[MiscDiagnostics] Downgrade plain type use error to a warning for certain positions

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.

Resolves: rdar://80270126

Author: xedin

lib/Sema/MiscDiagnostics.cpp

@@ -615,6 +615,11 @@ 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;
+
       // Allow references to types as a part of:
       // - member references T.foo, T.Type, T.self, etc.
       // - constructor calls T()
@@ -634,11 +639,22 @@ static void diagSyntacticUseRestrictions(const Expr *E, const DeclContext *DC,
             isa<SubscriptExpr>(ParentExpr)) {
           return;
         }
+
+        if (!Ctx.LangOpts.isSwiftVersionAtLeast(6)) {
+          auto argument = CallArgs.find(ParentExpr);
+          if (argument != CallArgs.end()) {
+            auto *callExpr = argument->second;
+            if (isa<SubscriptExpr>(callExpr) ||
+                isa<DynamicSubscriptExpr>(callExpr) ||
+                isa<ObjectLiteralExpr>(callExpr))
+              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)
+          .limitBehavior(behavior);
 
       // Add fix-it to insert '()', only if this is a metatype of
       // non-existential type and has any initializers.

test/Constraints/dynamic_lookup.swift

@@ -433,3 +433,20 @@ func testCallAsFunctionAnyObject(_ x: AnyObject) {
   x() // expected-error {{cannot call value of non-function type 'AnyObject'}}
   x.callAsFunction() // Okay.
 }
+
+// Note: In Swift >= 6 mode this would become an error.
+func test_dynamic_subscript_accepts_type_name_argument() {
+  @objc class A {
+    @objc subscript(a: A.Type) -> Int { get { 42 } }
+  }
+
+  func test(a: AnyObject, optA: AnyObject?) {
+    let _ = a[A] // expected-warning {{expected member name or constructor call after type name}}
+    // 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}}
+    // 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

@@ -217,14 +217,19 @@ func rdar61084565() {
   a[] // expected-error {{value of type 'Foo' has no subscripts}}
 }
 
+// Note: In Swift >= 6 mode this would become an error.
 func test_subscript_accepts_type_name_argument() {
   struct A {
     subscript(a: A.Type) -> Int { get { 42 } }
   }
 
-  func test(a: A?) {
-    let _ = a?[A] // expected-error {{expected member name or constructor call after type name}}
-    // expected-note@-1 {{add arguments after the type to construct a value of the type}} {{17-17=()}}
-    // expected-note@-2 {{use '.self' to reference the type object}} {{17-17=.self}}
+  func test(a: A, optA: A?) {
+    let _ = a[A] // expected-warning {{expected member name or constructor call after type name}}
+    // 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}}
+    // 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}}
   }
 }