[dev.typeparams] go/types: implement delete(m, k) where m is of type parameter type

This is a port of CL 333729 to go/types.

Change-Id: I8682f549a7a15124b1b338f8c73e83a57d138368
Reviewed-on: https://go-review.googlesource.com/c/go/+/335078
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/builtins.go

@@ -369,25 +369,40 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 		x.typ = Typ[Int]
 
 	case _Delete:
-		// delete(m, k)
-		m := asMap(x.typ)
-		if m == nil {
+		// delete(map_, key)
+		// map_ must be a map type or a type parameter describing map types.
+		// The key cannot be a type parameter for now.
+		map_ := x.typ
+		var key Type
+		if !underIs(map_, func(u Type) bool {
+			map_, _ := u.(*Map)
+			if map_ == nil {
 				check.invalidArg(x, _InvalidDelete, "%s is not a map", x)
+				return false
+			}
+			if key != nil && !Identical(map_.key, key) {
+				check.invalidArg(x, _Todo, "maps of %s must have identical key types", x)
+				return false
+			}
+			key = map_.key
+			return true
+		}) {
 			return
 		}
+
 		arg(x, 1) // k
 		if x.mode == invalid {
 			return
 		}
 
-		check.assignment(x, m.key, "argument to delete")
+		check.assignment(x, key, "argument to delete")
 		if x.mode == invalid {
 			return
 		}
 
 		x.mode = novalue
 		if check.Types != nil {
-			check.recordBuiltinType(call.Fun, makeSig(nil, m, m.key))
+			check.recordBuiltinType(call.Fun, makeSig(nil, map_, key))
 		}
 
 	case _Imag, _Real:

src/go/types/testdata/check/builtins.go2

@@ -43,6 +43,43 @@ func _[T C5[X], X any](ch T) {
 	close(ch)
 }
 
+// delete
+
+type M0 interface{ int }
+type M1 interface{ map[string]int }
+type M2 interface { map[string]int | map[string]float64 }
+type M3 interface{ map[string]int | map[rune]int }
+type M4[K comparable, V any] interface{ map[K]V | map[rune]V }
+
+func _[T any](m T) {
+	delete(m /* ERROR not a map */, "foo")
+}
+
+func _[T M0](m T) {
+	delete(m /* ERROR not a map */, "foo")
+}
+
+func _[T M1](m T) {
+	delete(m, "foo")
+}
+
+func _[T M2](m T) {
+	delete(m, "foo")
+	delete(m, 0 /* ERROR cannot use .* as string */)
+}
+
+func _[T M3](m T) {
+	delete(m /* ERROR must have identical key types */, "foo")
+}
+
+func _[T M4[rune, V], V any](m T) {
+	delete(m, 'k')
+}
+
+func _[T M4[K, V], K comparable, V any](m T) {
+	delete(m /* ERROR must have identical key types */, "foo")
+}
+
 // make
 
 type Bmc interface {

src/go/types/type.go

@@ -110,11 +110,6 @@ func asSignature(t Type) *Signature {
 	return op
 }
 
-func asMap(t Type) *Map {
-	op, _ := optype(t).(*Map)
-	return op
-}
-
 // If the argument to asInterface, asNamed, or asTypeParam is of the respective type
 // (possibly after expanding an instance type), these methods return that type.
 // Otherwise the result is nil.