[Cheng (chengchuu)]

Summary

Fixes #62127

Fix false TS2454 ("used before being assigned") diagnostics on writes through as and angle-bracket assertions in the Go compiler. This follows #62128, including the const and readonly assignment coverage requested by Jake Bailey and Ryan Cavanaugh. The following code now compiles without errors:

function example() {
  let value: number;
  (value as number) = 1;
}

Assignment checks now apply through assertions. Previously accepted writes to constants, readonly properties, class bindings, and restricted generic indexes can therefore report errors. JavaScript emission is unchanged.

Added regression tests for ordinary and nested writes, uninitialized reads, compound assignments, const/readonly targets, and mutable-object casts. Existing scenarios are preserved with updated baselines.

Read-after-assignment flow analysis remains unchanged: a subsequent read can still report TS2454 after an assertion-wrapped assignment.

Validation

I manually ran the following focused tests from the repository root on macOS ARM64. All passed:

GOWORK=off go -C tsc test -mod=readonly ./internal/testrunner \
  -run 'TestLocal/(usedBeforeAssignedTypeAssertion|classNameReferencesInStaticElements|assertedReadonlyParameterPropertyAssignment|assertedGenericPartialIndexedAssignment)' -count=1

The standalone reproduction reported TS2454 and exited 2 before the fix. I reran it with a compiler built from 4a2cd92c47d744d003a81503982430914c677b0d: no diagnostics, exit 0.

AI assistance: Copilot helped generate the regression tests. I reviewed the changes and will handle any review feedback or revisions myself.

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