lib: fix sourcemaps with ts module mocking

lib/internal/test_runner/coverage.js

@@ -408,6 +408,10 @@ class TestCoverage {
           }
           const mappedStartOffset = this.entryToOffset(startEntry, mappedLines);
           const mappedEndOffset = this.entryToOffset(endEntry, mappedLines) + 1;
+          if (mappedStartOffset < 0 || mappedEndOffset < 1) {
+            // The range is not mappable. Skip it.
+            continue;
+          }
           for (let l = startEntry.originalLine; l <= endEntry.originalLine; l++) {
             mappedLines[l].count = count;
           }
@@ -432,7 +436,12 @@ class TestCoverage {
 
   entryToOffset(entry, lines) {
     const line = MathMax(entry.originalLine, 0);
-    return MathMin(lines[line].startOffset + entry.originalColumn, lines[line].endOffset);
+    const mappedLine = lines[line];
+    if (!mappedLine) {
+      // Return -1 if the line is not mappable.
+      return -1;
+    }
+    return MathMin(mappedLine.startOffset + entry.originalColumn, mappedLine.endOffset);
   }
 
   mergeCoverage(merged, coverage) {

test/fixtures/test-runner/coverage/bar.mts

@@ -0,0 +1,3 @@
+export default function bar() {
+    return 'original bar';
+}

test/fixtures/test-runner/coverage/foo.mts

@@ -0,0 +1,3 @@
+import bar from './bar.mts';
+
+export const foo = () => bar();

test/fixtures/test-runner/output/typescript-coverage.mts

@@ -0,0 +1,25 @@
+import assert from 'node:assert/strict';
+import { before, describe, it, mock } from 'node:test';
+
+describe('foo', { concurrency: true }, () => {
+    let barMock = mock.fn();
+    let foo;
+
+    before(async () => {
+        const barNamedExports = await import('../coverage/bar.mts')
+            .then(({ default: _, ...rest }) => rest);
+
+        mock.module('../coverage/bar.mts', {
+            defaultExport: barMock,
+            namedExports: barNamedExports,
+        });
+
+        ({ foo } = await import('../coverage/foo.mts'));
+    });
+
+    it('should do the thing', () => {
+        barMock.mock.mockImplementationOnce(() => 42);
+
+        assert.equal(foo(), 42);
+    });
+});

test/fixtures/test-runner/output/typescript-coverage.snapshot

@@ -0,0 +1,39 @@
+TAP version 13
+# Subtest: foo
+    # Subtest: should do the thing
+    ok 1 - should do the thing
+      ---
+      duration_ms: *
+      type: 'test'
+      ...
+    1..1
+ok 1 - foo
+  ---
+  duration_ms: *
+  type: 'suite'
+  ...
+1..1
+# tests 1
+# suites 1
+# pass 1
+# fail 0
+# cancelled 0
+# skipped 0
+# todo 0
+# duration_ms *
+# start of coverage report
+# ----------------------------------------------------------------------------
+# file                        | line % | branch % | funcs % | uncovered lines
+# ----------------------------------------------------------------------------
+# test                        |        |          |         | 
+#  fixtures                   |        |          |         | 
+#   test-runner               |        |          |         | 
+#    coverage                 |        |          |         | 
+#     bar.mts                 |   0.00 |   100.00 |  100.00 | 1-3
+#     foo.mts                 | 100.00 |   100.00 |  100.00 | 
+#    output                   |        |          |         | 
+#     typescript-coverage.mts | 100.00 |   100.00 |  100.00 | 
+# ----------------------------------------------------------------------------
+# all files                   |  85.29 |   100.00 |   85.71 | 
+# ----------------------------------------------------------------------------
+# end of coverage report

test/parallel/test-runner-output.mjs

@@ -313,6 +313,15 @@ const tests = [
     flags: ['--test-reporter=tap', '--test-coverage-exclude=../output/**'],
     cwd: fixtures.path('test-runner/coverage-snap'),
   } : false,
+  process.features.inspector ? {
+    name: 'test-runner/output/typescript-coverage.mts',
+    flags: ['--disable-warning=ExperimentalWarning',
+            '--test-reporter=tap',
+            '--experimental-transform-types',
+            '--experimental-test-module-mocks',
+            '--experimental-test-coverage',
+            '--test-coverage-exclude=!test/**']
+  } : false,
 ]
 .filter(Boolean)
 .map(({ flags, name, tty, transform, cwd }) => ({