeOfSymbolWithDeferredType`
+	CheckFlagsHasNeverType           CheckFlags = 1 << 20 // Synthetic property with at least one never type in constituents
+	CheckFlagsMapped                 CheckFlags = 1 << 21 // Property of mapped type
+	CheckFlagsStripOptional          CheckFlags = 1 << 22 // Strip optionality in mapped property
+	CheckFlagsUnresolved             CheckFlags = 1 << 23 // Unresolved type alias symbol
+	CheckFlagsIsDiscriminantComputed CheckFlags = 1 << 24 // IsDiscriminant flags has been computed
+	CheckFlagsIsDiscriminant         CheckFlags = 1 << 25 // Discriminant property
+	CheckFlagsIndexSymbol            CheckFlags = 1 << 26 // Synthetic property created from index signature
 	CheckFlagsSynthetic                         = CheckFlagsSyntheticProperty | CheckFlagsSyntheticMethod
 	CheckFlagsNonUniformAndLiteral              = CheckFlagsHasNonUniformType | CheckFlagsHasLiteralType
 	CheckFlagsPartial                           = CheckFlagsReadPartial | CheckFlagsWritePartial
diff --git a/tsc/internal/checker/checker.go b/tsc/internal/checker/checker.go
index ba4c3fa2718b6..9c9cb64c78a31 100644
--- a/tsc/internal/checker/checker.go
+++ b/tsc/internal/checker/checker.go
@@ -11922,10 +11922,14 @@ func (c *Checker) checkPropertyAccessibilityAtLocation(location *ast.Node, isSup
 	// Property is known to be private or protected at this point
 	// Private property is accessible if the property is within the declaring class
 	if flags&ast.ModifierFlagsPrivate != 0 {
-		declaringClassDeclaration := ast.GetClassLikeDeclarationOfSymbol(c.getParentOfSymbol(prop))
-		if !c.isNodeWithinClass(location, declaringClassDeclaration) {
+		var declaringClassDeclaration *ast.Node
+		if parent := c.getParentOfSymbol(prop); parent != nil {
+			declaringClassDeclaration = ast.GetClassLikeDeclarationOfSymbol(parent)
+		}
+		if declaringClassDeclaration == nil || !c.isNodeWithinClass(location, declaringClassDeclaration) {
 			if errorNode != nil {
-				c.error(errorNode, diagnostics.Property_0_is_private_and_only_accessible_within_class_1, c.symbolToString(prop), c.TypeToString(c.getDeclaringClass(prop)))
+				class := core.OrElse(c.getDeclaringClass(prop), containingType)
+				c.error(errorNode, diagnostics.Property_0_is_private_and_only_accessible_within_class_1, c.symbolToString(prop), c.TypeToString(class))
 			}
 			return false
 		}
@@ -11958,10 +11962,7 @@ func (c *Checker) checkPropertyAccessibilityAtLocation(location *ast.Node, isSup
 		}
 		if flags&ast.ModifierFlagsStatic != 0 || enclosingClass == nil {
 			if errorNode != nil {
-				class := c.getDeclaringClass(prop)
-				if class == nil {
-					class = containingType
-				}
+				class := core.OrElse(c.getDeclaringClass(prop), containingType)
 				c.error(errorNode, diagnostics.Property_0_is_protected_and_only_accessible_within_class_1_and_its_subclasses, c.symbolToString(prop), c.TypeToString(class))
 			}
 			return false
@@ -21585,6 +21586,7 @@ func (c *Checker) createUnionOrIntersectionProperty(containingType *Type, name s
 			var modifiers ast.ModifierFlags
 			if prop != nil {
 				modifiers = getDeclarationModifierFlagsFromSymbol(prop)
+				writeModifiers := getDeclarationModifierFlagsFromSymbolEx(prop, true /*isWrite*/)
 				if prop.Flags&ast.SymbolFlagsClassMember != 0 {
 					if isUnion {
 						optionalFlag |= prop.Flags & ast.SymbolFlagsOptional
@@ -21624,14 +21626,19 @@ func (c *Checker) createUnionOrIntersectionProperty(containingType *Type, name s
 				} else if !isUnion && !c.isReadonlySymbol(prop) {
 					checkFlags &^= ast.CheckFlagsReadonly
 				}
-				if modifiers&ast.ModifierFlagsNonPublicAccessibilityModifier == 0 {
-					checkFlags |= ast.CheckFlagsContainsPublic
-				}
-				if modifiers&ast.ModifierFlagsProtected != 0 {
+				if modifiers&ast.ModifierFlagsProtected != 0 && modifiers&ast.ModifierFlagsPublic == 0 {
 					checkFlags |= ast.CheckFlagsContainsProtected
-				}
-				if modifiers&ast.ModifierFlagsPrivate != 0 {
+				} else if modifiers&ast.ModifierFlagsPrivate != 0 && modifiers&ast.ModifierFlagsPublic == 0 {
 					checkFlags |= ast.CheckFlagsContainsPrivate
+				} else {
+					checkFlags |= ast.CheckFlagsContainsPublic
+				}
+				if writeModifiers&ast.ModifierFlagsProtected != 0 && writeModifiers&ast.ModifierFlagsPublic == 0 {
+					checkFlags |= ast.CheckFlagsContainsWriteProtected
+				} else if writeModifiers&ast.ModifierFlagsPrivate != 0 && writeModifiers&ast.ModifierFlagsPublic == 0 {
+					checkFlags |= ast.CheckFlagsContainsWritePrivate
+				} else {
+					checkFlags |= ast.CheckFlagsContainsWritePublic
 				}
 				if modifiers&ast.ModifierFlagsStatic != 0 {
 					checkFlags |= ast.CheckFlagsContainsStatic
@@ -21665,13 +21672,27 @@ func (c *Checker) createUnionOrIntersectionProperty(containingType *Type, name s
 			}
 		}
 	}
-	if singleProp == nil || isUnion &&
+	if singleProp == nil {
+		// No property was found
+		return nil
+	}
+	if isUnion &&
 		(propSet.Size() != 0 || checkFlags&ast.CheckFlagsPartial != 0) &&
-		checkFlags&(ast.CheckFlagsContainsPrivate|ast.CheckFlagsContainsProtected) != 0 &&
+		checkFlags&(ast.CheckFlagsContainsPrivate|ast.CheckFlagsContainsProtected|ast.CheckFlagsContainsWritePrivate|ast.CheckFlagsContainsWriteProtected) != 0 &&
 		!(propSet.Size() != 0 && c.hasCommonDeclaration(&propSet)) {
-		// No property was found, or, in a union, a property has a private or protected declaration in one
-		// constituent, but is missing or has a different declaration in another constituent.
