cmd/internal/gc: improve "type *X has no field or method M" message

Try to provide hints for common areas, either *interface
were interface would have been better, and note incorrect
capitalization (but don't be more ambitious than that, at
least not today).

Added code and test for cases

  ptrInterface.ExistingMethod
  ptrInterface.unexportedMethod
  ptrInterface.MissingMethod
  ptrInterface.withwRongcASEdMethod
  interface.withwRongcASEdMethod
  ptrStruct.withwRongcASEdMethod
  struct.withwRongcASEdMethod

also included tests for related errors to check for
unintentional changes and consistent wording.

Somewhat simplified from previous versions to avoid second-
guessing user errors, yet also biased to point out most-likely
root cause.

Fixes #10700

Change-Id: I16693e93cc8d8ca195e7742a222d640c262105b4
Reviewed-on: https://go-review.googlesource.com/9731Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/internal/gc/typecheck.go

@@ -915,12 +915,27 @@ OpSwitch:
 			return
 		}
 
-		if !lookdot(n, t, 0) {
-			if lookdot(n, t, 1) {
+		if lookdot(n, t, 0) == nil {
+			// Legitimate field or method lookup failed, try to explain the error
+			switch {
+			case isnilinter(t):
+				Yyerror("%v undefined (type %v is interface with no methods)", n, n.Left.Type)
+
+			case Isptr[t.Etype] && Isinter(t.Type):
+				// Pointer to interface is almost always a mistake.
+				Yyerror("%v undefined (type %v is pointer to interface, not interface)", n, n.Left.Type)
+
+			case lookdot(n, t, 1) != nil:
+				// Field or method matches by name, but it is not exported.
 				Yyerror("%v undefined (cannot refer to unexported field or method %v)", n, n.Right.Sym)
+
+			default:
+				if mt := lookdot(n, t, 2); mt != nil { // Case-insensitive lookup.
+					Yyerror("%v undefined (type %v has no field or method %v, but does have %v)", n, n.Left.Type, n.Right.Sym, mt.Sym)
 				} else {
 					Yyerror("%v undefined (type %v has no field or method %v)", n, n.Left.Type, n.Right.Sym)
 				}
+			}
 			n.Type = nil
 			return
 		}
@@ -2391,6 +2406,9 @@ func lookdot1(errnode *Node, s *Sym, t *Type, f *Type, dostrcmp int) *Type {
 		if dostrcmp != 0 && f.Sym.Name == s.Name {
 			return f
 		}
+		if dostrcmp == 2 && strings.EqualFold(f.Sym.Name, s.Name) {
+			return f
+		}
 		if f.Sym != s {
 			continue
 		}
@@ -2461,7 +2479,7 @@ func derefall(t *Type) *Type {
 	return t
 }
 
-func lookdot(n *Node, t *Type, dostrcmp int) bool {
+func lookdot(n *Node, t *Type, dostrcmp int) *Type {
 	s := n.Right.Sym
 
 	dowidth(t)
@@ -2481,6 +2499,10 @@ func lookdot(n *Node, t *Type, dostrcmp int) bool {
 	}
 
 	if f1 != nil {
+		if dostrcmp > 1 {
+			// Already in the process of diagnosing an error.
+			return f1
+		}
 		if f2 != nil {
 			Yyerror("%v is both field and method", n.Right.Sym)
 		}
@@ -2500,10 +2522,14 @@ func lookdot(n *Node, t *Type, dostrcmp int) bool {
 			n.Op = ODOTINTER
 		}
 
-		return true
+		return f1
 	}
 
 	if f2 != nil {
+		if dostrcmp > 1 {
+			// Already in the process of diagnosing an error.
+			return f2
+		}
 		tt := n.Left.Type
 		dowidth(tt)
 		rcvr := getthisx(f2.Type).Type.Type
@@ -2543,7 +2569,7 @@ func lookdot(n *Node, t *Type, dostrcmp int) bool {
 				// It is invalid to automatically dereference a named pointer type when selecting a method.
 				// Make n->left == ll to clarify error message.
 				n.Left = ll
-				return false
+				return nil
 			}
 		}
 
@@ -2554,10 +2580,10 @@ func lookdot(n *Node, t *Type, dostrcmp int) bool {
 		//		print("lookdot found [%p] %T\n", f2->type, f2->type);
 		n.Op = ODOTMETH
 
