No widening to any[] when parent of expando has type annotation

[ahejlsberg]

With this PR we don't widen empty array literals to any[] in expando property assignments when the expando object has a type annotation. This mirrors the behavior in Strada. Part of me wants to just treat such expando property assignments the same as property assignments in object literals, meaning that we'd type the literals as never[] when noImplicitAny: true, but that would be a breaking change.

function f1() {}
f1.a = [];  // Implicit any error

const f2 = function() {};
f2.a = [];  // Implicit any error

const f3 = () => {};
f3.a = [];  // Implicit any error

const f4: { (): void, a: string[] } = () => {};
f4.a = [];

const f5: { (): void, a: string[] } = function() {};
f5.a = [];

Fixes #3976.

[Copilot]

Pull request overview

This PR adjusts the checker’s expando-property typing so that assigning an empty array literal ([]) to an expando property does not widen to any[] when the expando object’s containing declaration has an explicit type annotation. This aligns tsgo behavior with Strada and fixes the regression reported in #3976.

Changes:

• Updates expando assignment initializer typing to skip the “empty array => widen to any[] + implicit-any error” path when the expando parent is type-annotated.
• Adds a compiler regression test covering callable objects with fields (typed vs untyped) and expected TS7008 diagnostics/baselines.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

File	Description
internal/checker/checker.go	Skips empty-array widening to any[] for expando assignment declarations when the expando parent is type-annotated.
testdata/tests/cases/compiler/expandoPropertyEmptyArrayWidening.ts	New regression test reproducing #3976 and asserting correct behavior for typed vs untyped expandos.
testdata/baselines/reference/compiler/expandoPropertyEmptyArrayWidening.*	New reference baselines (types/symbols/errors) validating the updated behavior.