Be more lenient about iteration when lib=es5 / noLib

[jakebailey]

When lib=es5 or noLib, but the target is ES2015+, we issue errors on iterations complaining that [Symbol.iterator]() is missing. Which is impossible, because there's no lib.

See these breaks: #63067 (comment)

But, we already know how to handle iteration when target is ES5 and there's no iteration libs; we just say "okay, you can iterate over arrays and strings".

This PR tweaks things so that we more or less fall back to ES5-ish behavior when the iterator types are missing, re-allowing iteration on arrays/strings (when Array exists; in noLib, things are universally weird) but preventing other kinds of iterations.

This is not dissimilar to us allowing async/await to work when Promise libs are missing.

[Copilot]

Pull request overview

Adjusts TypeScript’s iteration type-checking so that when iterator/Iterable types are unavailable (e.g. lib=es5 or noLib) but target is ES2015+, the checker falls back to ES5-style “array/string-only” iteration rules instead of emitting [Symbol.iterator]-missing errors.

Changes:

• Gate “uplevel iteration” semantics on the presence of the global Iterable type; otherwise fall back to array/string handling.
• Add a compiler test matrix covering lib=es5 vs noLib and multiple target values for for..of iteration.
• Update reference baselines to reflect the new iteration behavior (fewer TS2488/TS2504 iterator errors; more ES5-style diagnostics and improved element typing).