-		return true
+		return f2
 	}
 
-	return false
+	return nil
 }
 
 func nokeys(l *NodeList) bool {

test/fixedbugs/issue10700.dir/other.go

+// Copyright 2015 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 other
+
+type Exported interface {
+	Do()
+	secret()
+}

test/fixedbugs/issue10700.dir/test.go

+// errorcheck -0 -m -l
+
+// Copyright 2015 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 "./other"
+
+type Imported interface {
+	Do()
+}
+
+type HasAMethod struct {
+	x int
+}
+
+func (me *HasAMethod) Do() {
+	println(me.x)
+}
+
+func InMyCode(x *Imported, y *HasAMethod, z *other.Exported) {
+	x.Do() // ERROR "x\.Do undefined \(type \*Imported is pointer to interface, not interface\)"
+	x.do() // ERROR "x\.do undefined \(type \*Imported is pointer to interface, not interface\)"
+	(*x).Do()
+	x.Dont()    // ERROR "x\.Dont undefined \(type \*Imported is pointer to interface, not interface\)"
+	(*x).Dont() // ERROR "\(\*x\)\.Dont undefined \(type Imported has no field or method Dont\)"
+
+	y.Do()
+	y.do() // ERROR "y\.do undefined \(type \*HasAMethod has no field or method do, but does have Do\)"
+	(*y).Do()
+	(*y).do()   // ERROR "\(\*y\)\.do undefined \(type HasAMethod has no field or method do, but does have Do\)"
+	y.Dont()    // ERROR "y\.Dont undefined \(type \*HasAMethod has no field or method Dont\)"
+	(*y).Dont() // ERROR "\(\*y\)\.Dont undefined \(type HasAMethod has no field or method Dont\)"
+
+	z.Do() // ERROR "z\.Do undefined \(type \*other\.Exported is pointer to interface, not interface\)"
+	z.do() // ERROR "z\.do undefined \(type \*other\.Exported is pointer to interface, not interface\)"
+	(*z).Do()
+	(*z).do()     // ERROR "\(\*z\)\.do undefined \(type other.Exported has no field or method do, but does have Do\)"
+	z.Dont()      // ERROR "z\.Dont undefined \(type \*other\.Exported is pointer to interface, not interface\)"
+	(*z).Dont()   // ERROR "\(\*z\)\.Dont undefined \(type other\.Exported has no field or method Dont\)"
+	z.secret()    // ERROR "z\.secret undefined \(type \*other\.Exported is pointer to interface, not interface\)"
+	(*z).secret() // ERROR "\(\*z\)\.secret undefined \(cannot refer to unexported field or method secret\)"
+
+}
+
+func main() {
+}

test/fixedbugs/issue10700.go

+// errorcheckdir
+
+// Copyright 2015 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

test/interface/embed2.go

@@ -12,20 +12,25 @@ import "os"
 
 const Value = 1e12
 
-type Inter interface { M() int64 }
+type Inter interface {
+	M() int64
+}
 
 type T int64
+
 func (t T) M() int64 { return int64(t) }
+
 var t = T(Value)
 var pt = &t
 var ti Inter = t
 var pti = &ti
 
-type S struct { Inter }
-var s = S{ ti }
+type S struct{ Inter }
+
+var s = S{ti}
 var ps = &s
 
-type SP struct { *Inter }	// ERROR "interface"
+type SP struct{ *Inter } // ERROR "interface"
 
 var i Inter
 var pi = &i
@@ -43,25 +48,25 @@ func main() {
 	check("t.M()", t.M())
 	check("pt.M()", pt.M())
 	check("ti.M()", ti.M())
-	check("pti.M()", pti.M())	// ERROR "method"
+	check("pti.M()", pti.M()) // ERROR "pointer to interface, not interface"
 	check("s.M()", s.M())
 	check("ps.M()", ps.M())
 
 	i = t
 	check("i = t; i.M()", i.M())
-	check("i = t; pi.M()", pi.M())	// ERROR "method"
+	check("i = t; pi.M()", pi.M()) // ERROR "pointer to interface, not interface"
 
 	i = pt
 	check("i = pt; i.M()", i.M())
-	check("i = pt; pi.M()", pi.M())	// ERROR "method"
+	check("i = pt; pi.M()", pi.M()) // ERROR "pointer to interface, not interface"
 
 	i = s
 	check("i = s; i.M()", i.M())
-	check("i = s; pi.M()", pi.M())	// ERROR "method"
+	check("i = s; pi.M()", pi.M()) // ERROR "pointer to interface, not interface"
 
 	i = ps
 	check("i = ps; i.M()", i.M())
-	check("i = ps; pi.M()", pi.M())	// ERROR "method"
+	check("i = ps; pi.M()", pi.M()) // ERROR "pointer to interface, not interface"
 
 	if !ok {
 		println("BUG: interface10")