Why were duplicate type parameters causing the crash?

The visitor checks all parameters and adds each parameter type to the result. for the following example

type Foo<T> = {
    fn: (a: T, b: T, c: T) => void;
}

ExtractType tries to create a type type NewType<T, T, T> which causes the error. @jessetrinity does that make sense?

Yes, that makes sense. Thanks for the explanation and the PR!

I'm still not clear on why it crashes but this seems to fix another future bug.

At some point it would be good to add a variety of tests that pump template and pattern literal types through codefixes/refactors. When @jessetrinity and I were debugging this, we definitely saw the formatting scanner get tripped up on that node. I still think there might be a missing condition to reScan something. I’m surprised this fixes that crash based on the behavior we saw, but the formatter works in mysterious ways 🤷

@andrewbranch I'm ok to revert these changes, in favor of adding changes to formatter.

Strictly speaking it doesn't fix the template type bug, #40738 did.

Previous to that, extract type on

type a = `${a}` 

would cause a crash.

You should actually be able to remove the string templates from the tests in this PR and still get the crash previous to this change.

I'm ok to revert these changes, in favor of adding changes to formatter.

Oh no, these changes are clearly correct, I was just surprised they fixed the bug.

Strictly speaking it doesn't fix the template type bug, #40738 did.

Ah, that makes more sense 👍

#40738 did

@jessetrinity Yep, I was waiting for the merging of this PR., because my first PR contained changes related to missing nodes and duplicated type references.

@andrewbranch @jessetrinity Does that make sense to drop the test tests/cases/fourslash/refactorExtractType69.ts?

I thought it was good to have a test on the code that surfaced the bug, but maybe it's a little confusing since there were two bugs..?

If anything, we might want a more minimal test for the template string issue covering with span:

type a = \*a*\`${extractMe}`\*b*\