Sema: Allow member type expressions rooted on non-identifier qualifiers to omit .self until Swift 6

Author: AnthonyLatsis

include/swift/AST/DiagnosticsSema.def

@@ -4188,7 +4188,8 @@ ERROR(value_of_module_type,none,
       "expected module member name after module name", ())
 
 ERROR(value_of_metatype_type,none,
-      "expected member name or constructor call after type name", ())
+      "expected member name or constructor call after type name%select{|; this "
+      "will be an error in Swift 6}0", (bool))
 
 NOTE(add_parens_to_type,none,
      "add arguments after the type to construct a value of the type", ())

lib/Sema/MiscDiagnostics.cpp

@@ -701,15 +701,17 @@ static void diagSyntacticUseRestrictions(const Expr *E, const DeclContext *DC,
       if (!AlreadyDiagnosedMetatypes.insert(E).second)
         return;
 
-      // In Swift < 6 warn about plain type name passed as an
-      // argument to a subscript, dynamic subscript, or ObjC
-      // literal since it used to be accepted.
       DiagnosticBehavior behavior = DiagnosticBehavior::Error;
 
       if (auto *ParentExpr = Parent.getAsExpr()) {
         if (ParentExpr->isValidParentOfTypeExpr(E))
           return;
 
+        // In Swift < 6 warn about
+        // - plain type name passed as an argument to a subscript, dynamic
+        //   subscript, or ObjC literal since it used to be accepted.
+        // - member type expressions rooted on non-identifier types, e.g.
+        //   '[X].Y' since they used to be accepted without the '.self'.
         if (!Ctx.LangOpts.isSwiftVersionAtLeast(6)) {
           if (isa<SubscriptExpr>(ParentExpr) ||
               isa<DynamicSubscriptExpr>(ParentExpr) ||
@@ -718,12 +720,21 @@ static void diagSyntacticUseRestrictions(const Expr *E, const DeclContext *DC,
             assert(argList);
             if (argList->isUnlabeledUnary())
               behavior = DiagnosticBehavior::Warning;
+          } else if (auto *TE = dyn_cast<TypeExpr>(E)) {
+            if (auto *TR =
+                    dyn_cast_or_null<MemberTypeRepr>(TE->getTypeRepr())) {
+              if (!isa<IdentTypeRepr>(TR->getBaseComponent())) {
+                behavior = DiagnosticBehavior::Warning;
+              }
+            }
           }
         }
       }
 
       // Is this a protocol metatype?
-      Ctx.Diags.diagnose(E->getStartLoc(), diag::value_of_metatype_type)
+      Ctx.Diags
+          .diagnose(E->getStartLoc(), diag::value_of_metatype_type,
+                    behavior == DiagnosticBehavior::Warning)
           .limitBehavior(behavior);
 
       // Add fix-it to insert '()', only if this is a metatype of

test/Constraints/dynamic_lookup.swift

@@ -454,11 +454,11 @@ func test_dynamic_subscript_accepts_type_name_argument() {
   }
 
   func test(a: AnyObject, optA: AnyObject?) {
-    let _ = a[A] // expected-warning {{expected member name or constructor call after type name}}
+    let _ = a[A] // expected-warning {{expected member name or constructor call after type name; this will be an error in Swift 6}}
     // expected-note@-1 {{add arguments after the type to construct a value of the type}} {{16-16=()}}
     // expected-note@-2 {{use '.self' to reference the type object}} {{16-16=.self}}
 
-    let _ = optA?[A] // expected-warning {{expected member name or constructor call after type name}}
+    let _ = optA?[A] // expected-warning {{expected member name or constructor call after type name; this will be an error in Swift 6}}
     // expected-note@-1 {{add arguments after the type to construct a value of the type}} {{20-20=()}}
     // expected-note@-2 {{use '.self' to reference the type object}} {{20-20=.self}}
   }

test/Constraints/subscript.swift

@@ -230,11 +230,11 @@ func test_subscript_accepts_type_name_argument() {
   }
 
   func test(a: A, optA: A?) {
-    let _ = a[A] // expected-warning {{expected member name or constructor call after type name}}
+    let _ = a[A] // expected-warning {{expected member name or constructor call after type name; this will be an error in Swift 6}}
     // expected-note@-1 {{add arguments after the type to construct a value of the type}} {{16-16=()}}
     // expected-note@-2 {{use '.self' to reference the type object}} {{16-16=.self}}
 
-    let _ = optA?[A] // expected-warning {{expected member name or constructor call after type name}}
+    let _ = optA?[A] // expected-warning {{expected member name or constructor call after type name; this will be an error in Swift 6}}
     // expected-note@-1 {{add arguments after the type to construct a value of the type}} {{20-20=()}}
     // expected-note@-2 {{use '.self' to reference the type object}} {{20-20=.self}}
   }

test/type/nested_types.swift

@@ -84,3 +84,33 @@ do {
   let _: (Int, Int).Undef // expected-error {{'Undef' is not a member type of type '(Swift.Int, Swift.Int)'}}
   let _: ((Int) -> Void).Undef // expected-error {{'Undef' is not a member type of type '(Swift.Int) -> Swift.Void'}}
 }
+
+// Accept member type expressions rooted on 'Self' or non-identifier qualifiers
+// until Swift 6.
+do {
+  struct Test {
+    enum E {}
+
+    func blackHole<T>(_: T.Type) {}
+
+    func test() {
+      blackHole(Self.E)
+      // expected-warning@-1 {{expected member name or constructor call after type name; this will be an error in Swift 6}}
+      // expected-note@-2 {{use '.self' to reference the type object}}
+      blackHole((Test).E)
+      // expected-warning@-1 {{expected member name or constructor call after type name; this will be an error in Swift 6}}
+      // expected-note@-2 {{use '.self' to reference the type object}}
+      blackHole([Test].Element)
+      // expected-warning@-1 {{expected member name or constructor call after type name; this will be an error in Swift 6}}
+      // expected-note@-2 {{use '.self' to reference the type object}}
+      // expected-note@-3 {{add arguments after the type to construct a value of the type}}
+      blackHole([Int : Test].Element)
+      // expected-warning@-1 {{expected member name or constructor call after type name; this will be an error in Swift 6}}
+      // expected-note@-2 {{use '.self' to reference the type object}}
+      blackHole(Test?.Wrapped)
+      // expected-warning@-1 {{expected member name or constructor call after type name; this will be an error in Swift 6}}
+      // expected-note@-2 {{use '.self' to reference the type object}}
+      // expected-note@-3 {{add arguments after the type to construct a value of the type}}
+    }
+  }
+}