-		return nil
+		// A property in a union has a private or protected declaration in one constituent, but is missing
+		// or has a different declaration in another constituent. If the private or protected declaration is
+		// for reading, we don't create a property.
+		if checkFlags&(ast.CheckFlagsContainsPrivate|ast.CheckFlagsContainsProtected) != 0 {
+			return nil
+		}
+		// Otherwise, if the private or protected declaration is for writing, reduce accessibility to that of
+		// the most restricted constituent.
+		if checkFlags&ast.CheckFlagsContainsWritePrivate != 0 {
+			checkFlags &^= ast.CheckFlagsContainsWritePublic | ast.CheckFlagsContainsWriteProtected
+		} else if checkFlags&ast.CheckFlagsContainsWriteProtected != 0 {
+			checkFlags &^= ast.CheckFlagsContainsWritePublic
+		}
 	}
 	if propSet.Size() == 0 && checkFlags&ast.CheckFlagsReadPartial == 0 && len(indexTypes) == 0 {
 		if !mergedInstantiations {
diff --git a/tsc/internal/checker/utilities.go b/tsc/internal/checker/utilities.go
index 69b2362c8fb58..9c486a5bb90ab 100644
--- a/tsc/internal/checker/utilities.go
+++ b/tsc/internal/checker/utilities.go
@@ -715,6 +715,21 @@ func getDeclarationModifierFlagsFromSymbol(s *ast.Symbol) ast.ModifierFlags {
 }
 
 func getDeclarationModifierFlagsFromSymbolEx(s *ast.Symbol, isWrite bool) ast.ModifierFlags {
+	if s.CheckFlags&ast.CheckFlagsSynthetic != 0 {
+		var accessModifier ast.ModifierFlags
+		switch {
+		case !isWrite && s.CheckFlags&ast.CheckFlagsContainsPublic != 0 || isWrite && s.CheckFlags&ast.CheckFlagsContainsWritePublic != 0:
+			accessModifier = ast.ModifierFlagsPublic
+		case !isWrite && s.CheckFlags&ast.CheckFlagsContainsProtected != 0 || isWrite && s.CheckFlags&ast.CheckFlagsContainsWriteProtected != 0:
+			accessModifier = ast.ModifierFlagsProtected
+		case !isWrite && s.CheckFlags&ast.CheckFlagsContainsPrivate != 0 || isWrite && s.CheckFlags&ast.CheckFlagsContainsWritePrivate != 0:
+			accessModifier = ast.ModifierFlagsPrivate
+		}
+		if s.CheckFlags&ast.CheckFlagsContainsStatic != 0 {
+			return accessModifier | ast.ModifierFlagsStatic
+		}
+		return accessModifier
+	}
 	if s.ValueDeclaration != nil {
 		var declaration *ast.Node
 		if isWrite {
@@ -732,22 +747,6 @@ func getDeclarationModifierFlagsFromSymbolEx(s *ast.Symbol, isWrite bool) ast.Mo
 		}
 		return flags & ^ast.ModifierFlagsAccessibilityModifier
 	}
-	if s.CheckFlags&ast.CheckFlagsSynthetic != 0 {
-		var accessModifier ast.ModifierFlags
-		switch {
-		case s.CheckFlags&ast.CheckFlagsContainsPrivate != 0:
-			accessModifier = ast.ModifierFlagsPrivate
-		case s.CheckFlags&ast.CheckFlagsContainsPublic != 0:
-			accessModifier = ast.ModifierFlagsPublic
-		default:
-			accessModifier = ast.ModifierFlagsProtected
-		}
-		var staticModifier ast.ModifierFlags
-		if s.CheckFlags&ast.CheckFlagsContainsStatic != 0 {
-			staticModifier = ast.ModifierFlagsStatic
-		}
-		return accessModifier | staticModifier
-	}
 	if s.Flags&ast.SymbolFlagsPrototype != 0 {
 		return ast.ModifierFlagsPublic | ast.ModifierFlagsStatic
 	}
diff --git a/tsc/testdata/baselines/reference/compiler/syntheticProtectedProperties.errors.txt b/tsc/testdata/baselines/reference/compiler/syntheticProtectedProperties.errors.txt
new file mode 100644
index 0000000000000..57504522a12bd
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/syntheticProtectedProperties.errors.txt
@@ -0,0 +1,168 @@
+syntheticProtectedProperties.ts(65,6): error TS2445: Property 'foo' is protected and only accessible within class 'C1 | C2' and its subclasses.
+syntheticProtectedProperties.ts(68,6): error TS2339: Property 'foo' does not exist on type 'C1 | C3'.
+syntheticProtectedProperties.ts(69,6): error TS2339: Property 'foo' does not exist on type 'C1 | C3'.
+syntheticProtectedProperties.ts(72,6): error TS2339: Property 'foo' does not exist on type 'C2 | C3'.
+syntheticProtectedProperties.ts(73,6): error TS2339: Property 'foo' does not exist on type 'C2 | C3'.
+syntheticProtectedProperties.ts(77,6): error TS2341: Property 'foo' is private and only accessible within class 'P1 | P2'.
+syntheticProtectedProperties.ts(80,6): error TS2339: Property 'foo' does not exist on type 'P1 | P3'.
+syntheticProtectedProperties.ts(81,6): error TS2339: Property 'foo' does not exist on type 'P1 | P3'.
+syntheticProtectedProperties.ts(84,6): error TS2339: Property 'foo' does not exist on type 'P2 | P3'.
+syntheticProtectedProperties.ts(85,6): error TS2339: Property 'foo' does not exist on type 'P2 | P3'.
+syntheticProtectedProperties.ts(99,6): error TS2445: Property 'foo' is protected and only accessible within class 'C2 & C3' and its subclasses.
+syntheticProtectedProperties.ts(106,6): error TS2339: Property 'foo' does not exist on type 'never'.
+  The intersection 'P1 & P3' was reduced to 'never' because property 'foo' exists in multiple constituents and is private in some.
+syntheticProtectedProperties.ts(107,6): error TS2339: Property 'foo' does not exist on type 'never'.
+  The intersection 'P1 & P3' was reduced to 'never' because property 'foo' exists in multiple constituents and is private in some.
