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Ignore source files / module declarations in couldContainTypeVariables - #65481
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Jake Bailey (jakebailey) merged 2 commits intoNov 14, 2026
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The object type wrapping a module for a synthetic default import has
the module's declarations, so that types are ordered the same way
wherever it is compared. Having declarations made it an anonymous type
that could reference type parameters, so inference explored its
members. For import("cloudflare:workers").then(mod => mod), inferring
from the callback erased the type parameter of a generic function the
module exports, and instantiating its return type with any never
ended, reporting TS5115. A module is never within the scope of a type
parameter, so the wrapper is again known not to contain any.
Fixes 65445
Jake Bailey (jakebailey)
requested review from
Anders Hejlsberg (ahejlsberg)
and
a balanced review from Copilot
November 13, 2026 05:37
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🟢 Approval recommended
No active review issues were identified for the dynamic-import inference fix.
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What changed in this PR
Addresses #65445, a TS5115 regression during dynamic-import callback inference, by preventing the native compiler from exploring synthetic module wrappers that cannot reference type parameters.
Changes:
- Exclude wrappers backed by source-file or module declarations from
couldContainTypeVariables. - Add a regression test combining dynamic import, callback inference, and recursive conditional types.
- Add expected type and symbol baselines.
| File | Description |
|---|---|
tsc/testdata/tests/cases/compiler/syntheticDefaultImportTypeInference.ts |
Adds the focused dynamic-import inference regression test. |
tsc/testdata/baselines/reference/compiler/syntheticDefaultImportTypeInference.types |
Records expected inferred types, including Promise<number>. |
tsc/testdata/baselines/reference/compiler/syntheticDefaultImportTypeInference.symbols |
Records expected symbol bindings for the regression test. |
tsc/internal/checker/checker.go |
Identifies module wrappers and excludes them from type-variable traversal. |
Review effort: Balanced
Anders Hejlsberg (ahejlsberg)
approved these changes
Nov 13, 2026
Jake Bailey (jakebailey)
deleted the
synthetic-default-import-type-variables
branch
November 14, 2026 00:00
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Fixes #65445
This traces back to microsoft/typescript-go#4687. That PR gave a synthetic module wrapper symbol a declaration so it could be sorted, but then that made
couldContainTypeVariableswalk into it when it shouldn't. Strada didn't give that symbol a declaration so it didn't either.Fix it by skipping decls that are source files / module declarations.