go/types: add detail to missing method error messages

When a concrete type doesn't exactly implement an interface, the error
messages produced by go/types are often unhelpful. The compiler shows
the expected signature versus the one found, which is useful, so add
this behavior here.

Fixes golang/go#38475

Change-Id: I8b780b7e1f1f433a0efe670de3b1437053f42fba
Reviewed-on: https://go-review.googlesource.com/c/go/+/228457
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/expr.go

@@ -1568,12 +1568,12 @@ func (check *Checker) typeAssertion(pos token.Pos, x *operand, xtyp *Interface,
 	}
 
 	var msg string
-	if wrongType {
-		msg = "wrong type for method"
+	if wrongType != nil {
+		msg = fmt.Sprintf("wrong type for method %s (have %s, want %s)", method.name, wrongType.typ, method.typ)
 	} else {
-		msg = "missing method"
+		msg = "missing method " + method.name
 	}
-	check.errorf(pos, "%s cannot have dynamic type %s (%s %s)", x, T, msg, method.name)
+	check.errorf(pos, "%s cannot have dynamic type %s (%s)", x, T, msg)
 }
 
 func (check *Checker) singleValue(x *operand) {

src/go/types/lookup.go

@@ -263,14 +263,17 @@ func (check *Checker) lookupType(m map[Type]int, typ Type) (int, bool) {
 // x is of interface type V).
 //
 func MissingMethod(V Type, T *Interface, static bool) (method *Func, wrongType bool) {
-	return (*Checker)(nil).missingMethod(V, T, static)
+	m, typ := (*Checker)(nil).missingMethod(V, T, static)
+	return m, typ != nil
 }
 
 // missingMethod is like MissingMethod but accepts a receiver.
 // The receiver may be nil if missingMethod is invoked through
 // an exported API call (such as MissingMethod), i.e., when all
 // methods have been type-checked.
-func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method *Func, wrongType bool) {
+// If the type has the correctly names method, but with the wrong
+// signature, the existing method is returned as well.
+func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method, wrongType *Func) {
 	check.completeInterface(T)
 
 	// fast path for common case
@@ -286,10 +289,10 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method *
 			switch {
 			case obj == nil:
 				if static {
-					return m, false
+					return m, nil
 				}
 			case !check.identical(obj.Type(), m.typ):
-				return m, true
+				return m, obj
 			}
 		}
 		return
@@ -302,7 +305,7 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method *
 		// we must have a method (not a field of matching function type)
 		f, _ := obj.(*Func)
 		if f == nil {
-			return m, false
+			return m, nil
 		}
 
 		// methods may not have a fully set up signature yet
@@ -311,7 +314,7 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method *
 		}
 
 		if !check.identical(f.typ, m.typ) {
-			return m, true
+			return m, f
 		}
 	}
 
@@ -323,7 +326,7 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method *
 // method required by V and whether it is missing or just has the wrong type.
 // The receiver may be nil if assertableTo is invoked through an exported API call
 // (such as AssertableTo), i.e., when all methods have been type-checked.
-func (check *Checker) assertableTo(V *Interface, T Type) (method *Func, wrongType bool) {
+func (check *Checker) assertableTo(V *Interface, T Type) (method, wrongType *Func) {
 	// no static check is required if T is an interface
 	// spec: "If T is an interface type, x.(T) asserts that the
 	//        dynamic type of x implements the interface T."

src/go/types/operand.go

@@ -8,6 +8,7 @@ package types
 
 import (
 	"bytes"
+	"fmt"
 	"go/ast"
 	"go/constant"
 	"go/token"
@@ -254,8 +255,8 @@ func (x *operand) assignableTo(check *Checker, T Type, reason *string) bool {
 	if Ti, ok := Tu.(*Interface); ok {
 		if m, wrongType := check.missingMethod(V, Ti, true); m != nil /* Implements(V, Ti) */ {
 			if reason != nil {
-				if wrongType {
-					*reason = "wrong type for method " + m.Name()
+				if wrongType != nil {
+					*reason = fmt.Sprintf("wrong type for method %s (have %s, want %s)", m.Name(), wrongType.typ, m.typ)
 				} else {
 					*reason = "missing method " + m.Name()
 				}

src/go/types/testdata/issues.src

@@ -129,6 +129,9 @@ func issue10260() {
 		t1 *T1
 		t2 *T2
 	)
+
+	_ = i2 /* ERROR i2 .* cannot have dynamic type \*T1 \(wrong type for method foo \(have func\(\), want func\(x int\)\)\) */ .(*T1)
+
 	i1 = i0 /* ERROR cannot use .* missing method foo */
 	i1 = t0 /* ERROR cannot use .* missing method foo */
 	i1 = i2 /* ERROR cannot use .* wrong type for method foo */
@@ -146,7 +149,7 @@ func issue10260() {
 	// a few more - less exhaustive now
 
 	f := func(I1, I2){}
-	f(i0 /* ERROR cannot use .* missing method foo */ , i1 /* ERROR cannot use .* wrong type for method foo */)
+	f(i0 /* ERROR cannot use .* missing method foo */ , i1 /* ERROR cannot use .* wrong type for method foo \(have func\(\), want func\(x int\)\) */ )
 
 	_ = [...]I1{i0 /* ERROR cannot use .* missing method foo */ }
 	_ = [...]I1{i2 /* ERROR cannot use .* wrong type for method foo */ }