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

首先复现 issue 中的嵌套泛型示例，比较将内部泛型命名为 T 与使用外部 T 的版本所产生的诊断信息。跟踪 TypeScript 类型检查器针对这些错误的诊断路径，并以能够区分嵌套的相同泛型标签且不改变运行时输出作为工作完成的标准。

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