cmd/compile: need better error message for unsatisfied constraints due to type mismatch

[griesemer]

Compiling

package p

type SliceConstraint[T any] interface {
	[]T
}

func Map[S SliceConstraint[E], E any](s S, f func(E) E) S {
	return s
}

type MySlice []int

func f(s MySlice) {
	Map[MySlice, int](s, nil)
}

produces

x.go:14:19: MySlice does not satisfy SliceConstraint[E]

It's hard to see what the error is. It should have been

type SliceConstraint[T any] interface {
	~[]T // <<< need ~ here!
}

We need to produce a better error message in cases like these. This will frustrate a lot of users.

cc: @findleyr

[gopherbot]

Change https://golang.org/cl/366757 mentions this issue: cmd/compile/internal/types2: better error position for instantiation failure

[gopherbot]

Change https://golang.org/cl/366758 mentions this issue: cmd/compile/internal/types2: better error message for missing ~ in constraint

[gopherbot]

Change https://golang.org/cl/367198 mentions this issue: go/types: better error position for instantiation failure

[gopherbot]

Change https://golang.org/cl/367199 mentions this issue: go/types: better error message for missing ~ in constraint