From fb0632eb7d817e95b706d7841c5192343a08826a Mon Sep 17 00:00:00 2001
From: Robert Griesemer <gri@golang.org>
Date: Thu, 31 Oct 2013 12:01:00 -0700
Subject: [PATCH] go.tools/go/types: Provide explicit type checker Error.

R=adonovan
CC=golang-dev
https://golang.org/cl/20400044
---
 go/types/api.go    | 19 ++++++++++++++++---
 go/types/errors.go |  5 +++--
 2 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/go/types/api.go b/go/types/api.go
index a733581d..3f4c9967 100644
--- a/go/types/api.go
+++ b/go/types/api.go
@@ -23,6 +23,7 @@
 package types
 
 import (
+	"fmt"
 	"go/ast"
 	"go/token"
 
@@ -44,6 +45,20 @@ func Check(path string, fset *token.FileSet, files []*ast.File) (*Package, error
 	return pkg, nil
 }
 
+// An Error describes a type-checking error;
+// it implements the error interface.
+type Error struct {
+	Fset *token.FileSet // file set for interpretation of Pos
+	Pos  token.Pos      // error position
+	Msg  string         // error message
+}
+
+// Error returns an error string formatted as follows:
+// filename:line:column: message
+func (err Error) Error() string {
+	return fmt.Sprintf("%s: %s", err.Fset.Position(err.Pos), err.Msg)
+}
+
 // A Config specifies the configuration for type checking.
 // The zero value for Config is a ready-to-use default configuration.
 type Config struct {
@@ -66,9 +81,7 @@ type Config struct {
 	Packages map[string]*Package
 
 	// If Error != nil, it is called with each error found
-	// during type checking. The error strings of errors with
-	// detailed position information are formatted as follows:
-	// filename:line:column: message
+	// during type checking; err has dynamic type Error.
 	Error func(err error)
 
 	// If Import != nil, it is called for each imported package.
diff --git a/go/types/errors.go b/go/types/errors.go
index 7173774a..66e8e95b 100644
--- a/go/types/errors.go
+++ b/go/types/errors.go
@@ -53,7 +53,8 @@ func (check *checker) dump(format string, args ...interface{}) {
 	fmt.Println(check.sprintf(format, args...))
 }
 
-func (check *checker) err(err error) {
+func (check *checker) err(pos token.Pos, msg string) {
+	err := Error{check.fset, pos, msg}
 	if check.firstErr == nil {
 		check.firstErr = err
 	}
@@ -65,7 +66,7 @@ func (check *checker) err(err error) {
 }
 
 func (check *checker) errorf(pos token.Pos, format string, args ...interface{}) {
-	check.err(fmt.Errorf("%s: %s", check.fset.Position(pos), check.sprintf(format, args...)))
+	check.err(pos, check.sprintf(format, args...))
 }
 
 func (check *checker) invalidAST(pos token.Pos, format string, args ...interface{}) {