cmd/compile/internal/syntax: unclear error message for function type have type parameters

[cuonglm]

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

$ go version
go version devel go1.18-ee91bb8319 Tue Sep 14 02:53:17 2021 +0000 darwin/arm64

Does this issue reproduce with the latest release?

What operating system and processor architecture are you using (go env)?

go env Output

$ go env

What did you do?

Compile this program:

package p

type I func[T any]()

What did you expect to see?

Friendly error message, like gotype:

$ gotype p.go
p.go:3:12: function type cannot have type parameters

What did you see instead?

$ go tool compile p.go
p.go:3:15: syntax error: unexpected any, expecting ]

cc @griesemer @findleyr

[griesemer]

The fix is not hard but it's also subtle due to the way func type parsing is used in various places. If we address this, it needs to be a comprehensive fix which requires some careful parser changes.

This looks straight-forward to address, after all.

Not urgent.

[gopherbot]

Change https://golang.org/cl/361259 mentions this issue: cmd/compile/internal/syntax: better error message when type parameters are not permitted