gc: explain why invalid receiver types are invalid

Fixes #1680.

R=ken2
CC=golang-dev
https://golang.org/cl/4446061

src/cmd/gc/dcl.c

@@ -1139,6 +1139,32 @@ addmethod(Sym *sf, Type *t, int local)
 	pa = pa->type;
 	f = methtype(pa);
 	if(f == T) {
+		t = pa;
+		if(t != T) {
+			if(isptr[t->etype]) {
+				if(t->sym != S) {
+					yyerror("invalid receiver type %T (%T is a pointer type)", pa, t);
+					return;
+				}
+				t = t->type;
+			}
+		}
+		if(t != T) {
+			if(t->sym == S) {
+				yyerror("invalid receiver type %T (%T is an unnamed type)", pa, t);
+				return;
+			}
+			if(isptr[t->etype]) {
+				yyerror("invalid receiver type %T (%T is a pointer type)", pa, t);
+				return;
+			}
+			if(t->etype == TINTER) {
+				yyerror("invalid receiver type %T (%T is an interface type)", pa, t);
+				return;
+			}
+		}
+		// Should have picked off all the reasons above,
+		// but just in case, fall back to generic error.
 		yyerror("invalid receiver type %T", pa);
 		return;
 	}

test/method2.go

@@ -12,8 +12,14 @@ type T struct {
 type P *T
 type P1 *T
 
-func (p P) val() int   { return 1 } // ERROR "receiver"
-func (p *P1) val() int { return 1 } // ERROR "receiver"
+func (p P) val() int   { return 1 } // ERROR "receiver.* pointer"
+func (p *P1) val() int { return 1 } // ERROR "receiver.* pointer"
+
+type I interface{}
+type I1 interface{}
+
+func (p I) val() int { return 1 } // ERROR "receiver.*interface"
+func (p *I1) val() int { return 1 } // ERROR "receiver.*interface"
 
 type Val interface {
 	val() int