gc: better error for method non-call

was
x.go:7: must call (&b).*Buffer·Write

now
x.go:7: method b.Write is not an expression, must be called

Fixes #1171.

R=ken2
CC=golang-dev
https://golang.org/cl/2384042

src/cmd/gc/go.h

@@ -210,8 +210,9 @@ struct	Node
 	uchar	dodata;		// compile literal assignment as data statement
 	uchar	used;
 	uchar	isddd;
-	uchar	pun;		// dont registerize variable ONAME
+	uchar	pun;		// don't registerize variable ONAME
 	uchar	readonly;
+	uchar	implicit;	// don't show in printout
 
 	// most nodes
 	Node*	left;

src/cmd/gc/print.c

@@ -23,6 +23,7 @@ void
 exprfmt(Fmt *f, Node *n, int prec)
 {
 	int nprec;
+	char *p;
 
 	nprec = 0;
 	if(n == nil) {
@@ -30,6 +31,11 @@ exprfmt(Fmt *f, Node *n, int prec)
 		return;
 	}
 	
+	if(n->implicit) {
+		exprfmt(f, n->left, prec);
+		return;
+	}
+
 	switch(n->op) {
 	case ONAME:
 	case ONONAME:
@@ -298,8 +304,15 @@ exprfmt(Fmt *f, Node *n, int prec)
 		exprfmt(f, n->left, 7);
 		if(n->right == N || n->right->sym == S)
 			fmtprint(f, ".<nil>");
+		else {
+			// skip leading type· in method name
+			p = utfrrune(n->right->sym->name, 0xb7);
+			if(p)
+				p+=2;
 			else
-			fmtprint(f, ".%s", n->right->sym->name);
+				p = n->right->sym->name;
+			fmtprint(f, ".%s", p);
+		}
 		break;
 
 	case ODOTTYPE:

src/cmd/gc/typecheck.c

@@ -1254,7 +1254,7 @@ ret:
 		goto error;
 	}
 	if((ok & Ecall) && !(top & Ecall)) {
-		yyerror("must call %#N", n);
+		yyerror("method %#N is not an expression, must be called", n);
 		goto error;
 	}
 	// TODO(rsc): simplify
@@ -1483,9 +1483,11 @@ lookdot(Node *n, Type *t, int dostrcmp)
 				checklvalue(n->left, "call pointer method on");
 				addrescapes(n->left);
 				n->left = nod(OADDR, n->left, N);
+				n->left->implicit = 1;
 				typecheck(&n->left, Etype|Erv);
 			} else if(tt->etype == tptr && eqtype(tt->type, rcvr)) {
 				n->left = nod(OIND, n->left, N);
+				n->left->implicit = 1;
 				typecheck(&n->left, Etype|Erv);
 			} else {
 				// method is attached to wrong type?