Go: 1.24 support - Tolerate type parameters on alias types

[owen-mc]

This is a small part of #18306 that can be merged in advance and means that the extractor won't fail on code using type paramters on type aliases, which is a new language feature in go 1.24. It is not tested in this PR as that would involve the test using 1.24, but it is tested in the original PR.

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Comments suppressed due to low confidence (1)

go/extractor/extractor.go:482

• The newly introduced code path for type aliases is not covered by tests in this PR. Consider adding tests to verify the functionality of type parameter parent population for alias types.

} else if tp, ok := typeNameObj.Type().(*types.Alias); ok {

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[smowton]

Add a test case to check what the comment describes re: the proper parent of a type parameter doesn't cause trouble?

For example, I think if we had A with type param T and then type B = A, it sounds like there was a risk that T would have ambiguous parents, A and B?

[owen-mc]

After staring at it for a long time, I think you're right that we need more of a check for the go 1.23 situation. I think the situation is this:

• a *types.TypeName is the kind of object for an alias or a defined type.
• in go 1.23 and before, calling Type() on a *types.TypeName for an alias gave you the type on the rhs of the alias declaration. So it could well be a defined type. I've moved the old check into the first conditional, which is intended just for named types. The second conditional, intended for alias types, won't be triggered in these versions of go.
• in go 1.24, calling Type() on a *types.TypeName gives you the alias type back again, so that will be caught be the second conditional.

As for tests:

• If A has a type parameter and you try to write type B = A you get the error "cannot use generic type A[T any] without instantiation" (in 1.23 and 1.24).
• type B[T any] = A[T] is only valid in go 1.24 (and it is tested in the go 1.24 PR).
• What you can do is type B = A[int], and that is already tested here.

[smowton]

👍 lmk when ready for re-review re adding an extra check if needed

[owen-mc]

@smowton I added the extra check already in Retain previous check for alias types. I also ran DCA, which showed nothing significant.