[kwy404]

Summary

When a Haste name has more than one provider, DependencyGraph turns the DuplicateHasteCandidatesError from metro-file-map into an AmbiguousModuleResolutionError. formatBundlingError (the error sent to the app) and TerminalReporter._logBundlingError (the terminal output) both build their own message for it, and list the candidates with Object.keys(he.duplicatesSet). But duplicatesSet is a Map (DuplicatesSet = Map<string, number> in metro-file-map, and HastePlugin throws with a Map), and Object.keys() of a Map is []. So both messages end with "You can delete or fix them:" and nothing after it.

The dev server hits this when two files share a Haste name, for example with resolver.enableGlobalPackages and two package.json files outside node_modules with the same name (the "multiple packages with the same name" case in resolver-test.js). Calling the formatter directly shows it:

const error = new AmbiguousModuleResolutionError(
  '/root/index.js',
  new DuplicateHasteCandidatesError(
    'shared',
    H.GENERIC_PLATFORM,
    false,
    new Map([
      ['/root/a/package.json', H.PACKAGE],
      ['/root/b/package.json', H.PACKAGE],
    ]),
  ),
);
formatBundlingError(error).message;
// "Ambiguous resolution: module '/root/index.js' tries to require 'shared', but there
// are several files providing this module. You can delete or fix them: \n\n"

The fix reads the paths from the map keys ([...he.duplicatesSet.keys()]) in both places. In formatBundlingError this also drops a .map() that only turned each path into the same string.

Changelog: [Fix] Ambiguous module resolution errors list the conflicting files instead of an empty list, in the terminal and in the app

Test plan

Added an AmbiguousModuleResolutionError test to packages/metro/src/lib/__tests__/formatBundlingError-test.js. It builds the error from a real DuplicateHasteCandidatesError with two paths and checks the whole message.

• Before the fix it fails: the message ends right after "fix them: \n\n".
• After the fix it passes, with both paths listed.
• TerminalReporter has no test file, so I checked it with a throwaway test that calls _logBundlingError with the same error. Before the fix nothing follows "fix them:", after it the output lists * `/root/a.js` and * `/root/b.js` .
• yarn jest packages/metro/src/lib/__tests__ packages/metro/src/__tests__/HmrServer-test.js packages/metro/src/Server/__tests__/Server-test.js: 20 suites, 274 tests pass.
• eslint and prettier --check on the three changed files: clean.
• flow focus-check on the three files: no new errors. My local flow-bin is still 0.333, so I ran it with --ignore-version; the only error it reports (the asserts syntax in flow-typed/npm/invariant_v2.x.x.js) is the same on main.