Introduce missing Sendable conformances for existential conversions

When performing conversions to an existential that involves Sendable,
introducing missing conformances as needed to allow the type-check to
succeed and then (later) they'll be diagnosed appropriately.

Fixes rdar://89992095.

Author: DougGregor

lib/Sema/CSApply.cpp

@@ -5401,7 +5401,7 @@ collectExistentialConformances(Type fromType, Type toType,
   SmallVector<ProtocolConformanceRef, 4> conformances;
   for (auto proto : layout.getProtocols()) {
     conformances.push_back(TypeChecker::containsProtocol(
-        fromType, proto, module));
+        fromType, proto, module, false, /*allowMissing=*/true));
   }
 
   return toType->getASTContext().AllocateCopy(conformances);

lib/Sema/CSSimplify.cpp

@@ -7106,7 +7106,8 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyConformsToConstraint(
   case ConstraintKind::SelfObjectOfProtocol: {
     auto conformance = TypeChecker::containsProtocol(
         type, protocol, DC->getParentModule(),
-        /*skipConditionalRequirements=*/true);
+        /*skipConditionalRequirements=*/true,
+        /*allowMissing=*/true);
     if (conformance) {
       return recordConformance(conformance);
     }

lib/Sema/TypeCheckProtocol.cpp

@@ -5538,7 +5538,8 @@ void ConformanceChecker::emitDelayedDiags() {
 
 ProtocolConformanceRef
 TypeChecker::containsProtocol(Type T, ProtocolDecl *Proto, ModuleDecl *M,
-                              bool skipConditionalRequirements) {
+                              bool skipConditionalRequirements,
+                              bool allowMissing) {
   // Existential types don't need to conform, i.e., they only need to
   // contain the protocol.
   if (T->isExistentialType()) {
@@ -5560,8 +5561,9 @@ TypeChecker::containsProtocol(Type T, ProtocolDecl *Proto, ModuleDecl *M,
     if (auto superclass = layout.getSuperclass()) {
       auto result =
           (skipConditionalRequirements
-           ? M->lookupConformance(superclass, Proto)
-           : TypeChecker::conformsToProtocol(superclass, Proto, M));
+           ? M->lookupConformance(superclass, Proto, allowMissing)
+           : TypeChecker::conformsToProtocol(
+               superclass, Proto, M, allowMissing));
       if (result) {
         return result;
       }
@@ -5579,13 +5581,22 @@ TypeChecker::containsProtocol(Type T, ProtocolDecl *Proto, ModuleDecl *M,
         return ProtocolConformanceRef(Proto);
     }
 
+    // FIXME: Unify with shouldCreateMissingConformances
+    if (allowMissing &&
+        Proto->isSpecificProtocol(KnownProtocolKind::Sendable)) {
+      return ProtocolConformanceRef(
+          M->getASTContext().getBuiltinConformance(
+            T, Proto, GenericSignature(), { },
+            BuiltinConformanceKind::Missing));
+    }
+
     return ProtocolConformanceRef::forInvalid();
   }
 
   // For non-existential types, this is equivalent to checking conformance.
   return (skipConditionalRequirements
-          ? M->lookupConformance(T, Proto)
-          : TypeChecker::conformsToProtocol(T, Proto, M));
+          ? M->lookupConformance(T, Proto, allowMissing)
+          : TypeChecker::conformsToProtocol(T, Proto, M, allowMissing));
 }
 
 ProtocolConformanceRef

lib/Sema/TypeChecker.h

@@ -788,7 +788,8 @@ Expr *addImplicitLoadExpr(
 /// an empty optional.
 ProtocolConformanceRef containsProtocol(Type T, ProtocolDecl *Proto,
                                         ModuleDecl *M,
-                                        bool skipConditionalRequirements=false);
+                                        bool skipConditionalRequirements=false,
+                                        bool allowMissing=false);
 
 /// Determine whether the given type conforms to the given protocol.
 ///

test/Concurrency/sendable_existentials.swift

@@ -0,0 +1,54 @@
+// RUN: %target-typecheck-verify-swift -strict-concurrency=targeted
+// REQUIRES: concurrency
+// REQUIRES: OS=macosx
+
+@preconcurrency func send(_: Sendable) { }
+func sendOpt(_: Sendable?) { }
+
+enum E {
+  case something(Sendable)
+}
+
+@available(SwiftStdlib 5.1, *)
+func testE(a: Any, aOpt: Any?) async {
+  send(a) // expected-warning{{type 'Any' does not conform to the 'Sendable' protocol}}
+  sendOpt(a) // expected-warning{{type 'Any' does not conform to the 'Sendable' protocol}}
+  sendOpt(aOpt) // expected-warning{{type 'Any' does not conform to the 'Sendable' protocol}}
+
+  let _: E = .something(a) // expected-warning{{type 'Any' does not conform to the 'Sendable' protocol}}
+  _ = E.something(a) // expected-warning{{type 'Any' does not conform to the 'Sendable' protocol}}
+
+  var sendable: Sendable
+  sendable = a // expected-warning{{type 'Any' does not conform to the 'Sendable' protocol}}
+
+  var arrayOfSendable: [Sendable]
+  arrayOfSendable = [a, a] // expected-warning 2{{type 'Any' does not conform to the 'Sendable' protocol}}
+
+  func localFunc() { }
+  sendable = localFunc // expected-warning{{type '() -> ()' does not conform to the 'Sendable' protocol}}
+  // expected-note@-1{{a function type must be marked '@Sendable' to conform to 'Sendable'}}
+
+  _ = sendable
+  _ = arrayOfSendable
+}
+
+func testESilently(a: Any, aOpt: Any?) {
+  send(a)
+  sendOpt(a)
+  sendOpt(aOpt)
+
+  let _: E = .something(a)
+  _ = E.something(a)
+
+  var sendable: Sendable
+  sendable = a
+
+  var arrayOfSendable: [Sendable]
+  arrayOfSendable = [a, a]
+
+  func localFunc() { }
+  sendable = localFunc
+
+  _ = sendable
+  _ = arrayOfSendable
+}