Error Messages Related to Nested Identical Generic Names Could Be More Explicit

Suggestion

🔍 Search Terms

identical generics
overlapping generics
nested generics

✅ Viability Checklist

My suggestion meets these guidelines (please notify me if my checklist is incorrect):

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Modify generic-related error messages, that involve 2 or more nested, identically named generic labels, to be more explicit.
e.g. Append "(which is a different type from the first 'T')" (or something similar) to the original message.

📃 Motivating Example

The following error message was caused by a function nested within a function using the same generic label (as well as some erroneous code.) I suggest we modify generic-related error messages where there are 2 or more nested generic labels that have the same name. A more explicit message would help nudge the developer towards the real error source.

Here is a snippet of the actual source code:

export function useForm<T extends FormFieldData>() {
  // ...
  function<T extends FormFieldData>(input: FormFieldKeys<T>) {
    // ...
  }
}

The solution in this case was removing the unnecessary nested generic and using the useForm scoped generic:

export function useForm<T extends FormFieldData>() {
  // ...
  function (input: FormFieldKeys<T>) {
    // ...
  }
}

@DanielRosenwasser @sandersn

Start by reproducing the nested generic example from the issue, comparing the diagnostics produced when the inner generic is named T with the version that uses the outer T. Trace the TypeScript type-checker diagnostic path for these errors and define the work as complete when nested identical generic labels are distinguished without changing runtime output.