TypeScript checker parity: eliminate the remaining 30 failures

[chrisbbreuer]

Parent: #67

Scope

Make ts_checker fully green by porting the remaining TypeScript semantics and exact diagnostic behavior. Do not weaken assertions or encode fixture-specific outcomes.

Current verified state

After fixing distributive-conditional union member ownership:

• 2,976 / 3,006 pass
• 30 fail
• 0 crash (down from 2 crashes)

The crash fix copies union member IDs before recursive conditional evaluation so interner reallocations cannot invalidate a borrowed slice.

Remaining failure clusters

• Enum/member literal assignment compatibility.
• Global-script this indexed access diagnostics.
• Generic defaults, conditional distribution, NonNullable, mapped and template types.
• Variadic tuple inference and callback arity.
• Overload diagnostic anchoring and CheckJS/JSDoc arity.
• Accessor read/write and spread typing.
• Alias/generic-instance diagnostic rendering.
• Recursive type depth/circular mapped diagnostics.

Completion gate

• ./pantry/.bin/zig build test -Dfilter=ts_checker --summary failures reports 3,006 / 3,006.
• No crash, timeout, or skipped parity assertion replaces a failure.
• Each semantic cluster lands in a focused conventional commit with regression coverage.

[chrisbbreuer]

Crash-removal chunk landed in 5ed86afd3.

Verified:

• conditional union call signatures do not leak excess properties: 2/2 pass.
• distributive conditional yielding union of callables is callable: 2/2 pass.
• Full ts_checker: 2,976/3,006 pass, 30 fail, 0 crash.
• Net movement: 2 crashes flipped to passes; remaining failure list is unchanged.

Root cause was a borrowed unionMembers slice surviving recursive interning that could reallocate its backing pool. The evaluator now owns a copy for the recursive loop.

[chrisbbreuer]

Enum-member identity chunk landed in 8ca140fee.

Verified delta:

• Checker regression enum assignment compat models enum value object and member literals: 2/2 pass.
• Compiler exact enumAssignmentCompat* cluster: 4/7 -> 6/7.
• enumAssignmentCompat and enumAssignmentCompat2 now pass exactly.
• Remaining enum fixture is enumAssignmentCompat5, whose only mismatch is the TypeScript rule allowing a broad number value to assign to numeric enum member type E.A.

[chrisbbreuer]

Pushed 9f04059c0 (fix(ts-checker): allow number for numeric enum members).

This ports tsgo's bit-flag assignability rule: broad number is assignable to numeric enum-member types, while numeric literal sources still require an exact value match. enumAssignmentCompat5 flips to exact, taking the focused enum cluster from 6/7 to 7/7. Focused regression passes; full checker is 2,978/3,007 passing, 29 known failures, zero crashes.

[chrisbbreuer]

Pushed 19c1c7aa5 (fix(ts-checker): diagnose global this computed writes).

Top-level this/globalThis computed writes now bypass the synthetic global-object index under noImplicitAny, preserving TS7053 with typeof globalThis. Full checker moves from 2,978/3,007 (29 failures) to 2,979/3,007 (28 failures), zero crashes.

[chrisbbreuer]

Pushed 252b3902c (fix(ts-checker): elaborate generic parameter defaults).

Unsafe parameter defaults targeting bare type parameters now use the existing instantiation-mismatch diagnostics; constrained defaults such as U extends T remain valid. All 12 focused parameter-default tests pass. Full checker moves from 2,979/3,007 (28 failures) to 2,980/3,007 (27 failures), zero crashes.

[chrisbbreuer]

Pushed 2b0e9d516 (fix(ts-checker): anchor overload errors on arguments).

TS2769 now carries the incompatible argument node out of overload applicability checks, preserving per-overload chains and related declarations while anchoring the diagnostic like tsgo. Corrected a legacy boolean-call assertion accidentally changed by an earlier construct-overload commit. All 4 focused overload-match tests pass. Full checker moves from 2,980/3,007 (27 failures) to 2,981/3,007 (26 failures), zero crashes.