gc: avoid package name ambiguity in error messages

Fixes #2006.

R=ken2
CC=golang-dev
https://golang.org/cl/4643056

src/cmd/gc/go.h

@@ -302,6 +302,7 @@ struct	Sym
 	uchar	flags;
 	uchar	sym;		// huffman encoding in object file
 	Sym*	link;
+	int32	npkg;	// number of imported packages with this name
 
 	// saved and restored by dcopy
 	Pkg*	pkg;
@@ -777,6 +778,7 @@ EXTERN	int32	nhunk;
 EXTERN	int32	thunk;
 
 EXTERN	int	exporting;
+EXTERN	int	erroring;
 EXTERN	int	noargnames;
 
 EXTERN	int	funcdepth;

src/cmd/gc/go.y

@@ -238,6 +238,7 @@ import_package:
 	LPACKAGE sym import_safety ';'
 	{
 		importpkg->name = $2->name;
+		pkglookup($2->name, nil)->npkg++;
 		importpkg->direct = 1;
 		
 		if(safemode && !curio.importsafe)
@@ -1658,6 +1659,7 @@ hidden_import:
 
 		p = mkpkg($3.u.sval);
 		p->name = $2->name;
+		pkglookup($2->name, nil)->npkg++;
 	}
 |	LVAR hidden_pkg_importsym hidden_type ';'
 	{

src/cmd/gc/subr.c

@@ -45,10 +45,12 @@ adderr(int line, char *fmt, va_list arg)
 	Fmt f;
 	Error *p;
 
+	erroring++;
 	fmtstrinit(&f);
 	fmtprint(&f, "%L: ", line);
 	fmtvprint(&f, fmt, arg);
 	fmtprint(&f, "\n");
+	erroring--;
 
 	if(nerr >= merr) {
 		if(merr == 0)
@@ -1123,6 +1125,13 @@ Sconv(Fmt *fp)
 	}
 
 	if(s->pkg != localpkg || longsymnames || (fp->flags & FmtLong)) {
+		// This one is for the user.  If the package name
+		// was used by multiple packages, give the full
+		// import path to disambiguate.
+		if(erroring && pkglookup(s->pkg->name, nil)->npkg > 1) {
+			fmtprint(fp, "\"%Z\".%s", s->pkg->path, s->name);
+			return 0;
+		}
 		fmtprint(fp, "%s.%s", s->pkg->name, s->name);
 		return 0;
 	}

test/fixedbugs/bug345.dir/io.go

+// Copyright 2011 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package io
+
+type Writer interface {
+	WrongWrite()
+}
+
+type SectionReader struct {
+	X int
+}
+
+func SR(*SectionReader) {}

test/fixedbugs/bug345.dir/main.go

+// Copyright 2011 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"bufio"
+	"./io"
+	goio "io"
+)
+
+func main() {
+	// The errors here complain that io.X != io.X
+	// for different values of io so they should be
+	// showing the full import path, which for the
+	// "./io" import is really ..../go/test/io.
+	// For example:
+	//
+	// main.go:25: cannot use w (type "/Users/rsc/g/go/test/fixedbugs/bug345.dir/io".Writer) as type "io".Writer in function argument:
+	//	io.Writer does not implement io.Writer (missing Write method)
+	// main.go:27: cannot use &x (type *"io".SectionReader) as type *"/Users/rsc/g/go/test/fixedbugs/bug345.dir/io".SectionReader in function argument
+
+	var w io.Writer
+	bufio.NewWriter(w)  // ERROR "test/io"
+	var x goio.SectionReader
+	io.SR(&x)  // ERROR "test/io"
+}

test/fixedbugs/bug345.go

+// $G $D/$F.dir/io.go && errchk $G -e $D/$F.dir/main.go
+
+// Copyright 2011 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ignored