test_runner: source-mapped coverage marks an unexecuted return as covered #67099

Version

v26.12.1

Platform

Darwin arm64

Subsystem

test_runner coverage

Reproduction

Create these two files in an empty directory. No packages or module hooks are required. The inline source map contains its TypeScript source.

compiled.mjs

function choose(value) {
  if (value) {
    return "yes";
  }
  return "no";
}
export {
  choose
};
//# sourceMappingURL=data:application/json;base64,ewogICJ2ZXJzaW9uIjogMywKICAic291cmNlcyI6IFsiZXhhbXBsZS50cyJdLAogICJzb3VyY2VzQ29udGVudCI6IFsiZXhwb3J0IGZ1bmN0aW9uIGNob29zZSh2YWx1ZTogYm9vbGVhbik6IHN0cmluZyB7XG4gIGlmICh2YWx1ZSkge1xuICAgIHJldHVybiBcInllc1wiO1xuICB9XG4gIHJldHVybiBcIm5vXCI7XG59XG4iXSwKICAibWFwcGluZ3MiOiAiQUFBTyxTQUFTLE9BQU8sT0FBd0I7QUFDN0MsTUFBSSxPQUFPO0FBQ1QsV0FBTztBQUFBLEVBQ1Q7QUFDQSxTQUFPO0FBQ1Q7IiwKICAibmFtZXMiOiBbXQp9Cg==

compiled.test.mjs

import assert from "node:assert/strict";
import test from "node:test";
import { choose } from "./compiled.mjs";

test("branch", () => assert.equal(choose(true), "yes"));

Run:

node --enable-source-maps --experimental-test-coverage --test compiled.test.mjs

The embedded original source is:

export function choose(value: boolean): string {
  if (value) {
    return "yes";
  }
  return "no";
}

Expected behavior

The test only calls choose(true). Original line 5 (return "no") should be uncovered. With the source-map comment removed, the JavaScript report correctly shows line 5 uncovered.

Actual behavior

The source-mapped report shows example.ts at 100% lines, 66.67% branches, and 100% functions, with no uncovered lines. The lcov reporter also emits DA:5,1. This overstates execution of a branch that the test never takes.

This reproduces with a single test process and a precompiled module. The example was generated using esbuild 0.28.2, but the reproduction above does not require esbuild. Related source-map coverage reports include #65946 and #67077; those describe off-thread hooks or merging multiple test processes, while this case needs neither.