go.tools/ssa: emit ChangeType when using method as function in f := T.meth.

Previously: typeOf(f).Signature.Recv == T
       Now: typeOf(f).Signature.Params.At(0) == T

Added test.

BUG=5781

R=gri
CC=golang-dev
https://golang.org/cl/10622043

ssa/builder.go

@@ -735,7 +735,7 @@ func (b *builder) expr(fn *Function, e ast.Expr) Value {
 			id := MakeId(e.Sel.Name, fn.Pkg.Types)
 			typ := fn.Pkg.typeOf(e.X)
 			if m := fn.Prog.MethodSet(typ)[id]; m != nil {
-				return m
+				return emitConv(fn, m, fn.Pkg.typeOf(e))
 			}
 
 			// T must be an interface; return wrapper.

ssa/emit.go

@@ -124,8 +124,7 @@ func isValuePreserving(ut_src, ut_dst types.Type) bool {
 		return ok
 
 	case *types.Signature:
-		// Conversion between f(T) function and (T) func f() method?
-		// TODO(adonovan): is this sound?  Discuss with gri.
+		// Conversion from (T) func f() method to f(T) function?
 		_, ok := ut_src.(*types.Signature)
 		return ok
 	}

ssa/interp/testdata/boundmeth.go

@@ -79,10 +79,21 @@ func anonStruct() {
 	assert(get() == 3)
 }
 
+func typeCheck() {
+	var i interface{}
+	i = (*S).incr
+	_ = i.(func(*S)) // type assertion: receiver type prepended to params
+
+	var s S
+	i = s.incr
+	_ = i.(func()) // type assertion: receiver type disappears
+}
+
 func main() {
 	valueReceiver()
 	pointerReceiver()
 	addressibleValuePointerReceiver()
 	promotedReceiver()
 	anonStruct()
+	typeCheck()
 }