Port TypeScript#63071: Mark downlevelIteration as removed

[Copilot]

Ports microsoft/TypeScript#63071. Rather than adding a deprecation diagnostic (TS5101) as in the upstream PR, this adds a "removed" diagnostic consistent with how the Go port handles other removed options.

• internal/compiler/program.go: Emit removed-option diagnostic when downlevelIteration is explicitly set (true or false)
• internal/testutil/harnessutil/harnessutil.go: Skip submodule tests that set downlevelIteration
• Baselines: Delete 118 now-unused baseline files from newly skipped tests

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[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Ports the upstream TypeScript change to treat downlevelIteration as a removed compiler option (TS5102) in the Go port, and updates tests/baselines accordingly.

Changes:

• Emit a removed-option diagnostic when downlevelIteration is explicitly set in config.
• Remove downlevelIteration-driven iteration behavior from the checker.
• Update reference baselines to reflect TS5102 (and related output changes).

Reviewed changes

Copilot reviewed 49 out of 49 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
internal/core/compileroptions.go	Moves DownlevelIteration into the deprecated/options-parsing section of CompilerOptions while keeping JSON parsing support.
internal/compiler/program.go	Adds validation to emit a removed-option diagnostic (TS5102) when downlevelIteration is explicitly provided.
internal/checker/checker.go	Removes downlevelIteration handling from iteration typing logic and refactors iterable gating/diagnostics.
testdata/baselines/reference/submodule/conformance/restParameterInDownlevelGenerator(target=es2015).errors.txt.diff	Updates baseline diff to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/restParameterInDownlevelGenerator(target=es2015).errors.txt	Adds/updates baseline to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/forAwaitPerIterationBindingDownlevel(target=es2015).errors.txt.diff	Updates baseline diff to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/forAwaitPerIterationBindingDownlevel(target=es2015).errors.txt	Adds/updates baseline to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/emptyVariableDeclarationBindingPatterns02_ES5iterable(target=es2015).errors.txt.diff	Updates baseline diff to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/emptyVariableDeclarationBindingPatterns02_ES5iterable(target=es2015).errors.txt	Updates baseline to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/emptyVariableDeclarationBindingPatterns01_ES5iterable(target=es2015).errors.txt.diff	Updates baseline diff to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/emptyVariableDeclarationBindingPatterns01_ES5iterable(target=es2015).errors.txt	Adds/updates baseline to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/emptyAssignmentPatterns04_ES5iterable(target=es2015).errors.txt.diff	Updates baseline diff to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/emptyAssignmentPatterns04_ES5iterable(target=es2015).errors.txt	Adds/updates baseline to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/emptyAssignmentPatterns03_ES5iterable(target=es2015).errors.txt.diff	Updates baseline diff to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/emptyAssignmentPatterns03_ES5iterable(target=es2015).errors.txt	Adds/updates baseline to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/emptyAssignmentPatterns02_ES5iterable(target=es2015).errors.txt.diff	Updates baseline diff to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/emptyAssignmentPatterns02_ES5iterable(target=es2015).errors.txt	Adds/updates baseline to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/emptyAssignmentPatterns01_ES5iterable(target=es2015).errors.txt.diff	Updates baseline diff to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/emptyAssignmentPatterns01_ES5iterable(target=es2015).errors.txt	Adds/updates baseline to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/destructuringVariableDeclaration1ES5iterable(target=es2015).errors.txt.diff	Updates baseline diff to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/destructuringVariableDeclaration1ES5iterable(target=es2015).errors.txt	Updates baseline to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/destructuringParameterDeclaration7ES5iterable(target=es2015).errors.txt.diff	Updates baseline diff to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/destructuringParameterDeclaration7ES5iterable(target=es2015).errors.txt	Adds/updates baseline to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/destructuringParameterDeclaration3ES5iterable(target=es2015).errors.txt.diff	Updates baseline diff to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/destructuringParameterDeclaration3ES5iterable(target=es2015).errors.txt	Updates baseline to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/destructuringParameterDeclaration1ES5iterable.errors.txt.diff	Updates baseline diff to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/destructuringParameterDeclaration1ES5iterable.errors.txt	Updates baseline to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/destructuringArrayBindingPatternAndAssignment4(target=es2015).errors.txt.diff	Updates baseline diff to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/destructuringArrayBindingPatternAndAssignment4(target=es2015).errors.txt	Updates baseline to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/destructuringArrayBindingPatternAndAssignment1ES5iterable.errors.txt.diff	Updates baseline diff to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/destructuringArrayBindingPatternAndAssignment1ES5iterable.errors.txt	Updates baseline to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/asyncArrowFunction11_es5(target=es2015).errors.txt.diff	Updates baseline diff to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/asyncArrowFunction11_es5(target=es2015).errors.txt	Adds/updates baseline to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/arrayLiteralSpreadES5iterable(target=es2015).errors.txt.diff	Updates baseline diff to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/arrayLiteralSpreadES5iterable(target=es2015).errors.txt	Adds/updates baseline to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/ES5For-of37(target=es2015).errors.txt.diff	Updates baseline diff to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/ES5For-of37(target=es2015).errors.txt	Adds/updates baseline to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/ES5For-of36(target=es2015).errors.txt.diff	Updates baseline diff to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/ES5For-of36(target=es2015).errors.txt	Updates baseline to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/ES5For-of35(target=es2015).errors.txt.diff	Updates baseline diff to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/ES5For-of35(target=es2015).errors.txt	Updates baseline to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/ES5For-of34(target=es2015).errors.txt.diff	Updates baseline diff to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/ES5For-of34(target=es2015).errors.txt	Updates baseline to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/ES5For-of33(target=es2015).errors.txt.diff	Updates baseline diff to TS5102 removed-option output.
testdata/baselines/reference/submodule/conformance/ES5For-of33(target=es2015).errors.txt	Adds/updates baseline to TS5102 removed-option output.
testdata/baselines/reference/submodule/compiler/sourceMapValidationVarInDownLevelGenerator(target=es2015).errors.txt.diff	Updates baseline diff to TS5102 removed-option output.
testdata/baselines/reference/submodule/compiler/sourceMapValidationVarInDownLevelGenerator(target=es2015).errors.txt	Adds/updates baseline to TS5102 removed-option output.
testdata/baselines/reference/submodule/compiler/blockScopedBindingsInDownlevelGenerator(target=es2015).errors.txt.diff	Updates baseline diff to TS5102 removed-option output.
testdata/baselines/reference/submodule/compiler/blockScopedBindingsInDownlevelGenerator(target=es2015).errors.txt	Adds/updates baseline to TS5102 removed-option output.

