Inconsistent diagnostic and misleading error span for 'override declare' (TS1040 vs TS1243) #65399

🔎 Search Terms

override declare, TS1040 override, override modifier cannot be used in an ambient context

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about class modifiers.

⏯ Playground Link

No response

💻 Code

class Base {
p1: number = 1;
p2: number = 2;
}

class Derived extends Base {
declare override p1: number;
override declare p2: number;
}

🙁 Actual behavior

When written as override declare, TypeScript reports:

override declare p2: number; ~~~~~~~ error TS1040: 'override' modifier cannot be used in an ambient context.

1. Incorrect message: override is explicitly allowed in ambient contexts (declare class Derived extends Base { override p: number; } is valid in both .ts and .d.ts files).
2. Misplaced error anchor: The squiggly underlines declare, but mentions override and claims the context is ambient (even in an ordinary non-ambient class).
3. Order asymmetry: Writing declare override reports TS1243: 'override' modifier cannot be used with 'declare' modifier., but flipping the order yields TS1040.

🙂 Expected behavior

Combining declare and override on a class property should report TS1243: '{0}' modifier cannot be used with '{1}' modifier. symmetrically regardless of order:

class Derived extends Base {
    declare override p1: number; // TS1243: 'override' modifier cannot be used with 'declare' modifier.
    override declare p2: number; // Expected TS1243: 'declare' modifier cannot be used with 'override' modifier.
}

In tsc/internal/checker/grammarchecks.go, line 451 currently calls diagnostics.X_0_modifier_cannot_be_used_in_an_ambient_context with "override". Updating it to diagnostics.X_0_modifier_cannot_be_used_with_1_modifier, "declare", "override" restores symmetry with other mutually exclusive modifier combinations (such as declare accessor / accessor declare).

Additional information about the issue

No response