+syntheticProtectedProperties.ts(110,6): error TS2339: Property 'foo' does not exist on type 'never'.
+  The intersection 'P2 & P3' was reduced to 'never' because property 'foo' exists in multiple constituents and is private in some.
+syntheticProtectedProperties.ts(111,6): error TS2339: Property 'foo' does not exist on type 'never'.
+  The intersection 'P2 & P3' was reduced to 'never' because property 'foo' exists in multiple constituents and is private in some.
+
+
+==== syntheticProtectedProperties.ts (15 errors) ====
+    // https://github.com/microsoft/TypeScript/issues/63749
+    
+    declare class Dummy {
+        a: number;
+    }
+    
+    type Public = Base | Dummy;
+    
+    declare class Base {
+        protected get content(): string;
+        protected set content(value: string);
+    }
+    
+    declare class Mock {
+        get content(): string;
+        set content(value: string);
+    }
+    
+    declare const w: Public
+    if (w instanceof Mock) {
+        w.content;
+    }
+    declare const w2: Mock & Public
+    w2.content;
+    
+    declare const w3: Public & Mock
+    w3.content;
+    
+    // private/protected set accessors are tracked in unions and intersections
+    
+    declare class C1 {
+        get foo(): number;
+        set foo(value: number);
+    }
+    
+    declare class C2 {
+        get foo(): number;
+        protected set foo(value: number);
+    }
+    
+    declare class C3 {
+        protected get foo(): number;
+        protected set foo(value: number);
+    }
+    
+    declare class P1 {
+        get foo(): number;
+        set foo(value: number);
+    }
+    
+    declare class P2 {
+        get foo(): number;
+        private set foo(value: number);
+    }
+    
+    declare class P3 {
+        private get foo(): number;
+        private set foo(value: number);
+    }
+    
+    // Unions properties have most restricted accessibility
+    
+    declare const cu12: C1 | C2;
+    cu12.foo;
+    cu12.foo = 123;  // Error, protected
+         ~~~
+!!! error TS2445: Property 'foo' is protected and only accessible within class 'C1 | C2' and its subclasses.
+    
+    declare const cu13: C1 | C3;
+    cu13.foo;  // Error, no property
+         ~~~
+!!! error TS2339: Property 'foo' does not exist on type 'C1 | C3'.
+    cu13.foo = 123;  // Error, no property
+         ~~~
+!!! error TS2339: Property 'foo' does not exist on type 'C1 | C3'.
+    
+    declare const cu23: C2 | C3;
+    cu23.foo;  // Error, no property
+         ~~~
+!!! error TS2339: Property 'foo' does not exist on type 'C2 | C3'.
+    cu23.foo = 123;  // Error, no property
+         ~~~
+!!! error TS2339: Property 'foo' does not exist on type 'C2 | C3'.
+    
+    declare const pu12: P1 | P2;
+    pu12.foo;
+    pu12.foo = 123;  // Error, private
+         ~~~
+!!! error TS2341: Property 'foo' is private and only accessible within class 'P1 | P2'.
+    
+    declare const pu13: P1 | P3;
+    pu13.foo;  // Error, no property
+         ~~~
+!!! error TS2339: Property 'foo' does not exist on type 'P1 | P3'.
+    pu13.foo = 123;  // Error, no property
+         ~~~
+!!! error TS2339: Property 'foo' does not exist on type 'P1 | P3'.
+    
+    declare const pu23: P2 | P3;
+    pu23.foo;  // Error, no property
+         ~~~
+!!! error TS2339: Property 'foo' does not exist on type 'P2 | P3'.
+    pu23.foo = 123;  // Error, no property
+         ~~~
+!!! error TS2339: Property 'foo' does not exist on type 'P2 | P3'.
+    
+    // Intersection properties have most permissive accessibility
+    
+    declare const ci12: C1 & C2;
+    ci12.foo;
+    ci12.foo = 123;
+    
+    declare const ci13: C1 & C3;
+    ci13.foo;
+    ci13.foo = 123;
+    
+    declare const ci23: C2 & C3;
+    ci23.foo;
+    ci23.foo = 123;  // Error, protected
+         ~~~
+!!! error TS2445: Property 'foo' is protected and only accessible within class 'C2 & C3' and its subclasses.
+    
+    declare const pi12: P1 & P2;
+    pi12.foo;
+    pi12.foo = 123;
+    
+    declare const pi13: P1 & P3;
+    pi13.foo;  // Error, reduced to never
+         ~~~
+!!! error TS2339: Property 'foo' does not exist on type 'never'.
+!!! error TS2339:   The intersection 'P1 & P3' was reduced to 'never' because property 'foo' exists in multiple constituents and is private in some.
+    pi13.foo = 123;  // Error, reduced to never
+         ~~~
+!!! error TS2339: Property 'foo' does not exist on type 'never'.
+!!! error TS2339:   The intersection 'P1 & P3' was reduced to 'never' because property 'foo' exists in multiple constituents and is private in some.
+    
+    declare const pi23: P2 & P3;
+    pi23.foo;  // Error, reduced to never
+         ~~~
+!!! error TS2339: Property 'foo' does not exist on type 'never'.
+!!! error TS2339:   The intersection 'P2 & P3' was reduced to 'never' because property 'foo' exists in multiple constituents and is private in some.
+    pi23.foo = 123;  // Error, reduced to never
+         ~~~
+!!! error TS2339: Property 'foo' does not exist on type 'never'.
+!!! error TS2339:   The intersection 'P2 & P3' was reduced to 'never' because property 'foo' exists in multiple constituents and is private in some.
