[Typechecker] Covariant subscript check must also take into consideration that there may be a type mismatch (#29828)

Author: theblixguy

lib/Sema/TypeCheckDeclOverride.cpp

@@ -1157,7 +1157,8 @@ bool OverrideMatcher::checkOverride(ValueDecl *baseDecl,
     // Otherwise, if this is a subscript, validate that covariance is ok.
     // If the parent is non-mutable, it's okay to be covariant.
     auto parentSubscript = cast<SubscriptDecl>(baseDecl);
-    if (parentSubscript->supportsMutation()) {
+    if (parentSubscript->supportsMutation() &&
+        attempt != OverrideCheckingAttempt::MismatchedTypes) {
       diags.diagnose(subscript, diag::override_mutable_covariant_subscript,
                      declTy, baseTy);
       diags.diagnose(baseDecl, diag::subscript_override_here);

test/attr/attr_override.swift

@@ -59,8 +59,7 @@ class A {
     }
   }
 
-  // FIXME(SR-10323): The second note is wrong; it should be "potential overridden class subscript 'subscript(_:)' here". This is a preexisting bug.
-  class subscript (i: String) -> String { // expected-note{{overridden declaration is here}} expected-note{{attempt to override subscript here}}
+  class subscript (i: String) -> String { // expected-note{{overridden declaration is here}} expected-note{{potential overridden class subscript 'subscript(_:)' here}}
     get {
       return "hello"
     }
@@ -144,8 +143,7 @@ class B : A {
     }
   }
 
-  // FIXME(SR-10323): This error is wrong; it should be "subscript does not override any subscript from its superclass". This is a preexisting bug.
-  override class subscript (i: Int) -> String { // expected-error{{cannot override mutable subscript of type '(Int) -> String' with covariant type '(String) -> String'}}
+  override class subscript (i: Int) -> String { // expected-error{{subscript does not override any subscript from its superclass}}
     get {
       return "hello"
     }
@@ -647,4 +645,3 @@ public extension SR_11740_Base where F: SR_11740_Q {
 extension SR_11740_Derived where F: SR_11740_P {
     public static func foo(_: A) {}
 }
-