From 4b27c9d72eaa501fae86d6365205eacbfd89e83f Mon Sep 17 00:00:00 2001
From: Russ Cox <rsc@golang.org>
Date: Thu, 5 Feb 2015 12:31:13 -0500
Subject: [PATCH] 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/4643
Reviewed-by: Austin Clements <austin@google.com>
---
 src/cmd/gc/go.h        |  1 +
 src/cmd/gc/subr.c      | 13 +++++++++++++
 src/cmd/gc/typecheck.c |  8 ++++++--
 3 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/src/cmd/gc/go.h b/src/cmd/gc/go.h
index 38edaea747..0efd40d2bf 100644
--- a/src/cmd/gc/go.h
+++ b/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
diff --git a/src/cmd/gc/subr.c b/src/cmd/gc/subr.c
index 486e1d8dc9..c91b14d151 100644
--- a/src/cmd/gc/subr.c
+++ b/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)
 {
diff --git a/src/cmd/gc/typecheck.c b/src/cmd/gc/typecheck.c
index 64b80a88cd..c73c34804f 100644
--- a/src/cmd/gc/typecheck.c
+++ b/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;
-- 
2.50.0