cmd/compile: assigning to interface variable mentions only first missing method

[snakeru]

Consider the following code (http://play.golang.org/p/0Sm13cKcq5):

type A interface {
    Read()
    Write()
}
var a A = 1

This fails to compile with the following message:
"cannot use 1 (type int) as type A in assignment: int does not implement A (missing Read method)"

The problem is that 'int' also misses the 'Write' method, but it is not disclosed until user works around by defining a custom type and implementing the missing method (only to realize that it was not the only one).

This behavior delivers surprises when using complex libraries and writing compartibility layers for them.

[mdempsky]

If there's only a couple missing methods, it seems reasonable to mention them all. But I don't think we want to list all 29 missing methods for:

var x reflect.Type = 1

Maybe something like this though?

cannot use 1 (type int) as type reflect.Type in assignment: int does not implement reflect.Type (missing Align, FieldAlign, Method, MethodByName, and 25 more methods)

[odeke-em]

Also a duplicate of #8095.

This issue seems to be a hot topic (second duplicate of that issue)

Closing it in favor of #8095.