Declaration emit incorrectly prints uninstantiated generic type

[marikaner]

Bug Report

I have a type with an unused generic type parameter.*
This type annotates an optional parameter property in the constructor of a class.
Starting with Version 4.2.X of TypeScript, this class is incorrectly transpiled, so that TS code using the transpiled sources (.d.ts) does not transpile anymore.
The error was:

error TS2304: Cannot find name 'T'

No wonder it cannot be found, it was not in the original code (please see the minimal example below).

During my investigation I found that this issue occurs, when these prerequisites are met:

1. The given type has an unused generic parameter.
2. The given type represents a union type.
3. You have a class with a constructor argument using the given type.
4. The constructor argument is a parameter property.
5. The constructor argument is optional.

This list might not be complete or exact.

* (Tried something. Didn't work. Postponed it, but left the parameter for later use.)

🔎 Search Terms

generics, transpilation, missing parameter

🕗 Version & Regression Information

• This changed between versions 4.1.X and 4.2.X

⏯ Playground Link

Playground link with relevant code

💻 Code

Before transpilation:

type BrokenType<T> = 'a' | 'b';

class MyClass {
    constructor(readonly arg?: BrokenType<any>) {}
}

After transpilation

declare type BrokenType<T> = 'a' | 'b';
declare class MyClass {
    readonly arg?: BrokenType<T> | undefined;
    constructor(arg?: BrokenType<T> | undefined); // <----- T should be any
}

🙁 Actual behavior

The code is transpiled to contain the generic parameter name, instead of the actual type that was passed as argument.

🙂 Expected behavior

I would expect the result to contain the original type as specified in the original code. The generic parameter is not known when consuming the type. This worked before version 4.2.X.

[weswigham]

Fixed by #42211 - a dupe of #42079 (though, admittedly, a simpler repro)