Port PR #61376: Fix spurious "used before being assigned" errors in for of/in loops

[Copilot]

Port of microsoft/TypeScript#61376.

Variables declared in for-in/for-of loops were incorrectly triggering TS2454 ("used before being assigned") when referenced in closures within the loop body.

for (let x of [1, 2]) {
  function f() {
    x;  // Error: Variable 'x' is used before being assigned (spurious)
  }
}

Changes

• Add !ast.IsForInOrOfStatement(immediateDeclaration.Parent.Parent) check to isNeverInitialized in internal/checker/checker.go
• Add local test unusedLocalsInForInOrOf1.ts from upstream PR with baselines

Variables in for-in/for-of loops are implicitly initialized by the loop iteration and should not be treated as uninitialized.

Original prompt

Port microsoft/TypeScript#61376

Custom agent used: Strada to Corsa Port Expert
A Go and TypeScript expert who can easily figure out how to port PRs from one language to another

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[RyanCavanaugh]

Diff looks wonky because of the line breaks, what's actually happening is this

-isNeverInitialized := immediateDeclaration != nil && ast.IsVariableDeclaration(immediateDeclaration)                                                                  && immediateDeclaration.Initializer() == nil && immediateDeclaration.AsVariableDeclaration().ExclamationToken == nil && c.isMutableLocalVariableDeclaration(immediateDeclaration) && !c.isSymbolAssignedDefinitely(symbol)
+isNeverInitialized := immediateDeclaration != nil && ast.IsVariableDeclaration(immediateDeclaration) && !ast.IsForInOrOfStatement(immediateDeclaration.Parent.Parent) && immediateDeclaration.Initializer() == nil && immediateDeclaration.AsVariableDeclaration().ExclamationToken == nil && c.isMutableLocalVariableDeclaration(immediateDeclaration) && !c.isSymbolAssignedDefinitely(symbol)

[Copilot]

Pull request overview

This pull request ports TypeScript PR #61376, which fixes spurious "used before being assigned" errors (TS2454) for variables declared in for-in/for-of loops when they are referenced in closures within the loop body.

Key Changes:

• Added a check !ast.IsForInOrOfStatement(immediateDeclaration.Parent.Parent) to the isNeverInitialized condition in checkIdentifier function
• Variables in for-in/for-of loops are now correctly recognized as implicitly initialized by the loop iteration

[RyanCavanaugh]

@copilot also add the new tests from the upstream PR as local tests in this repo

[Copilot]

@copilot also add the new tests from the upstream PR as local tests in this repo

Added the test from upstream PR as a local test in testdata/tests/cases/compiler/unusedLocalsInForInOrOf1.ts with corresponding baselines in commit 9208fd4.