Port TypeScript PR #60528: Fix crash on index type deferral for generic mapped types with name types

[Copilot]

Ports microsoft/TypeScript#60528, which fixes crashes when computing index types for generic mapped types with name types. Previously, the type checker would crash or incorrectly defer index type resolution in cases like:

type Values<T> = T[keyof T];

type Mapped<T> = {
  [K in keyof T as K & string]: {
    src: K;
    logic: T[K];
  };
};

// This would crash or produce incorrect types
type Result = keyof Mapped<SomeType>;

Changes

• Added TypeFlagsIndex to simplifiable types - Enables simplification of index types for mapped types
• Removed hasDistributiveNameType function - The distributive check is no longer needed; simplified to just checking if a name type exists
• Updated shouldDeferIndexType - Changed from complex distributive checks to simple presence check: getNameTypeFromMappedType(t) != nil
• Added getSimplifiedIndexType - New simplification path for index types on generic mapped types with name types but without keyof constraint declarations
• Simplified checkIndexedAccessIndexType - Removed conditional logic, now always uses getIndexTypeEx
• Added regression tests - Two new Go tests in internal/checker/checker_test.go:
  • TestKeyRemappingKeyofResult2 - Validates fix for keyof FilteringMappedType can't compute its constraint TypeScript#56239
  • TestMappedTypeAsClauseRecursiveNoCrash - Validates fix for Regression: Mapped type with recursive key remapping crashes tsc with "RangeError: Maximum call stack size exceeded" in TS@5.4+ TypeScript#60476

Impact

Error messages are now more readable:

• Before: Type 'Mapped6<K>[string] | Mapped6<K>[number] | Mapped6<K>[symbol]' is not assignable to...
• After: Type 'Mapped6<K>[\_${K}`]' is not assignable to...`

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

Original prompt

Port microsoft/TypeScript#60528

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]

@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 two regression tests from the upstream PR in commit 5675e7e:

• TestKeyRemappingKeyofResult2 for issue #56239
• TestMappedTypeAsClauseRecursiveNoCrash for issue #60476

Both tests verify that the type checker doesn't crash when processing the complex mapped types that previously caused issues.