Infer void for statement-less function bodies in isolatedDeclarations syntactic inference

[Wesley Wigham (weswigham)]

Per discussion in today's design meeting, pending any objections from the community. Since we no longer infer any for statementless function bodies in JS, we always, uniformly, infer void for these in the checker - making it a no-brainer to do the inference syntactically and avoid an isolatedDeclarations error requiring an explicit : void in the common case of an empty/noop function/method.

We'll move this to the 7.1 milestone once we have one~

[Copilot]

Pull request overview

Updates the isolatedDeclarations syntactic inference path to treat statement-less (empty) function bodies as returning void, aligning with the checker’s behavior and reducing spurious “explicit return type required” diagnostics for empty/no-op functions.

Changes:

• Added a void pseudo-type (PseudoTypeKindVoid/PseudoTypeVoid) and mapped it through the node/type builders.
• Updated pseudo-checker return-type inference to return void for empty block bodies.
• Refreshed a large set of isolated-declarations diagnostic baselines to reflect the new inference behavior (notably removing TS9007 in these cases).
• Updated the fourslash “known failing tests” list to include additional failures.

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
internal/pseudochecker/type.go	Adds a new PseudoTypeKindVoid and PseudoTypeVoid singleton.
internal/pseudochecker/lookup.go	Returns PseudoTypeVoid for empty block-bodied functions during single-return inference.
internal/checker/pseudotypenodebuilder.go	Maps PseudoTypeKindVoid to void type nodes and to the checker’s voidType.
internal/fourslash/_scripts/failingTests.txt	Adds newly failing code-fix fourslash tests to the skip list.
testdata/baselines/reference/submoduleTriaged/compiler/isolatedDeclarationErrorsReturnTypes.errors.txt.diff	Baseline updates reflecting changed isolated-declarations diagnostics (TS9007→other outcomes).
testdata/baselines/reference/submoduleTriaged/compiler/isolatedDeclarationErrorsExpandoFunctions.errors.txt.diff	Baseline updates reflecting changed diagnostics for expando-function scenarios.
testdata/baselines/reference/submoduleTriaged/compiler/isolatedDeclarationErrorsClasses.errors.txt.diff	Baseline updates removing method-return-type requirements in some empty-body cases.
testdata/baselines/reference/submoduleTriaged/compiler/isolatedDeclarationErrors.errors.txt.diff	Baseline updates reflecting fewer TS9007 diagnostics for empty functions.
testdata/baselines/reference/submodule/compiler/isolatedDeclarationLazySymbols.errors.txt.diff	Baseline diff updates reflecting removed TS9007 for empty exported function.
testdata/baselines/reference/submodule/compiler/isolatedDeclarationLazySymbols.errors.txt	Baseline updates reflecting removed TS9007 for empty exported function.
testdata/baselines/reference/submodule/compiler/isolatedDeclarationErrorsReturnTypes.errors.txt	Baseline updates reflecting new diagnostic set in return-type-related fixtures.
testdata/baselines/reference/submodule/compiler/isolatedDeclarationErrorsObjects.errors.txt.diff	Baseline diff updates reflecting removed TS9008 for empty methods in object literals.
testdata/baselines/reference/submodule/compiler/isolatedDeclarationErrorsObjects.errors.txt	Baseline updates reflecting removed TS9008 for empty methods in object literals.
testdata/baselines/reference/submodule/compiler/isolatedDeclarationErrorsFunctionDeclarations.errors.txt.diff	Baseline diff updates reflecting removed TS9007 for empty exported function decl.
testdata/baselines/reference/submodule/compiler/isolatedDeclarationErrorsFunctionDeclarations.errors.txt	Baseline updates reflecting removed TS9007 for empty exported function decl.
testdata/baselines/reference/submodule/compiler/isolatedDeclarationErrorsExpandoFunctions.errors.txt	Baseline updates reflecting removed TS9007 for the top-level empty exported function.
testdata/baselines/reference/submodule/compiler/isolatedDeclarationErrorsClasses.errors.txt	Baseline updates reflecting removed TS9008 for empty method bodies in classes.
testdata/baselines/reference/submodule/compiler/isolatedDeclarationErrors.errors.txt	Baseline updates reflecting removed TS9007 for const x = () => {}-style empty functions.

[Ryan Cavanaugh (RyanCavanaugh)]

Looks OK, fix merge conflicts?

[Andrew Branch (andrewbranch)]

When we talked about this at the design meeting, it was suggested/remembered at the very end of the meeting that we probably didn't do this originally in order to give us room in the future to make design changes around microsoft/TypeScript#42709 without breaking third-party isolatedDeclarations emitters.

[Wesley Wigham (weswigham)]

Yeah, except if we change that inference, we'd just change the isolatedDeclarations inference rule to match. It's not really any worse for a 3rd party emitter than if we, say, add a new syntax construct type kind. I don't think it'd preclude us from changing anything there (however unlikely that is).