If a `name_mapper` points to a non-existent file, it should tell user that the mapped path doesn't exist (instead of the original path) · facebook/flow#7427

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enhancementerror messagesgood first issuemodule resolution

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module.name_mapper='.*\.css$' -> '<PROJECT_ROOT>/css-modules-stub.js.flow'

import foo from `foo.css`

Flow version: 0.91.

Error should say:

Cannot find module 'foo.css' (after it was remapped to '<PROJECT_ROOT>/css-modules-stub.js.flow', check your .flowconfig mapper paths exist)

Cannot find module 'foo.css'

Another example:

module.name_mapper='debug' -> 'empty/object'

If empty/object doesn't exist, there is no message to say that is the cause of Cannot resolve module 'debug' error.

There are many cases where name_mapper breaks things by the user wouldn't know. Try this to show the errors that are generated:

module.name_mapper='.*' -> '<PROJECT_ROOT>/some-empty-stub.js'

Then you get errors like this:

Cannot import Component because there is no Component export in react. Did you mean import Component from "..."?

But the cause is that the name_mapper has mapped to a file that doesn't export anything.

Research direction: Locate the module resolution logic in Flow's source (likely in src/services/module resolver.ml or similar). Understand how e.g. name mapper is applied and where 'Cannot find module' errors are generated. Modify the error message to include the remapped path and a hint when the mapped file does not exist. Check linked issues #3889, #1068, and #1090 for similar patterns.
Tech stack: javascript
Domain: clideveloper experiencetooling
Issue type: Feature
Difficulty: 3
Estimated time: 1-3 hours
Activity status: Stale
Clarity: Clear
Prerequisites: familiarity with Flowbasic understanding of .flowconfigknowledge of JavaScript module systems
Newbie friendliness: 60