cmd/compile: misleading error message when struct lacks expected type parameters

[rogpeppe]

commit 1ec5108

I compiled the following (incorrect) code: https://gotipplay.golang.org/p/NF9UzMFrJG1

type S[A, B any] struct {
	F int
}

func f[A any](s S[A]) {
	s.F = 1
}

I see these error messages:

./prog.go:10:17: cannot infer B (prog.go:6:11)
./prog.go:10:17: got 1 arguments but 2 type parameters
./prog.go:11:4: s.F undefined (type S[A] has no field or method F)

The last error message is misleading because S actually does have a field named F even if it hasn't been instantiated with the required type parameters.

[rogpeppe]

Here another example, probably related: https://gotipplay.golang.org/p/FBFHzcZOTmX

package main

func main() {
	X(Interface[*F[string]](Impl{}))
}

func X[Q Qer](fs Interface[Q]) {
}

type Impl struct{}

func (Impl) M() {}

type Interface[Q Qer] interface {
	M()
}

type Qer interface {
	Q()
}

type F[A, B any] struct{}

func (f *F[A, B]) Q() {}

This produces the errors:

./prog.go:4:3: *F[string] does not satisfy Qer (missing method Q)
./prog.go:4:14: *F[string] does not satisfy Qer (missing method Q)
./prog.go:4:15: cannot infer B (prog.go:22:11)
./prog.go:4:15: got 1 arguments but 2 type parameters

The type F does have a method Q so the first two error messages are misleading.
Also, those messages come before the actual cause of the error (that we're only passing
one type parameter when we should be passing two), which adds to the confusion.

[cagedmantis]

/cc @griesemer

[griesemer]

The underlying issue here is that some of the checks must be done in a delayed fashion, leading to some confusing scenarios like this one.

This is just a bug.

cc: @findleyr

[gopherbot]

Change https://golang.org/cl/378175 mentions this issue: go/types, types2: avoid field/method lookup error on invalid types