+    
\ No newline at end of file
diff --git a/tsc/testdata/baselines/reference/compiler/syntheticProtectedProperties.symbols b/tsc/testdata/baselines/reference/compiler/syntheticProtectedProperties.symbols
new file mode 100644
index 0000000000000..25260556ee2f5
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/syntheticProtectedProperties.symbols
@@ -0,0 +1,300 @@
+//// [tests/cases/compiler/syntheticProtectedProperties.ts] ////
+
+=== syntheticProtectedProperties.ts ===
+// https://github.com/microsoft/TypeScript/issues/63749
+
+declare class Dummy {
+>Dummy : Symbol(Dummy, Decl(syntheticProtectedProperties.ts, 0, 0))
+
+    a: number;
+>a : Symbol(Dummy.a, Decl(syntheticProtectedProperties.ts, 2, 21))
+}
+
+type Public = Base | Dummy;
+>Public : Symbol(Public, Decl(syntheticProtectedProperties.ts, 4, 1))
+>Base : Symbol(Base, Decl(syntheticProtectedProperties.ts, 6, 27))
+>Dummy : Symbol(Dummy, Decl(syntheticProtectedProperties.ts, 0, 0))
+
+declare class Base {
+>Base : Symbol(Base, Decl(syntheticProtectedProperties.ts, 6, 27))
+
+    protected get content(): string;
+>content : Symbol(Base.content, Decl(syntheticProtectedProperties.ts, 8, 20), Decl(syntheticProtectedProperties.ts, 9, 36))
+
+    protected set content(value: string);
+>content : Symbol(Base.content, Decl(syntheticProtectedProperties.ts, 8, 20), Decl(syntheticProtectedProperties.ts, 9, 36))
+>value : Symbol(value, Decl(syntheticProtectedProperties.ts, 10, 26))
+}
+
+declare class Mock {
+>Mock : Symbol(Mock, Decl(syntheticProtectedProperties.ts, 11, 1))
+
+    get content(): string;
+>content : Symbol(Mock.content, Decl(syntheticProtectedProperties.ts, 13, 20), Decl(syntheticProtectedProperties.ts, 14, 26))
+
+    set content(value: string);
+>content : Symbol(Mock.content, Decl(syntheticProtectedProperties.ts, 13, 20), Decl(syntheticProtectedProperties.ts, 14, 26))
+>value : Symbol(value, Decl(syntheticProtectedProperties.ts, 15, 16))
+}
+
+declare const w: Public
+>w : Symbol(w, Decl(syntheticProtectedProperties.ts, 18, 13))
+>Public : Symbol(Public, Decl(syntheticProtectedProperties.ts, 4, 1))
+
+if (w instanceof Mock) {
+>w : Symbol(w, Decl(syntheticProtectedProperties.ts, 18, 13))
+>Mock : Symbol(Mock, Decl(syntheticProtectedProperties.ts, 11, 1))
+
+    w.content;
+>w.content : Symbol(Mock.content, Decl(syntheticProtectedProperties.ts, 8, 20), Decl(syntheticProtectedProperties.ts, 9, 36), Decl(syntheticProtectedProperties.ts, 13, 20), Decl(syntheticProtectedProperties.ts, 14, 26))
+>w : Symbol(w, Decl(syntheticProtectedProperties.ts, 18, 13))
+>content : Symbol(Mock.content, Decl(syntheticProtectedProperties.ts, 8, 20), Decl(syntheticProtectedProperties.ts, 9, 36), Decl(syntheticProtectedProperties.ts, 13, 20), Decl(syntheticProtectedProperties.ts, 14, 26))
+}
+declare const w2: Mock & Public
+>w2 : Symbol(w2, Decl(syntheticProtectedProperties.ts, 22, 13))
+>Mock : Symbol(Mock, Decl(syntheticProtectedProperties.ts, 11, 1))
+>Public : Symbol(Public, Decl(syntheticProtectedProperties.ts, 4, 1))
+
+w2.content;
+>w2.content : Symbol(Mock.content, Decl(syntheticProtectedProperties.ts, 13, 20), Decl(syntheticProtectedProperties.ts, 14, 26), Decl(syntheticProtectedProperties.ts, 8, 20), Decl(syntheticProtectedProperties.ts, 9, 36))
+>w2 : Symbol(w2, Decl(syntheticProtectedProperties.ts, 22, 13))
+>content : Symbol(Mock.content, Decl(syntheticProtectedProperties.ts, 13, 20), Decl(syntheticProtectedProperties.ts, 14, 26), Decl(syntheticProtectedProperties.ts, 8, 20), Decl(syntheticProtectedProperties.ts, 9, 36))
+
+declare const w3: Public & Mock
+>w3 : Symbol(w3, Decl(syntheticProtectedProperties.ts, 25, 13))
+>Public : Symbol(Public, Decl(syntheticProtectedProperties.ts, 4, 1))
+>Mock : Symbol(Mock, Decl(syntheticProtectedProperties.ts, 11, 1))
+
+w3.content;
+>w3.content : Symbol(Mock.content, Decl(syntheticProtectedProperties.ts, 8, 20), Decl(syntheticProtectedProperties.ts, 9, 36), Decl(syntheticProtectedProperties.ts, 13, 20), Decl(syntheticProtectedProperties.ts, 14, 26))
+>w3 : Symbol(w3, Decl(syntheticProtectedProperties.ts, 25, 13))
+>content : Symbol(Mock.content, Decl(syntheticProtectedProperties.ts, 8, 20), Decl(syntheticProtectedProperties.ts, 9, 36), Decl(syntheticProtectedProperties.ts, 13, 20), Decl(syntheticProtectedProperties.ts, 14, 26))
+
+// private/protected set accessors are tracked in unions and intersections
+
+declare class C1 {
+>C1 : Symbol(C1, Decl(syntheticProtectedProperties.ts, 26, 11))
+
+    get foo(): number;
+>foo : Symbol(C1.foo, Decl(syntheticProtectedProperties.ts, 30, 18), Decl(syntheticProtectedProperties.ts, 31, 22))
+
+    set foo(value: number);
+>foo : Symbol(C1.foo, Decl(syntheticProtectedProperties.ts, 30, 18), Decl(syntheticProtectedProperties.ts, 31, 22))
+>value : Symbol(value, Decl(syntheticProtectedProperties.ts, 32, 12))
+}
+
+declare class C2 {
+>C2 : Symbol(C2, Decl(syntheticProtectedProperties.ts, 33, 1))
+
+    get foo(): number;
+>foo : Symbol(C2.foo, Decl(syntheticProtectedProperties.ts, 35, 18), Decl(syntheticProtectedProperties.ts, 36, 22))
+
+    protected set foo(value: number);
