cmd/compile: a little inconsistency on uses of type parameters in type parameter lists

[zigo101]

What version of Go are you using (go version)?

$ go version
go version go1.18rc1 linux/amd64

What did you do?

type C1[T []T] struct{} // okay
type C2[T *T] struct{}  // error: undefined: T

What did you expect to see?

Both compiles.

What did you see instead?

The second one doesn't compile.

Not sure if this is intended, just feel they should be consistent.

[ianlancetaylor]

CC @griesemer

For 1.19.

[griesemer]

This is working correctly according to the spec. The C2 type parameter list is ambiguous and per the spec it is parsed as an (invalid) array declaration since T is unknown. These work:

type C2[T *T,] struct{}
type C3[T interface{*T}] struct{}