Allow overloads with varying @sendable func types

Specifically, an override can validly have a parameter type that is not `@Sendable` or a function type that is `@Sendable` even if that contradicts the base declaration. That’s becuase it’s always valid to provide a sendable function when one is not actually needed, but not vice versa.

Author: beccadax

lib/AST/Type.cpp

@@ -3167,6 +3167,11 @@ static bool matchesFunctionType(CanAnyFunctionType fn1, CanAnyFunctionType fn2,
           matchMode.contains(TypeMatchFlags::AllowABICompatible))) {
       ext1 = ext1.withThrows(true);
     }
+
+    // Removing '@Sendable' is ABI-compatible because there's nothing wrong with
+    // a function being sendable when it doesn't need to be.
+    if (!ext2.isSendable())
+      ext1 = ext1.withConcurrent(false);
   }
 
   // If specified, allow an escaping function parameter to override a

test/attr/attr_concurrent.swift

@@ -125,3 +125,17 @@ func testExplicitConcurrentClosure() {
   }
   let _: String = fn // expected-error{{cannot convert value of type '@Sendable () -> Int' to specified type 'String'}}
 }
+
+class SuperSendable {
+  func runsInBackground(_: @Sendable () -> Void) {}
+  func runsInForeground(_: () -> Void) {} // expected-note {{potential overridden instance method 'runsInForeground' here}}
+  func runnableInBackground() -> @Sendable () -> Void { fatalError() } // expected-note {{potential overridden instance method 'runnableInBackground()' here}}
+  func runnableInForeground() -> () -> Void { fatalError() }
+}
+
+class SubSendable: SuperSendable {
+  override func runsInBackground(_: () -> Void) {}
+  override func runsInForeground(_: @Sendable () -> Void) {} // expected-error {{method does not override any method from its superclass}}
+  override func runnableInBackground() -> () -> Void { fatalError() }  // expected-error {{method does not override any method from its superclass}}
+  override func runnableInForeground() -> @Sendable () -> Void { fatalError() }
+}