+>foo : Symbol(C2.foo, Decl(syntheticProtectedProperties.ts, 35, 18), Decl(syntheticProtectedProperties.ts, 36, 22))
+>value : Symbol(value, Decl(syntheticProtectedProperties.ts, 37, 22))
+}
+
+declare class C3 {
+>C3 : Symbol(C3, Decl(syntheticProtectedProperties.ts, 38, 1))
+
+    protected get foo(): number;
+>foo : Symbol(C3.foo, Decl(syntheticProtectedProperties.ts, 40, 18), Decl(syntheticProtectedProperties.ts, 41, 32))
+
+    protected set foo(value: number);
+>foo : Symbol(C3.foo, Decl(syntheticProtectedProperties.ts, 40, 18), Decl(syntheticProtectedProperties.ts, 41, 32))
+>value : Symbol(value, Decl(syntheticProtectedProperties.ts, 42, 22))
+}
+
+declare class P1 {
+>P1 : Symbol(P1, Decl(syntheticProtectedProperties.ts, 43, 1))
+
+    get foo(): number;
+>foo : Symbol(P1.foo, Decl(syntheticProtectedProperties.ts, 45, 18), Decl(syntheticProtectedProperties.ts, 46, 22))
+
+    set foo(value: number);
+>foo : Symbol(P1.foo, Decl(syntheticProtectedProperties.ts, 45, 18), Decl(syntheticProtectedProperties.ts, 46, 22))
+>value : Symbol(value, Decl(syntheticProtectedProperties.ts, 47, 12))
+}
+
+declare class P2 {
+>P2 : Symbol(P2, Decl(syntheticProtectedProperties.ts, 48, 1))
+
+    get foo(): number;
+>foo : Symbol(P2.foo, Decl(syntheticProtectedProperties.ts, 50, 18), Decl(syntheticProtectedProperties.ts, 51, 22))
+
+    private set foo(value: number);
+>foo : Symbol(P2.foo, Decl(syntheticProtectedProperties.ts, 50, 18), Decl(syntheticProtectedProperties.ts, 51, 22))
+>value : Symbol(value, Decl(syntheticProtectedProperties.ts, 52, 20))
+}
+
+declare class P3 {
+>P3 : Symbol(P3, Decl(syntheticProtectedProperties.ts, 53, 1))
+
+    private get foo(): number;
+>foo : Symbol(P3.foo, Decl(syntheticProtectedProperties.ts, 55, 18), Decl(syntheticProtectedProperties.ts, 56, 30))
+
+    private set foo(value: number);
+>foo : Symbol(P3.foo, Decl(syntheticProtectedProperties.ts, 55, 18), Decl(syntheticProtectedProperties.ts, 56, 30))
+>value : Symbol(value, Decl(syntheticProtectedProperties.ts, 57, 20))
+}
+
+// Unions properties have most restricted accessibility
+
+declare const cu12: C1 | C2;
+>cu12 : Symbol(cu12, Decl(syntheticProtectedProperties.ts, 62, 13))
+>C1 : Symbol(C1, Decl(syntheticProtectedProperties.ts, 26, 11))
+>C2 : Symbol(C2, Decl(syntheticProtectedProperties.ts, 33, 1))
+
+cu12.foo;
+>cu12.foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 30, 18), Decl(syntheticProtectedProperties.ts, 31, 22), Decl(syntheticProtectedProperties.ts, 35, 18), Decl(syntheticProtectedProperties.ts, 36, 22))
+>cu12 : Symbol(cu12, Decl(syntheticProtectedProperties.ts, 62, 13))
+>foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 30, 18), Decl(syntheticProtectedProperties.ts, 31, 22), Decl(syntheticProtectedProperties.ts, 35, 18), Decl(syntheticProtectedProperties.ts, 36, 22))
+
+cu12.foo = 123;  // Error, protected
+>cu12.foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 30, 18), Decl(syntheticProtectedProperties.ts, 31, 22), Decl(syntheticProtectedProperties.ts, 35, 18), Decl(syntheticProtectedProperties.ts, 36, 22))
+>cu12 : Symbol(cu12, Decl(syntheticProtectedProperties.ts, 62, 13))
+>foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 30, 18), Decl(syntheticProtectedProperties.ts, 31, 22), Decl(syntheticProtectedProperties.ts, 35, 18), Decl(syntheticProtectedProperties.ts, 36, 22))
+
+declare const cu13: C1 | C3;
+>cu13 : Symbol(cu13, Decl(syntheticProtectedProperties.ts, 66, 13))
+>C1 : Symbol(C1, Decl(syntheticProtectedProperties.ts, 26, 11))
+>C3 : Symbol(C3, Decl(syntheticProtectedProperties.ts, 38, 1))
+
+cu13.foo;  // Error, no property
+>cu13 : Symbol(cu13, Decl(syntheticProtectedProperties.ts, 66, 13))
+
+cu13.foo = 123;  // Error, no property
+>cu13 : Symbol(cu13, Decl(syntheticProtectedProperties.ts, 66, 13))
+
+declare const cu23: C2 | C3;
+>cu23 : Symbol(cu23, Decl(syntheticProtectedProperties.ts, 70, 13))
+>C2 : Symbol(C2, Decl(syntheticProtectedProperties.ts, 33, 1))
+>C3 : Symbol(C3, Decl(syntheticProtectedProperties.ts, 38, 1))
+
+cu23.foo;  // Error, no property
+>cu23 : Symbol(cu23, Decl(syntheticProtectedProperties.ts, 70, 13))
+
+cu23.foo = 123;  // Error, no property
+>cu23 : Symbol(cu23, Decl(syntheticProtectedProperties.ts, 70, 13))
+
+declare const pu12: P1 | P2;
+>pu12 : Symbol(pu12, Decl(syntheticProtectedProperties.ts, 74, 13))
+>P1 : Symbol(P1, Decl(syntheticProtectedProperties.ts, 43, 1))
+>P2 : Symbol(P2, Decl(syntheticProtectedProperties.ts, 48, 1))
+
+pu12.foo;
+>pu12.foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 45, 18), Decl(syntheticProtectedProperties.ts, 46, 22), Decl(syntheticProtectedProperties.ts, 50, 18), Decl(syntheticProtectedProperties.ts, 51, 22))
+>pu12 : Symbol(pu12, Decl(syntheticProtectedProperties.ts, 74, 13))
+>foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 45, 18), Decl(syntheticProtectedProperties.ts, 46, 22), Decl(syntheticProtectedProperties.ts, 50, 18), Decl(syntheticProtectedProperties.ts, 51, 22))
+
+pu12.foo = 123;  // Error, private
