[Copilot]

Fixes #65287

Analysis

Type erasure wraps !, as, satisfies, and angle-bracket assertions in PartiallyEmittedExpression. Downlevel compound-assignment transforms miss the underlying member access, duplicating receiver or key evaluation.

obj[++key]! ||= 1;
// Before: obj[++key] || (obj[++key] = 1)
// After:  obj[_a = ++key] || (obj[_a] = 1)

Fix

• Target recognition: Skip parentheses and type-erasure wrappers before lowering ||=, &&=, ??=, and **=.
• super handling: Preserve the receiver directly while caching computed keys.
• Regression coverage: Exercise wrapped property, element, and super targets across ES2015, ES2020, and ES2021.

Copilot Checklist

I successfully ran the applicable command at the end of my session, and it completed without error:

• npx hereby validate
• npx hereby validate --api (for TypeScript API changes)

• Fixes Downlevel emit of ||=, &&=, ??= and **= evaluates a member target twice when it has !, as or satisfies #65287

[Copilot]

🟢 Approval recommended

No active review issues are identified for the changes preserving single evaluation of wrapped compound-assignment targets.

No open issues 🎉

What changed in this PR

Addresses #65287 in the native TypeScript compiler by preserving single evaluation of wrapped compound-assignment targets during downlevel emission.

Changes:

• Recognize member targets beneath parentheses and type-erasure wrappers for logical and exponentiation assignments.
• Preserve super directly during exponentiation lowering while caching computed keys.
• Add regression coverage across ES2015, ES2020, and ES2021, and update affected conformance baselines.

File	Description
tsc/​testdata/​tests/​cases/​compiler/​compoundAssignmentWithTypeOnlyExpressions.ts	Tests wrapped property, element, and super targets.
tsc/​testdata/​baselines/​reference/​conformance/​compoundExponentiationAssignmentLHSIsValue.js	Updates expected emit for value targets and super.
tsc/​testdata/​baselines/​reference/​conformance/​compoundExponentiationAssignmentLHSIsReference.js	Records caching for parenthesized member targets.
tsc/​testdata/​baselines/​reference/​compiler/​compoundAssignmentWithTypeOnlyExpressions(target=es2021).js	Records native compound-assignment emit after type erasure.
tsc/​testdata/​baselines/​reference/​compiler/​compoundAssignmentWithTypeOnlyExpressions(target=es2020).js	Records corrected logical-assignment lowering.
tsc/​testdata/​baselines/​reference/​compiler/​compoundAssignmentWithTypeOnlyExpressions(target=es2015).js	Records corrected logical and exponentiation lowering.
tsc/​internal/​transformers/​estransforms/​logicalassignment.go	Unwraps type-erasure expressions before recognizing targets.
tsc/​internal/​transformers/​estransforms/​exponentiation.go	Unwraps targets and avoids caching super as a value.

_{Review effort: Balanced}