Reviewed changes

Copilot reviewed 72 out of 72 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/compiler/checker.ts	Changes iteration semantics selection to depend on presence of Iterable (fallback to array/string iteration when missing).
tests/cases/compiler/arrayIterationLibES5TargetDifferent.ts	Adds coverage for for..of behavior under lib=es5 / noLib across targets.
tests/baselines/reference/useObjectValuesAndEntries1(target=es2015).types	Updates element type inference for for..of when falling back from missing iterator libs.
tests/baselines/reference/useObjectValuesAndEntries1(target=es2015).errors.txt	Removes iterator-missing error now that array fallback applies.
tests/baselines/reference/types.forAwait.es2018.3.errors.txt	Updates diagnostics emitted for for await..of when async iteration types are missing.
tests/baselines/reference/sourceMapValidationDestructuringVariableStatementArrayBindingPatternDefaultValues3(target=es2015).types	Baseline updates due to new iteration/destructuring behavior when iterator libs are missing.
tests/baselines/reference/sourceMapValidationDestructuringVariableStatementArrayBindingPatternDefaultValues3(target=es2015).errors.txt	Removes iterator-missing errors under new fallback behavior.
tests/baselines/reference/sourceMapValidationDestructuringVariableStatementArrayBindingPatternDefaultValues2(target=es2015).errors.txt	Removes iterator-missing errors under new fallback behavior.
tests/baselines/reference/sourceMapValidationDestructuringVariableStatementArrayBindingPatternDefaultValues(target=es2015).errors.txt	Removes iterator-missing errors under new fallback behavior.
tests/baselines/reference/sourceMapValidationDestructuringVariableStatementArrayBindingPattern3(target=es2015).types	Baseline updates due to new iteration/destructuring behavior when iterator libs are missing.
tests/baselines/reference/sourceMapValidationDestructuringVariableStatementArrayBindingPattern3(target=es2015).errors.txt	Removes iterator-missing errors under new fallback behavior.
tests/baselines/reference/sourceMapValidationDestructuringVariableStatementArrayBindingPattern2(target=es2015).errors.txt	Removes iterator-missing errors under new fallback behavior.
tests/baselines/reference/sourceMapValidationDestructuringVariableStatementArrayBindingPattern(target=es2015).errors.txt	Removes iterator-missing errors under new fallback behavior.
tests/baselines/reference/sourceMapValidationDestructuringParametertArrayBindingPatternDefaultValues2(target=es2015).types	Baseline updates due to new iteration/destructuring behavior when iterator libs are missing.
tests/baselines/reference/sourceMapValidationDestructuringParametertArrayBindingPatternDefaultValues2(target=es2015).errors.txt	Removes iterator-missing errors under new fallback behavior.
tests/baselines/reference/sourceMapValidationDestructuringParametertArrayBindingPatternDefaultValues(target=es2015).types	Baseline updates due to new iteration/destructuring behavior when iterator libs are missing.
tests/baselines/reference/sourceMapValidationDestructuringParametertArrayBindingPatternDefaultValues(target=es2015).errors.txt	Removes iterator-missing errors under new fallback behavior.
tests/baselines/reference/sourceMapValidationDestructuringParametertArrayBindingPattern2(target=es2015).types	Baseline updates due to new iteration/destructuring behavior when iterator libs are missing.
tests/baselines/reference/sourceMapValidationDestructuringParametertArrayBindingPattern2(target=es2015).errors.txt	Removes iterator-missing errors under new fallback behavior.
tests/baselines/reference/sourceMapValidationDestructuringParametertArrayBindingPattern(target=es2015).types	Baseline updates due to new iteration/destructuring behavior when iterator libs are missing.
tests/baselines/reference/sourceMapValidationDestructuringParametertArrayBindingPattern(target=es2015).errors.txt	Removes iterator-missing errors under new fallback behavior.
tests/baselines/reference/sourceMapValidationDestructuringForOfObjectBindingPatternDefaultValues2(target=es2015).types	Baseline updates due to new iteration behavior when iterator libs are missing.
tests/baselines/reference/sourceMapValidationDestructuringForOfObjectBindingPatternDefaultValues(target=es2015).types	Baseline updates due to new iteration behavior when iterator libs are missing.
tests/baselines/reference/sourceMapValidationDestructuringForOfObjectBindingPatternDefaultValues(target=es2015).symbols	Baseline symbol changes due to updated binding/typing under fallback iteration.
tests/baselines/reference/sourceMapValidationDestructuringForOfObjectBindingPatternDefaultValues(target=es2015).errors.txt	Removes iterator-missing errors under new fallback behavior.
tests/baselines/reference/sourceMapValidationDestructuringForOfObjectBindingPattern2(target=es2015).types	Baseline updates due to new iteration behavior when iterator libs are missing.
tests/baselines/reference/sourceMapValidationDestructuringForOfObjectBindingPattern2(target=es2015).errors.txt	Removes iterator-missing errors under new fallback behavior.
tests/baselines/reference/sourceMapValidationDestructuringForOfObjectBindingPattern(target=es2015).types	Baseline updates due to new iteration behavior when iterator libs are missing.
tests/baselines/reference/sourceMapValidationDestructuringForOfObjectBindingPattern(target=es2015).symbols	Baseline symbol changes due to updated binding/typing under fallback iteration.
tests/baselines/reference/sourceMapValidationDestructuringForOfObjectBindingPattern(target=es2015).errors.txt	Removes iterator-missing errors under new fallback behavior.
tests/baselines/reference/sourceMapValidationDestructuringForOfArrayBindingPatternDefaultValues2(target=es2015).types	Baseline updates due to new iteration/destructuring behavior when iterator libs are missing.
tests/baselines/reference/sourceMapValidationDestructuringForOfArrayBindingPatternDefaultValues2(target=es2015).errors.txt	Removes iterator-missing errors under new fallback behavior.
tests/baselines/reference/sourceMapValidationDestructuringForOfArrayBindingPatternDefaultValues(target=es2015).errors.txt	Removes iterator-missing errors under new fallback behavior.
tests/baselines/reference/sourceMapValidationDestructuringForOfArrayBindingPattern2(target=es2015).types	Baseline updates due to new iteration/destructuring behavior when iterator libs are missing.
tests/baselines/reference/sourceMapValidationDestructuringForOfArrayBindingPattern2(target=es2015).errors.txt	Removes iterator-missing errors under new fallback behavior.
tests/baselines/reference/sourceMapValidationDestructuringForOfArrayBindingPattern(target=es2015).errors.txt	Removes iterator-missing errors under new fallback behavior.
tests/baselines/reference/sourceMapValidationDestructuringForArrayBindingPatternDefaultValues2(target=es2015).types	Baseline updates due to new iteration/destructuring behavior when iterator libs are missing.
tests/baselines/reference/sourceMapValidationDestructuringForArrayBindingPatternDefaultValues2(target=es2015).errors.txt	Removes iterator-missing errors under new fallback behavior.
tests/baselines/reference/sourceMapValidationDestructuringForArrayBindingPatternDefaultValues(target=es2015).types	Baseline updates due to new iteration/destructuring behavior when iterator libs are missing.
tests/baselines/reference/sourceMapValidationDestructuringForArrayBindingPatternDefaultValues(target=es2015).errors.txt	Removes iterator-missing errors under new fallback behavior.
tests/baselines/reference/sourceMapValidationDestructuringForArrayBindingPattern2(target=es2015).types	Baseline updates due to new iteration/destructuring behavior when iterator libs are missing.
tests/baselines/reference/sourceMapValidationDestructuringForArrayBindingPattern2(target=es2015).errors.txt	Removes iterator-missing errors under new fallback behavior.
tests/baselines/reference/sourceMapValidationDestructuringForArrayBindingPattern(target=es2015).types	Baseline updates due to new iteration/destructuring behavior when iterator libs are missing.
tests/baselines/reference/sourceMapValidationDestructuringForArrayBindingPattern(target=es2015).errors.txt	Updates diagnostics summary due to fallback iteration (iterator-missing errors removed).
tests/baselines/reference/noCrashOnNoLib.errors.txt	Baseline updates for changed diagnostic behavior under noLib.
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=true,target=esnext).types	New baseline for test matrix (types).
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=true,target=esnext).symbols	New baseline for test matrix (symbols).
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=true,target=esnext).js	New baseline for test matrix (emit).
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=true,target=esnext).errors.txt	New baseline for test matrix (errors).
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=true,target=es5).types	New baseline for test matrix (types).
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=true,target=es5).symbols	New baseline for test matrix (symbols).
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=true,target=es5).js	New baseline for test matrix (emit).
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=true,target=es5).errors.txt	New baseline for test matrix (errors).
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=true,target=es2015).types	New baseline for test matrix (types).
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=true,target=es2015).symbols	New baseline for test matrix (symbols).
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=true,target=es2015).js	New baseline for test matrix (emit).
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=true,target=es2015).errors.txt	New baseline for test matrix (errors).
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=false,target=esnext).types	New baseline for test matrix (types).
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=false,target=esnext).symbols	New baseline for test matrix (symbols).
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=false,target=esnext).js	New baseline for test matrix (emit).
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=false,target=esnext).errors.txt	New baseline for test matrix (errors).
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=false,target=es5).types	New baseline for test matrix (types).
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=false,target=es5).symbols	New baseline for test matrix (symbols).
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=false,target=es5).js	New baseline for test matrix (emit).
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=false,target=es5).errors.txt	New baseline for test matrix (errors).
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=false,target=es2015).types	New baseline for test matrix (types).
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=false,target=es2015).symbols	New baseline for test matrix (symbols).
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=false,target=es2015).js	New baseline for test matrix (emit).
tests/baselines/reference/arrayIterationLibES5TargetDifferent(nolib=false,target=es2015).errors.txt	New baseline for test matrix (errors).