+>pu12.foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 45, 18), Decl(syntheticProtectedProperties.ts, 46, 22), Decl(syntheticProtectedProperties.ts, 50, 18), Decl(syntheticProtectedProperties.ts, 51, 22))
+>pu12 : Symbol(pu12, Decl(syntheticProtectedProperties.ts, 74, 13))
+>foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 45, 18), Decl(syntheticProtectedProperties.ts, 46, 22), Decl(syntheticProtectedProperties.ts, 50, 18), Decl(syntheticProtectedProperties.ts, 51, 22))
+
+declare const pu13: P1 | P3;
+>pu13 : Symbol(pu13, Decl(syntheticProtectedProperties.ts, 78, 13))
+>P1 : Symbol(P1, Decl(syntheticProtectedProperties.ts, 43, 1))
+>P3 : Symbol(P3, Decl(syntheticProtectedProperties.ts, 53, 1))
+
+pu13.foo;  // Error, no property
+>pu13 : Symbol(pu13, Decl(syntheticProtectedProperties.ts, 78, 13))
+
+pu13.foo = 123;  // Error, no property
+>pu13 : Symbol(pu13, Decl(syntheticProtectedProperties.ts, 78, 13))
+
+declare const pu23: P2 | P3;
+>pu23 : Symbol(pu23, Decl(syntheticProtectedProperties.ts, 82, 13))
+>P2 : Symbol(P2, Decl(syntheticProtectedProperties.ts, 48, 1))
+>P3 : Symbol(P3, Decl(syntheticProtectedProperties.ts, 53, 1))
+
+pu23.foo;  // Error, no property
+>pu23 : Symbol(pu23, Decl(syntheticProtectedProperties.ts, 82, 13))
+
+pu23.foo = 123;  // Error, no property
+>pu23 : Symbol(pu23, Decl(syntheticProtectedProperties.ts, 82, 13))
+
+// Intersection properties have most permissive accessibility
+
+declare const ci12: C1 & C2;
+>ci12 : Symbol(ci12, Decl(syntheticProtectedProperties.ts, 88, 13))
+>C1 : Symbol(C1, Decl(syntheticProtectedProperties.ts, 26, 11))
+>C2 : Symbol(C2, Decl(syntheticProtectedProperties.ts, 33, 1))
+
+ci12.foo;
+>ci12.foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 30, 18), Decl(syntheticProtectedProperties.ts, 31, 22), Decl(syntheticProtectedProperties.ts, 35, 18), Decl(syntheticProtectedProperties.ts, 36, 22))
+>ci12 : Symbol(ci12, Decl(syntheticProtectedProperties.ts, 88, 13))
+>foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 30, 18), Decl(syntheticProtectedProperties.ts, 31, 22), Decl(syntheticProtectedProperties.ts, 35, 18), Decl(syntheticProtectedProperties.ts, 36, 22))
+
+ci12.foo = 123;
+>ci12.foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 30, 18), Decl(syntheticProtectedProperties.ts, 31, 22), Decl(syntheticProtectedProperties.ts, 35, 18), Decl(syntheticProtectedProperties.ts, 36, 22))
+>ci12 : Symbol(ci12, Decl(syntheticProtectedProperties.ts, 88, 13))
+>foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 30, 18), Decl(syntheticProtectedProperties.ts, 31, 22), Decl(syntheticProtectedProperties.ts, 35, 18), Decl(syntheticProtectedProperties.ts, 36, 22))
+
+declare const ci13: C1 & C3;
+>ci13 : Symbol(ci13, Decl(syntheticProtectedProperties.ts, 92, 13))
+>C1 : Symbol(C1, Decl(syntheticProtectedProperties.ts, 26, 11))
+>C3 : Symbol(C3, Decl(syntheticProtectedProperties.ts, 38, 1))
+
+ci13.foo;
+>ci13.foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 30, 18), Decl(syntheticProtectedProperties.ts, 31, 22), Decl(syntheticProtectedProperties.ts, 40, 18), Decl(syntheticProtectedProperties.ts, 41, 32))
+>ci13 : Symbol(ci13, Decl(syntheticProtectedProperties.ts, 92, 13))
+>foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 30, 18), Decl(syntheticProtectedProperties.ts, 31, 22), Decl(syntheticProtectedProperties.ts, 40, 18), Decl(syntheticProtectedProperties.ts, 41, 32))
+
+ci13.foo = 123;
+>ci13.foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 30, 18), Decl(syntheticProtectedProperties.ts, 31, 22), Decl(syntheticProtectedProperties.ts, 40, 18), Decl(syntheticProtectedProperties.ts, 41, 32))
+>ci13 : Symbol(ci13, Decl(syntheticProtectedProperties.ts, 92, 13))
+>foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 30, 18), Decl(syntheticProtectedProperties.ts, 31, 22), Decl(syntheticProtectedProperties.ts, 40, 18), Decl(syntheticProtectedProperties.ts, 41, 32))
+
+declare const ci23: C2 & C3;
+>ci23 : Symbol(ci23, Decl(syntheticProtectedProperties.ts, 96, 13))
+>C2 : Symbol(C2, Decl(syntheticProtectedProperties.ts, 33, 1))
+>C3 : Symbol(C3, Decl(syntheticProtectedProperties.ts, 38, 1))
+
+ci23.foo;
+>ci23.foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 35, 18), Decl(syntheticProtectedProperties.ts, 36, 22), Decl(syntheticProtectedProperties.ts, 40, 18), Decl(syntheticProtectedProperties.ts, 41, 32))
+>ci23 : Symbol(ci23, Decl(syntheticProtectedProperties.ts, 96, 13))
+>foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 35, 18), Decl(syntheticProtectedProperties.ts, 36, 22), Decl(syntheticProtectedProperties.ts, 40, 18), Decl(syntheticProtectedProperties.ts, 41, 32))
+
+ci23.foo = 123;  // Error, protected
+>ci23.foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 35, 18), Decl(syntheticProtectedProperties.ts, 36, 22), Decl(syntheticProtectedProperties.ts, 40, 18), Decl(syntheticProtectedProperties.ts, 41, 32))
+>ci23 : Symbol(ci23, Decl(syntheticProtectedProperties.ts, 96, 13))
+>foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 35, 18), Decl(syntheticProtectedProperties.ts, 36, 22), Decl(syntheticProtectedProperties.ts, 40, 18), Decl(syntheticProtectedProperties.ts, 41, 32))
+
