From a6d2a4254ebe0154bc44ff9b8473b8738f0d68e8 Mon Sep 17 00:00:00 2001
From: Robert Griesemer <gri@golang.org>
Date: Mon, 8 Jun 2015 14:37:01 -0700
Subject: [PATCH] cmd/vet: backport of
 https://go-review.googlesource.com/#/c/10777/3

Change-Id: I8d63575f2053eda2d7f0a63f3ff862bf0de88d31
Reviewed-on: https://go-review.googlesource.com/10807
Reviewed-by: Alan Donovan <adonovan@google.com>
---
 cmd/vet/types.go | 24 +++++++++++-------------
 1 file changed, 11 insertions(+), 13 deletions(-)

diff --git a/cmd/vet/types.go b/cmd/vet/types.go
index 055f403ad7..084be85d68 100644
--- a/cmd/vet/types.go
+++ b/cmd/vet/types.go
@@ -19,8 +19,8 @@ var imports = make(map[string]*types.Package)
 
 var (
 	errorType     *types.Interface
-	stringerType  *types.Interface
-	formatterType *types.Interface
+	stringerType  *types.Interface // possibly nil
+	formatterType *types.Interface // possibly nil
 )
 
 func init() {
@@ -37,13 +37,12 @@ func init() {
 
 // importType returns the type denoted by the qualified identifier
 // path.name, and adds the respective package to the imports map
-// as a side effect.
+// as a side effect. In case of an error, importType returns nil.
 func importType(path, name string) types.Type {
 	pkg, err := types.DefaultImport(imports, path)
 	if err != nil {
-		// This can happen if fmt hasn't been compiled yet.
-		// Since nothing uses formatterType anyway, don't complain.
-		//warnf("import failed: %v", err)
+		// This can happen if the package at path hasn't been compiled yet.
+		warnf("import failed: %v", err)
 		return nil
 	}
 	if obj, ok := pkg.Scope().Lookup(name).(*types.TypeName); ok {
@@ -136,16 +135,13 @@ func (f *File) matchArgTypeInternal(t printfArgType, typ types.Type, arg ast.Exp
 		}
 	}
 	// If the type implements fmt.Formatter, we have nothing to check.
-	// But (see issue 6259) that's not easy to verify, so instead we see
-	// if its method set contains a Format function. We could do better,
-	// even now, but we don't need to be 100% accurate. Wait for 6259 to
-	// be fixed instead. TODO.
-	if f.hasMethod(typ, "Format") {
+	// formatterTyp may be nil - be conservative and check for Format method in that case.
+	if formatterType != nil && types.Implements(typ, formatterType) || f.hasMethod(typ, "Format") {
 		return true
 	}
 	// If we can use a string, might arg (dynamically) implement the Stringer or Error interface?
 	if t&argString != 0 {
-		if types.AssertableTo(errorType, typ) || types.AssertableTo(stringerType, typ) {
+		if types.AssertableTo(errorType, typ) || stringerType != nil && types.AssertableTo(stringerType, typ) {
 			return true
 		}
 	}
@@ -319,7 +315,9 @@ func (f *File) isErrorMethodCall(call *ast.CallExpr) bool {
 	if typ != nil {
 		// We know it's called "Error", so just check the function signature
 		// (stringerType has exactly one method, String).
-		return types.Identical(f.pkg.types[call.Fun].Type, stringerType.Method(0).Type())
+		if stringerType != nil && stringerType.NumMethods() == 1 {
+			return types.Identical(f.pkg.types[call.Fun].Type, stringerType.Method(0).Type())
+		}
 	}
 	// Without types, we can still check by hand.
 	// Is it a selector expression? Otherwise it's a function call, not a method call.