cmd/gc: add .y to error about missing x in x.y

If the Go source says x.y, and x is undefined, today we get

	undefined: x

Change to:

	undefined: x in x.y

Change-Id: I8ea95503bd469ea933c6bcbd675b7122a5d454f3
Reviewed-on: https://go-review.googlesource.com/4643Reviewed-by: Austin Clements <austin@google.com>

src/cmd/gc/go.h

@@ -1448,6 +1448,7 @@ void	warn(char *fmt, ...);
 void	warnl(int line, char *fmt, ...);
 void	yyerror(char *fmt, ...);
 void	yyerrorl(int line, char *fmt, ...);
+void	adderrorname(Node*);
 
 /*
  *	swt.c

src/cmd/gc/subr.c

@@ -38,6 +38,19 @@ parserline(void)
 	return lineno;
 }
 
+void
+adderrorname(Node *n)
+{
+	char *old;
+	
+	if(n->op != ODOT)
+		return;
+	old = smprint("%L: undefined: %N\n", n->lineno, n->left);
+	if(nerr > 0 && err[nerr-1].lineno == n->lineno && strcmp(err[nerr-1].msg, old) == 0)
+		err[nerr-1].msg = smprint("%L: undefined: %N in %N\n", n->lineno, n->left, n);
+	free(old);
+}
+
 static void
 adderr(int line, char *fmt, va_list arg)
 {

src/cmd/gc/typecheck.c

@@ -786,12 +786,14 @@ reswitch:
 	case ODOT:
 		typecheck(&n->left, Erv|Etype);
 		defaultlit(&n->left, T);
-		if((t = n->left->type) == T)
-			goto error;
 		if(n->right->op != ONAME) {
 			yyerror("rhs of . must be a name");	// impossible
 			goto error;
 		}
+		if((t = n->left->type) == T) {
+			adderrorname(n);
+			goto error;
+		}
 		r = n->right;
 
 		if(n->left->op == OTYPE) {
@@ -3303,6 +3305,8 @@ typecheckdef(Node *n)
 			n->diag = 1;
 			if(n->lineno != 0)
 				lineno = n->lineno;
+			// Note: adderrorname looks for this string and
+			// adds context about the outer expression
 			yyerror("undefined: %S", n->sym);
 		}
 		return n;