+declare const pi12: P1 & P2;
+>pi12 : Symbol(pi12, Decl(syntheticProtectedProperties.ts, 100, 13))
+>P1 : Symbol(P1, Decl(syntheticProtectedProperties.ts, 43, 1))
+>P2 : Symbol(P2, Decl(syntheticProtectedProperties.ts, 48, 1))
+
+pi12.foo;
+>pi12.foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 45, 18), Decl(syntheticProtectedProperties.ts, 46, 22), Decl(syntheticProtectedProperties.ts, 50, 18), Decl(syntheticProtectedProperties.ts, 51, 22))
+>pi12 : Symbol(pi12, Decl(syntheticProtectedProperties.ts, 100, 13))
+>foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 45, 18), Decl(syntheticProtectedProperties.ts, 46, 22), Decl(syntheticProtectedProperties.ts, 50, 18), Decl(syntheticProtectedProperties.ts, 51, 22))
+
+pi12.foo = 123;
+>pi12.foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 45, 18), Decl(syntheticProtectedProperties.ts, 46, 22), Decl(syntheticProtectedProperties.ts, 50, 18), Decl(syntheticProtectedProperties.ts, 51, 22))
+>pi12 : Symbol(pi12, Decl(syntheticProtectedProperties.ts, 100, 13))
+>foo : Symbol(foo, Decl(syntheticProtectedProperties.ts, 45, 18), Decl(syntheticProtectedProperties.ts, 46, 22), Decl(syntheticProtectedProperties.ts, 50, 18), Decl(syntheticProtectedProperties.ts, 51, 22))
+
+declare const pi13: P1 & P3;
+>pi13 : Symbol(pi13, Decl(syntheticProtectedProperties.ts, 104, 13))
+>P1 : Symbol(P1, Decl(syntheticProtectedProperties.ts, 43, 1))
+>P3 : Symbol(P3, Decl(syntheticProtectedProperties.ts, 53, 1))
+
+pi13.foo;  // Error, reduced to never
+>pi13 : Symbol(pi13, Decl(syntheticProtectedProperties.ts, 104, 13))
+
+pi13.foo = 123;  // Error, reduced to never
+>pi13 : Symbol(pi13, Decl(syntheticProtectedProperties.ts, 104, 13))
+
+declare const pi23: P2 & P3;
+>pi23 : Symbol(pi23, Decl(syntheticProtectedProperties.ts, 108, 13))
+>P2 : Symbol(P2, Decl(syntheticProtectedProperties.ts, 48, 1))
+>P3 : Symbol(P3, Decl(syntheticProtectedProperties.ts, 53, 1))
+
+pi23.foo;  // Error, reduced to never
+>pi23 : Symbol(pi23, Decl(syntheticProtectedProperties.ts, 108, 13))
+
+pi23.foo = 123;  // Error, reduced to never
+>pi23 : Symbol(pi23, Decl(syntheticProtectedProperties.ts, 108, 13))
+
diff --git a/tsc/testdata/baselines/reference/compiler/syntheticProtectedProperties.types b/tsc/testdata/baselines/reference/compiler/syntheticProtectedProperties.types
new file mode 100644
index 0000000000000..9d40cbc76fa03
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/syntheticProtectedProperties.types
@@ -0,0 +1,318 @@
+//// [tests/cases/compiler/syntheticProtectedProperties.ts] ////
+
+=== syntheticProtectedProperties.ts ===
+// https://github.com/microsoft/TypeScript/issues/63749
+
+declare class Dummy {
+>Dummy : Dummy
+
+    a: number;
+>a : number
+}
+
+type Public = Base | Dummy;
+>Public : Public
+
+declare class Base {
+>Base : Base
+
+    protected get content(): string;
+>content : string
+
+    protected set content(value: string);
+>content : string
+>value : string
+}
+
+declare class Mock {
+>Mock : Mock
+
+    get content(): string;
+>content : string
+
+    set content(value: string);
+>content : string
+>value : string
+}
+
+declare const w: Public
+>w : Public
+
+if (w instanceof Mock) {
+>w instanceof Mock : boolean
+>w : Public
+>Mock : typeof Mock
+
+    w.content;
+>w.content : string
+>w : Public & Mock
+>content : string
+}
+declare const w2: Mock & Public
+>w2 : Mock & Public
+
+w2.content;
+>w2.content : string
+>w2 : Mock & Public
+>content : string
+
+declare const w3: Public & Mock
+>w3 : Public & Mock
+
+w3.content;
+>w3.content : string
+>w3 : Public & Mock
+>content : string
+
+// private/protected set accessors are tracked in unions and intersections
+
+declare class C1 {
+>C1 : C1
+
+    get foo(): number;
+>foo : number
+
+    set foo(value: number);
+>foo : number
+>value : number
+}
+
+declare class C2 {
+>C2 : C2
+
+    get foo(): number;
+>foo : number
+
+    protected set foo(value: number);
+>foo : number
+>value : number
+}
+
+declare class C3 {
+>C3 : C3
+
+    protected get foo(): number;
+>foo : number
+
+    protected set foo(value: number);
+>foo : number
+>value : number
+}
+
+declare class P1 {
+>P1 : P1
+
+    get foo(): number;
+>foo : number
+
+    set foo(value: number);
+>foo : number
+>value : number
+}
+
+declare class P2 {
+>P2 : P2
+
+    get foo(): number;
+>foo : number
+
+    private set foo(value: number);
+>foo : number
+>value : number
+}
+
+declare class P3 {
+>P3 : P3
+
+    private get foo(): number;
+>foo : number
+
+    private set foo(value: number);
+>foo : number
+>value : number
+}
+
+// Unions properties have most restricted accessibility
+
+declare const cu12: C1 | C2;
+>cu12 : C1 | C2
+
+cu12.foo;
+>cu12.foo : number
+>cu12 : C1 | C2
+>foo : number
+
+cu12.foo = 123;  // Error, protected
+>cu12.foo = 123 : 123
+>cu12.foo : number
+>cu12 : C1 | C2
+>foo : number
+>123 : 123
+
+declare const cu13: C1 | C3;
+>cu13 : C1 | C3
+
+cu13.foo;  // Error, no property
+>cu13.foo : any
+>cu13 : C1 | C3
+>foo : any
+
+cu13.foo = 123;  // Error, no property
+>cu13.foo = 123 : 123
+>cu13.foo : any
+>cu13 : C1 | C3
+>foo : any
+>123 : 123
