cmd/compile: missing T.m is a field, not a method error upon assignments to interfaces

[griesemer]

The 1.17 compiler did report this extra error. See fixedbugs/bug231.go for a test case.

package main

type I interface{ m() }
type T struct{ m func() }
type M struct{}

func (M) m() {}

func main() {
	var t T
	var m M
	var i I

	i = m
	// types2 does not give extra error "T.m is a field, not a method"
	i = t // ERROR "not a method|has no methods|does not implement I"
	_ = i
}

cc: @findleyr

[griesemer]

This requires a bit of consolidation and clean-up around method/field lookups and error reporting. Moving forward.

[gopherbot]

Change https://go.dev/cl/438856 mentions this issue: go/types, types2: better error if there's a field with the name of a missing method