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Avoid duplicate assignability errors on literal unions - #64595
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When a union of literals generalizes to the same base type as its constituent check, elide the redundant child diagnostic on the error chain. Fixes microsoft#63050.
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Review effort: Lite
Findings: 2
chainArgsMatch(generalizedSourceType, targetType)` can match a chain entry that has more… · New
Prevents redundant nested diagnostics when assignability checks on literal unions are generalized to the same base type, which previously caused the same assignability error to be emitted twice.
Changes:
- Adds a compiler test case covering duplicate assignability errors for literal unions (string/number/bigint).
- Adjusts relation error reporting to elide a redundant child diagnostic for assignability/comparability when it matches the outer diagnostic.
- Updates multiple .errors baselines to reflect deduplicated error chains.
| File | Description |
|---|---|
| tsc/internal/checker/relater.go | Drops a redundant child entry from the error chain when assignability/comparability diagnostics match. |
| tsc/testdata/tests/cases/compiler/literalUnionAssignabilityDoubleError.ts | New regression test reproducing the duplicate-error scenario. |
| tsc/testdata/baselines/reference/compiler/literalUnionAssignabilityDoubleError.errors.txt | New baseline verifying only one assignability error is emitted per site. |
| tsc/testdata/baselines/reference/compiler/literalUnionAssignabilityDoubleError.types | New type baseline for the added test. |
| tsc/testdata/baselines/reference/compiler/literalUnionAssignabilityDoubleError.symbols | New symbol baseline for the added test. |
| tsc/testdata/baselines/reference/compiler/literalUnionAssignabilityDoubleError.js | New emit baseline for the added test. |
| tsc/testdata/baselines/reference/compiler/typeOfOperator1.errors.txt | Baseline updated to remove duplicated nested error line. |
| tsc/testdata/baselines/reference/compiler/recursiveIndexedAccessSimplification.errors.txt | Baseline updated to remove duplicated nested error segments. |
| tsc/testdata/baselines/reference/compiler/complicatedIndexedAccessKeyofReliesOnKeyofNeverUpperBound.errors.txt | Baseline updated to remove repeated nested assignability chain lines. |
| tsc/testdata/baselines/reference/compiler/bigintPropertyName.errors.txt | Baseline updated to remove duplicated nested error line. |
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tsc/internal/checker/relater.go
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| } | ||
| case diagnostics.Type_0_is_not_assignable_to_type_1, | ||
| diagnostics.Type_0_is_not_comparable_to_type_1: | ||
| if r.chainArgsMatch(generalizedSourceType, targetType) { |
Comment thread
tsc/internal/checker/relater.go
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Comment on lines
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| case diagnostics.Type_0_is_not_assignable_to_type_1, | ||
| diagnostics.Type_0_is_not_comparable_to_type_1: | ||
| if r.chainArgsMatch(generalizedSourceType, targetType) { | ||
| r.errorChain = r.errorChain.next | ||
| } |
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bodapatisaikrishna due to an error
October 2, 2026 16:35
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Fixes #63050.
When relation error reporting generalizes a literal union to the same base type as its constituent check, the error chain ends up emitting the exact same assignability error twice. This elides the redundant child diagnostic when its arguments match the outer error.