go.tools/go/types: dup check interface keys in composite map literals correctly

Specifically, take into account the key type.

LGTM=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/103080044

go/types/expr.go

@@ -1081,7 +1081,7 @@ func (check *checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 			check.indexedElts(e.Elts, utyp.elem, -1)
 
 		case *Map:
-			visited := make(map[interface{}]bool, len(e.Elts))
+			visited := make(map[interface{}][]Type, len(e.Elts))
 			for _, e := range e.Elts {
 				kv, _ := e.(*ast.KeyValueExpr)
 				if kv == nil {
@@ -1096,11 +1096,24 @@ func (check *checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 					continue
 				}
 				if x.mode == constant {
-					if visited[x.val] {
+					duplicate := false
+					// if the key is of interface type, the type is also significant when checking for duplicates
+					if _, ok := utyp.key.Underlying().(*Interface); ok {
+						for _, vtyp := range visited[x.val] {
+							if Identical(vtyp, x.typ) {
+								duplicate = true
+								break
+							}
+						}
+						visited[x.val] = append(visited[x.val], x.typ)
+					} else {
+						_, duplicate = visited[x.val]
+						visited[x.val] = nil
+					}
+					if duplicate {
 						check.errorf(x.pos(), "duplicate key %s in map literal", x.val)
 						continue
 					}
-					visited[x.val] = true
 				}
 				check.exprWithHint(x, kv.Value, utyp.elem)
 				if !check.assignment(x, utyp.elem) {

go/types/testdata/expr3.src

@@ -291,6 +291,9 @@ func slice_literals() {
 
 const index2 int = 2
 
+type N int
+func (N) f() {}
+
 func map_literals() {
 	type M0 map[string]int
 	type M1 map[bool]int
@@ -302,6 +305,23 @@ func map_literals() {
 	_ = M0{"foo": "bar" /* ERROR "cannot convert" */ }
 	_ = M0{"foo": 1, "bar": 2, "foo" /* ERROR "duplicate key" */ : 3 }
 
+	_ = map[interface{}]int{2: 1, 2 /* ERROR "duplicate key" */ : 1}
+	_ = map[interface{}]int{int(2): 1, int16(2): 1}
+	_ = map[interface{}]int{int16(2): 1, int16 /* ERROR "duplicate key" */ (2): 1}
+
+	type S string
+
+	_ = map[interface{}]int{"a": 1, "a" /* ERROR "duplicate key" */ : 1}
+	_ = map[interface{}]int{"a": 1, S("a"): 1}
+	_ = map[interface{}]int{S("a"): 1, S /* ERROR "duplicate key" */ ("a"): 1}
+
+	type I interface {
+		f()
+	}
+
+	_ = map[I]int{N(0): 1, N(2): 1}
+	_ = map[I]int{N(2): 1, N /* ERROR "duplicate key" */ (2): 1}
+
 	// map keys must be resolved correctly
 	key1 := "foo"
 	_ = M0{key1: 1}