go.tools/go/types: fix: the type of a method value should not have a reciever

e.g.
type T int
func (T) f() {}
var t T
_ = t.f  // method value: should have signature "func()", no receiver

Also:
- ssa: add sanity check that helped diagnose this.

R=gri
CC=golang-dev
https://golang.org/cl/12283043

go/types/call.go

@@ -305,7 +305,11 @@ func (check *checker) selector(x *operand, e *ast.SelectorExpr) {
 			}
 
 			x.mode = value
-			x.typ = obj.typ
+
+			// remove receiver
+			sig := *obj.typ.(*Signature)
+			sig.recv = nil
+			x.typ = &sig
 
 		default:
 			unreachable()

ssa/sanity.go

@@ -37,6 +37,7 @@ func sanityCheck(fn *Function, reporter io.Writer) bool {
 //
 func mustSanityCheck(fn *Function, reporter io.Writer) {
 	if !sanityCheck(fn, reporter) {
+		fn.DumpTo(os.Stderr)
 		panic("SanityCheck failed")
 	}
 }
@@ -141,7 +142,17 @@ func (s *sanity) checkInstr(idx int, instr Instruction) {
 	case *Lookup:
 	case *MakeChan:
 	case *MakeClosure:
-		// TODO(adonovan): check FreeVars count matches.
+		numFree := len(instr.Fn.(*Function).FreeVars)
+		numBind := len(instr.Bindings)
+		if numFree != numBind {
+			s.errorf("MakeClosure has %d Bindings for function %s with %d free vars",
+				numBind, instr.Fn, numFree)
+
+		}
+		if recv := instr.Type().(*types.Signature).Recv(); recv != nil {
+			s.errorf("MakeClosure's type includes receiver %s", recv.Type())
+		}
+
 	case *MakeInterface:
 	case *MakeMap:
 	case *MakeSlice: