Correct @sendable variance for witnesses

It previously treated parameters and return types as the same, instead of properly handling variance.

Author: beccadax

lib/Sema/TypeCheckProtocol.cpp

@@ -155,13 +155,25 @@ getTypesToCompare(ValueDecl *reqt, Type reqtType, bool reqtTypeIsIUO,
     // function types that aren't in a parameter can be Sendable or not.
     // FIXME: Should we check for a Sendable bound on the requirement type?
     bool inRequirement = (adjustment != TypeAdjustment::NoescapeToEscaping);
-    (void)adjustInferredAssociatedType(adjustment, reqtType, inRequirement);
+    Type adjustedReqtType =
+      adjustInferredAssociatedType(adjustment, reqtType, inRequirement);
 
     bool inWitness = false;
     Type adjustedWitnessType =
       adjustInferredAssociatedType(adjustment, witnessType, inWitness);
-    if (inWitness && !inRequirement)
-      witnessType = adjustedWitnessType;
+
+    switch (variance) {
+    case VarianceKind::None:
+      break;
+    case VarianceKind::Covariant:
+      if (inRequirement && !inWitness)
+        reqtType = adjustedReqtType;
+      break;
+    case VarianceKind::Contravariant:
+      if (inWitness && !inRequirement)
+        witnessType = adjustedWitnessType;
+      break;
+    }
   };
 
   applyAdjustment(TypeAdjustment::NoescapeToEscaping);

test/attr/attr_concurrent.swift

@@ -139,3 +139,17 @@ class SubSendable: SuperSendable {
   override func runnableInBackground() -> () -> Void { fatalError() }  // expected-error {{method does not override any method from its superclass}}
   override func runnableInForeground() -> @Sendable () -> Void { fatalError() }
 }
+
+protocol AbstractSendable {
+  func runsInBackground(_: @Sendable () -> Void)
+  func runsInForeground(_: () -> Void) // expected-note {{protocol requires function 'runsInForeground' with type '(() -> Void) -> ()'; do you want to add a stub?}}
+  func runnableInBackground() -> @Sendable () -> Void // expected-note {{protocol requires function 'runnableInBackground()' with type '() -> @Sendable () -> Void'; do you want to add a stub?}}
+  func runnableInForeground() -> () -> Void
+}
+
+struct ConcreteSendable: AbstractSendable { // expected-error {{type 'ConcreteSendable' does not conform to protocol 'AbstractSendable'}}
+  func runsInBackground(_: () -> Void) {}
+  func runsInForeground(_: @Sendable () -> Void) {} // expected-note {{candidate has non-matching type '(@Sendable () -> Void) -> ()'}}
+  func runnableInBackground() -> () -> Void { fatalError() } // expected-note {{candidate has non-matching type '() -> () -> Void'}}
+  func runnableInForeground() -> @Sendable () -> Void { fatalError() }
+}