+
+declare const cu23: C2 | C3;
+>cu23 : C2 | C3
+
+cu23.foo;  // Error, no property
+>cu23.foo : any
+>cu23 : C2 | C3
+>foo : any
+
+cu23.foo = 123;  // Error, no property
+>cu23.foo = 123 : 123
+>cu23.foo : any
+>cu23 : C2 | C3
+>foo : any
+>123 : 123
+
+declare const pu12: P1 | P2;
+>pu12 : P1 | P2
+
+pu12.foo;
+>pu12.foo : number
+>pu12 : P1 | P2
+>foo : number
+
+pu12.foo = 123;  // Error, private
+>pu12.foo = 123 : 123
+>pu12.foo : number
+>pu12 : P1 | P2
+>foo : number
+>123 : 123
+
+declare const pu13: P1 | P3;
+>pu13 : P1 | P3
+
+pu13.foo;  // Error, no property
+>pu13.foo : any
+>pu13 : P1 | P3
+>foo : any
+
+pu13.foo = 123;  // Error, no property
+>pu13.foo = 123 : 123
+>pu13.foo : any
+>pu13 : P1 | P3
+>foo : any
+>123 : 123
+
+declare const pu23: P2 | P3;
+>pu23 : P2 | P3
+
+pu23.foo;  // Error, no property
+>pu23.foo : any
+>pu23 : P2 | P3
+>foo : any
+
+pu23.foo = 123;  // Error, no property
+>pu23.foo = 123 : 123
+>pu23.foo : any
+>pu23 : P2 | P3
+>foo : any
+>123 : 123
+
+// Intersection properties have most permissive accessibility
+
+declare const ci12: C1 & C2;
+>ci12 : C1 & C2
+
+ci12.foo;
+>ci12.foo : number
+>ci12 : C1 & C2
+>foo : number
+
+ci12.foo = 123;
+>ci12.foo = 123 : 123
+>ci12.foo : number
+>ci12 : C1 & C2
+>foo : number
+>123 : 123
+
+declare const ci13: C1 & C3;
+>ci13 : C1 & C3
+
+ci13.foo;
+>ci13.foo : number
+>ci13 : C1 & C3
+>foo : number
+
+ci13.foo = 123;
+>ci13.foo = 123 : 123
+>ci13.foo : number
+>ci13 : C1 & C3
+>foo : number
+>123 : 123
+
+declare const ci23: C2 & C3;
+>ci23 : C2 & C3
+
+ci23.foo;
+>ci23.foo : number
+>ci23 : C2 & C3
+>foo : number
+
+ci23.foo = 123;  // Error, protected
+>ci23.foo = 123 : 123
+>ci23.foo : number
+>ci23 : C2 & C3
+>foo : number
+>123 : 123
+
+declare const pi12: P1 & P2;
+>pi12 : P1 & P2
+
+pi12.foo;
+>pi12.foo : number
+>pi12 : P1 & P2
+>foo : number
+
+pi12.foo = 123;
+>pi12.foo = 123 : 123
+>pi12.foo : number
+>pi12 : P1 & P2
+>foo : number
+>123 : 123
+
+declare const pi13: P1 & P3;
+>pi13 : never
+
+pi13.foo;  // Error, reduced to never
+>pi13.foo : any
+>pi13 : never
+>foo : any
+
+pi13.foo = 123;  // Error, reduced to never
+>pi13.foo = 123 : 123
+>pi13.foo : any
+>pi13 : never
+>foo : any
+>123 : 123
+
+declare const pi23: P2 & P3;
+>pi23 : never
+
+pi23.foo;  // Error, reduced to never
+>pi23.foo : any
+>pi23 : never
+>foo : any
+
+pi23.foo = 123;  // Error, reduced to never
+>pi23.foo = 123 : 123
+>pi23.foo : any
+>pi23 : never
+>foo : any
+>123 : 123
+
diff --git a/tsc/testdata/tests/cases/compiler/syntheticProtectedProperties.ts b/tsc/testdata/tests/cases/compiler/syntheticProtectedProperties.ts
new file mode 100644
index 0000000000000..7bb293f26ca30
--- /dev/null
+++ b/tsc/testdata/tests/cases/compiler/syntheticProtectedProperties.ts
@@ -0,0 +1,113 @@
+// @noEmit: true
+
+// https://github.com/microsoft/TypeScript/issues/63749
+
+declare class Dummy {
+    a: number;
+}
+
+type Public = Base | Dummy;
+
+declare class Base {
+    protected get content(): string;
+    protected set content(value: string);
+}
+
+declare class Mock {
+    get content(): string;
+    set content(value: string);
+}
+
+declare const w: Public
+if (w instanceof Mock) {
+    w.content;
+}
+declare const w2: Mock & Public
+w2.content;
+
+declare const w3: Public & Mock
+w3.content;
+
+// private/protected set accessors are tracked in unions and intersections
+
+declare class C1 {
+    get foo(): number;
+    set foo(value: number);
+}
+
+declare class C2 {
+    get foo(): number;
+    protected set foo(value: number);
+}
+
+declare class C3 {
+    protected get foo(): number;
+    protected set foo(value: number);
+}
+
+declare class P1 {
+    get foo(): number;
+    set foo(value: number);
+}
+
+declare class P2 {
+    get foo(): number;
+    private set foo(value: number);
+}
+
+declare class P3 {
+    private get foo(): number;
+    private set foo(value: number);
+}
+
+// Unions properties have most restricted accessibility
+
+declare const cu12: C1 | C2;
+cu12.foo;
+cu12.foo = 123;  // Error, protected
+
+declare const cu13: C1 | C3;
+cu13.foo;  // Error, no property
+cu13.foo = 123;  // Error, no property
+
+declare const cu23: C2 | C3;
+cu23.foo;  // Error, no property
+cu23.foo = 123;  // Error, no property
+
+declare const pu12: P1 | P2;
+pu12.foo;
+pu12.foo = 123;  // Error, private
+
+declare const pu13: P1 | P3;
+pu13.foo;  // Error, no property
+pu13.foo = 123;  // Error, no property
+
+declare const pu23: P2 | P3;
+pu23.foo;  // Error, no property
+pu23.foo = 123;  // Error, no property
+
+// Intersection properties have most permissive accessibility
+
+declare const ci12: C1 & C2;
+ci12.foo;
+ci12.foo = 123;
+
+declare const ci13: C1 & C3;
+ci13.foo;
+ci13.foo = 123;
+
+declare const ci23: C2 & C3;
+ci23.foo;
+ci23.foo = 123;  // Error, protected
+
+declare const pi12: P1 & P2;
+pi12.foo;
+pi12.foo = 123;
+
+declare const pi13: P1 & P3;
+pi13.foo;  // Error, reduced to never
+pi13.foo = 123;  // Error, reduced to never
+
+declare const pi23: P2 & P3;
+pi23.foo;  // Error, reduced to never
+pi23.foo = 123;  // Error, reduced to never