cmd/compile: improve error message for invalid use of interface with type constraints

[virtuald]

What did you do?

https://gotipplay.golang.org/p/K_OPRyt5aJo

package main

type C interface {
	~int
	Fn()
}

func UseInterface(c C) {}

type Foo struct {
	I C
}

What did you expect to see?

./prog.go:8:21: interface with type constraints used as function parameter
./prog.go:11:4: interface with type constraints cannot be used as field

What did you see instead?

./prog.go:8:21: interface contains type constraints
./prog.go:11:4: interface contains type constraints

[ianlancetaylor]

CC @griesemer

[griesemer]

The checker.varType function needs a context string so that it can report a context-aware error. Straight-forward to fix, needs some Fingerspitzengefühl for good error messages.

[griesemer]

This reports now:

./prog.go:8:21: cannot use type C outside a type constraint: interface contains type constraints
./prog.go:11:4: cannot use type C outside a type constraint: interface contains type constraints

not great, but a bit clearer. Closing this for now.