Report inferred type predicate inference fallback

[jakebailey]

Fixes #3193

[Copilot]

Pull request overview

Adds --isolatedDeclarations coverage for inferred type predicate functions and updates the declaration/type node builder to report inference fallback when a signature’s return type is a type predicate.

Changes:

• Added a new compiler test case for an inferred type guard under --isolatedDeclarations.
• Added new reference baselines for the test (errors, JS/DTs emit, types, symbols).
• Updated return-type serialization to report inference fallback when encountering a type predicate.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
testdata/tests/cases/compiler/isolatedDeclarationsTypePredicate.ts	New test reproducing #3193 scenario.
testdata/baselines/reference/compiler/isolatedDeclarationsTypePredicate.errors.txt	Expected TS9007 diagnostic baseline.
testdata/baselines/reference/compiler/isolatedDeclarationsTypePredicate.js	Emit + .d.ts baseline for the new test.
testdata/baselines/reference/compiler/isolatedDeclarationsTypePredicate.types	Type baseline showing inferred predicate typing.
testdata/baselines/reference/compiler/isolatedDeclarationsTypePredicate.symbols	Symbol baseline for the new test.
internal/checker/nodebuilderimpl.go	Reports inference fallback when serializing type predicate return types.

[Copilot]

ReportInferenceFallback is triggered for any signature with a type predicate, including when the predicate is explicitly annotated (i.e. signature.declaration.Type() is a TypePredicateNode). That would incorrectly raise TS9007 under --isolatedDeclarations even though an explicit return type is present. Gate this report to only inferred predicates (e.g. when the declaration has no return type annotation) or implement reuse/matching for explicit predicate return types and only report when that reuse fails.

Suggested change

	} else if typePredicate != nil && !b.ctx.suppressReportInferenceFallback {
	} else if typePredicate != nil && signature.declaration != nil && signature.declaration.Type() == nil && !b.ctx.suppressReportInferenceFallback {

[jakebailey]

Not sure about that... if there's an annotation, we surely won't even ever ask to build a type.

[Copilot]

ReportInferenceFallback is called with signature.declaration without checking for nil. If serializeReturnTypeForSignature is ever invoked for a signature with a type predicate but no associated declaration, this will likely panic in the declaration emitter's tracker (it assumes a non-nil node). Add a nil guard (or choose a safer fallback node) before reporting.

Suggested change

	b.ctx.tracker.ReportInferenceFallback(signature.declaration)
	if signature.declaration != nil {
	b.ctx.tracker.ReportInferenceFallback(signature.declaration)
	}

[Copilot]

This test covers the missing-annotation case, but it doesn't assert the complementary behavior that an explicitly annotated type predicate return type (e.g. : value is string) should not produce TS9007 under --isolatedDeclarations. Adding a second function in this file with an explicit type predicate return type would prevent regressions where all type predicates get flagged.

Suggested change

	}
	}
	export function isExplicitString(value: unknown): value is string {
	return typeof value === "string";
	}