Fix reachability of code after a try with a branching finally (#63583)

[CedricConday]

Fixes #63583.

Problem

A function whose try body is an exhaustive switch that returns in every case is wrongly flagged Function lacks ending return statement — but only when the finally block contains a branch:

function test(x: boolean): number {
    try {
        switch (x) {
            case true: return 1;
            case false: return 0;
        }
    } finally { y ||= true; }   // branch in finally => bogus TS2366
}

The original report framed it as "read+write in finally", but the real trigger is any branch in the finally block (y ||= true, y = y || true, if (y) {} all reproduce; a linear y = true or a non-branching read does not).

Root cause

Reachability caches results for shared flow nodes in the process-global flowNodeReachable cache. While a ReduceLabel is active (try/finally), isReachableFlowNodeWorker temporarily rewrites the target label's antecedent, so the reachability of nodes reached during that walk is dependent on the reduced context. The ReduceLabel handler cleared only the single-entry lastFlowNode cache — not the shared cache. A branch inside finally produces a shared merge node whose reachability was cached under one reduce context (e.g. the return-exit antecedents) and then reused under another (the normal-exit antecedents), corrupting both end-of-function and unreachable-code analysis. With no branch there is no shared node in the finally, so the bug doesn't surface.

This mirrors the type-flow path, which already scopes its shared cache per invocation (sharedFlowStart/sharedFlowCount); reachability had no equivalent guard.

Fix

Suppress the shared reachability cache while a label reduction is active, depth-counted to handle nested try/finally. Reachability under reduced antecedents is simply recomputed rather than served from / written to the global cache.

Tests

Added controlFlowSwitchReturnWithBranchingFinally.ts covering the branching-finally cases (now clean), a non-branching control, and nested try/finally. The accompanying return-after-exhaustive-switch case is still correctly reported as unreachable (this is finally-independent — verified with no try/finally present), confirming the only defect was the contradiction. No baseline changes in the control-flow / finally / reachability / try-catch suites (816 passing).

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Disclosure: I'm an AI engineer and use Claude Code in my workflow. The root-cause analysis and fix are mine and fully reviewed; happy to walk through any part.

[Copilot]

Pull request overview

This PR fixes a control-flow reachability caching bug in the checker where reachability results for shared flow nodes could be incorrectly reused across different reduced-label contexts (e.g. try/finally), producing contradictory diagnostics like TS2366 (“Function lacks ending return statement”) when finally contains branching.

Changes:

• Suppress the shared reachability cache for the duration of a FlowFlags.ReduceLabel walk (depth-counted for nested try/finally).
• Add a new compiler regression test covering exhaustive switch returns with branching finally (including a nested case and a non-branching control).
• Add new reference baselines for the test (types, symbols, and expected error output).

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/compiler/checker.ts	Disables shared reachability caching while reduce-label rewriting is active to avoid context-dependent cache pollution.
tests/cases/compiler/controlFlowSwitchReturnWithBranchingFinally.ts	New regression test covering branching-in-finally cases (and a non-branching control).
tests/baselines/reference/controlFlowSwitchReturnWithBranchingFinally.types	Baseline for type display/output of the new test.
tests/baselines/reference/controlFlowSwitchReturnWithBranchingFinally.symbols	Baseline for symbol output of the new test.
tests/baselines/reference/controlFlowSwitchReturnWithBranchingFinally.errors.txt	Baseline verifying only the intended unreachable-code error is reported.

[MartinJohns]

I'm an AI engineer and use Claude Code in my workflow.

Claude failed to read this: https://github.com/microsoft/TypeScript/blob/main/CLAUDE.md

[CedricConday]

Thanks all, and thanks @MartinJohns for the pointer — you're right, I missed AGENTS.md. This repo is in maintenance mode and isn't accepting general bug-fix PRs against the JS codebase (active work has moved to microsoft/typescript-go), so this change is out of scope here. Closing accordingly. Happy to re-target the fix at typescript-go if it'd still be useful there. Apologies for the noise.