Sema: Implied 'Sendable' conformance should be unconditional

A conditional conformance to a protocol does not usually imply
a conformance to the protocol's inherited protocols, because
we have no way to guess what the conditional requirements
should be.

A carveout was added for 'Sendable', so that protocols could
inherit from 'Sendable' retroactively. However, the 'Sendable'
conformance would become conditional, which causes us to
reject a _second_ conditional conformance to such a protocol:

    struct G<T> {}
    protocol P: Sendable {}
    protocol Q: Sendable {}

    extension G: P where T: P {}
    extension G: Q where T: Q {}

To make this work, tweak the code so that an implied conformance
has the same generic signature as the conforming type, that is,
we force it to be unconditional.

Fixes rdar://122754849
Fixes #71544

Author: slavapestov

lib/AST/ConformanceLookup.cpp

@@ -718,6 +718,12 @@ LookupConformanceInModuleRequest::evaluate(
       // specialized type.
       auto *normalConf = cast<NormalProtocolConformance>(conformance);
       auto *conformanceDC = normalConf->getDeclContext();
+
+      if (normalConf->getSourceKind() == ConformanceEntryKind::Implied &&
+          normalConf->getProtocol()->isSpecificProtocol(KnownProtocolKind::Sendable)) {
+        conformanceDC = conformanceDC->getSelfNominalTypeDecl();
+      }
+
       auto subMap = type->getContextSubstitutionMap(mod, conformanceDC);
       return ProtocolConformanceRef(
           ctx.getSpecializedConformance(type, normalConf, subMap));

lib/AST/ProtocolConformance.cpp

@@ -231,6 +231,10 @@ GenericSignature ProtocolConformance::getGenericSignature() const {
   case ProtocolConformanceKind::Self:
     // If we have a normal or inherited protocol conformance, look for its
     // generic signature.
+    if (getSourceKind() == ConformanceEntryKind::Implied &&
+        getProtocol()->isSpecificProtocol(KnownProtocolKind::Sendable)) {
+      return getDeclContext()->getSelfNominalTypeDecl()->getGenericSignature();
+    }
     return getDeclContext()->getGenericSignatureOfContext();
 
   case ProtocolConformanceKind::Builtin:
@@ -406,7 +410,7 @@ ConditionalRequirementsRequest::evaluate(Evaluator &evaluator,
     return {};
   }
 
-  const auto extensionSig = ext->getGenericSignature();
+  const auto extensionSig = NPC->getGenericSignature();
 
   // The extension signature should be a superset of the type signature, meaning
   // every thing in the type signature either is included too or is implied by

lib/Sema/TypeCheckConcurrency.cpp

@@ -6055,6 +6055,8 @@ bool swift::checkSendableConformance(
     }
   }
 
+  if (conformance->getSourceKind() == ConformanceEntryKind::Implied)
+    conformanceDC = nominal;
   return checkSendableInstanceStorage(nominal, conformanceDC, check);
 }
 

test/Concurrency/implied_sendable_conformance_swift5.swift

@@ -0,0 +1,23 @@
+// RUN: %target-typecheck-verify-swift -swift-version 5
+// RUN: %target-swift-emit-silgen %s -swift-version 5
+
+protocol P: Sendable {}
+protocol Q: Sendable {}
+
+struct One<T> {  // expected-note {{consider making generic parameter 'T' conform to the 'Sendable' protocol}}
+  var t: T  // expected-warning {{stored property 't' of 'Sendable'-conforming generic struct 'One' has non-sendable type 'T'; this is an error in the Swift 6 language mode}}
+}
+
+extension One: P where T: P {}
+
+struct Both<T> {  // expected-note {{consider making generic parameter 'T' conform to the 'Sendable' protocol}}
+  var t: T  // expected-warning {{stored property 't' of 'Sendable'-conforming generic struct 'Both' has non-sendable type 'T'; this is an error in the Swift 6 language mode}}
+}
+
+extension Both: P where T: P {}
+extension Both: Q where T: Q {}
+
+func takesSendable<T: Sendable>(_: T) {}
+
+takesSendable(One<Int>(t: 3))
+takesSendable(Both<Int>(t: 3))