Comments suppressed due to low confidence (3)

internal/checker/checker.go:1

• This change drops the c.languageVersion >= ScriptTargetES2015 gating and replaces it with iterableExists. That means ES5/ES3 targets can start treating values as iterables (and potentially accept for..of over non-array iterables) merely because the ES2015 iterable lib types are present, even though emit semantics for those targets won’t support iterator-based iteration now that downlevelIteration is removed. Consider restoring an explicit target/languageVersion gate (e.g., only taking the iterable path when targeting ES2015+), and use that gate (not iterableExists) to decide when to report invalid-iterable errors.
  internal/checker/checker.go:1
• This change drops the c.languageVersion >= ScriptTargetES2015 gating and replaces it with iterableExists. That means ES5/ES3 targets can start treating values as iterables (and potentially accept for..of over non-array iterables) merely because the ES2015 iterable lib types are present, even though emit semantics for those targets won’t support iterator-based iteration now that downlevelIteration is removed. Consider restoring an explicit target/languageVersion gate (e.g., only taking the iterable path when targeting ES2015+), and use that gate (not iterableExists) to decide when to report invalid-iterable errors.
  internal/checker/checker.go:1
• The diagnostic referenced here still tells users to use the downlevelIteration flag, but this PR makes that option a hard-removed error (TS5102). The message should be updated to remove the suggestion to use downlevelIteration and instead direct users to the supported alternative (e.g., requiring --target es2015 or higher). This likely requires updating the underlying diagnostics entry name/text and adjusting baselines accordingly.

[Copilot]

The removed-option diagnostic for downlevelIteration appears to provide only generic guidance (empty replacement/extra info args). Since removing downlevelIteration also removes the only ES5-compatible iterator-based iteration mode, it would be more actionable if the diagnostic included the recommended alternative (e.g., targeting ES2015+). If createRemovedOptionDiagnostic supports a hint/replacement string, consider populating it for this option.

Suggested change

	createRemovedOptionDiagnostic("downlevelIteration", "", "Target ES2015 or later to use native iterator-based iteration support.")