Port TS#60528: Fix index type deferral crash on generic mapped types with name types

[Copilot]

Ports microsoft/TypeScript#60528. Generic mapped types with as clause name types could crash when computing index types due to incorrect deferral logic and missing base constraint handling.

Changes

• shouldDeferIndexType: Simplify condition from !hasDistributiveNameType(t) || getMappedTypeNameTypeKind(t) == Remapping to getNameTypeFromMappedType(t) != nil — any generic mapped type with an as clause now defers
• Remove hasDistributiveNameType: No longer referenced after the above simplification
• computeBaseConstraint: For TypeFlagsIndex targeting a generic mapped type with a name type (non-homomorphic), recursively compute base constraint from the mapped type's index type instead of unconditionally returning stringNumberSymbolType
• getBaseConstraintOfType: Route TypeFlagsIndex through getResolvedBaseConstraint instead of short-circuiting
• TypeFlagsSimplifiable: Add TypeFlagsIndex to the flag set
• Unskip mappedTypeAsClauseRecursiveNoCrash1.ts test

// Previously crashed or produced incorrect results:
type Mapped<K extends string> = { [P in K as `_${P}`]: true };
function f<K extends string>(key: keyof Mapped<K>) {
    let s: `_${string}` = key; // now correctly resolves constraint
}

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[Copilot]

Pull request overview

Ports TypeScript PR #60528 to prevent crashes and improve constraint computation for index types on generic mapped types that use as clause name types.

Changes:

• Adjust index-type deferral for generic mapped types with as clauses and remove obsolete distributive-name-type logic.
• Improve base-constraint resolution for TypeFlagsIndex when targeting certain generic mapped types with name types.
• Re-enable the mappedTypeAsClauseRecursiveNoCrash1.ts regression test and update affected baselines.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
internal/checker/checker.go	Updates deferral/base-constraint logic for index types involving generic mapped types; removes hasDistributiveNameType.
internal/checker/types.go	Adds TypeFlagsIndex to TypeFlagsSimplifiable so index types participate in simplification/constraint flows.
internal/testrunner/compiler_runner.go	Unskips mappedTypeAsClauseRecursiveNoCrash1.ts.
testdata/baselines/reference/submodule/conformance/mappedTypeConstraints2.*	Updates baselines for mapped type constraint and error output changes.
testdata/baselines/reference/submodule/conformance/mappedTypeAsClauseRecursiveNoCrash1.*	Adds new baseline outputs for the re-enabled regression test.
testdata/baselines/reference/submodule/compiler/keyRemappingKeyofResult2.errors.txt*	Removes baseline error output (now empty/no longer applicable).

Comments suppressed due to low confidence (2)

internal/checker/checker.go:1

• This comment now states there is 'a key remapping', but the condition guarding it is isMappedTypeWithKeyofConstraintDeclaration(t) plus a generic index constraint; that doesn’t necessarily imply an as-clause remapping is present. Consider rewording the comment to describe the actual condition (generic index + homomorphic mapping via keyof constraint) or tighten the code to only enter this path when a name type exists (e.g., getNameTypeFromMappedType(t) != nil) if that’s what the comment intends.
  internal/checker/checker.go:1
• This block calls t.AsIndexType() twice and repeats .target access. Consider storing index := t.AsIndexType() and mappedType := index.target once to make the control flow clearer and avoid duplicated accessor calls.