Sema: Key the implied Sendable conformance behavior off of -swift-version instead of -strict-concurrency

Also add some comments to explain what in the world is going on
with the new checks.

Author: slavapestov

lib/AST/ConformanceLookup.cpp

@@ -718,6 +718,10 @@ LookupConformanceInModuleRequest::evaluate(
       auto *normalConf = cast<NormalProtocolConformance>(conformance);
       auto *conformanceDC = normalConf->getDeclContext();
 
+      // In -swift-version 5 mode, a conditional conformance to a protocol can imply
+      // a Sendable conformance. The implied conformance is unconditional so it uses
+      // the generic signature of the nominal type and not the generic signature of
+      // the extension that declared the (implying) conditional conformance.
       if (normalConf->getSourceKind() == ConformanceEntryKind::Implied &&
           normalConf->getProtocol()->isSpecificProtocol(KnownProtocolKind::Sendable)) {
         conformanceDC = conformanceDC->getSelfNominalTypeDecl();

lib/AST/ProtocolConformance.cpp

@@ -230,10 +230,16 @@ GenericSignature ProtocolConformance::getGenericSignature() const {
   case ProtocolConformanceKind::Self:
     // If we have a normal or inherited protocol conformance, look for its
     // generic signature.
+
+    // In -swift-version 5 mode, a conditional conformance to a protocol can imply
+    // a Sendable conformance. The implied conformance is unconditional so it uses
+    // the generic signature of the nominal type and not the generic signature of
+    // the extension that declared the (implying) conditional conformance.
     if (getSourceKind() == ConformanceEntryKind::Implied &&
         getProtocol()->isSpecificProtocol(KnownProtocolKind::Sendable)) {
       return getDeclContext()->getSelfNominalTypeDecl()->getGenericSignature();
     }
+
     return getDeclContext()->getGenericSignatureOfContext();
 
   case ProtocolConformanceKind::Builtin:
@@ -409,6 +415,10 @@ ConditionalRequirementsRequest::evaluate(Evaluator &evaluator,
     return {};
   }
 
+  // In -swift-version 5 mode, a conditional conformance to a protocol can imply
+  // a Sendable conformance. We ask the conformance for its generic signature,
+  // which will always be the generic signature of `ext` except in this case,
+  // where it's the generic signature of the extended nominal.
   const auto extensionSig = NPC->getGenericSignature();
 
   // The extension signature should be a superset of the type signature, meaning

lib/Sema/TypeCheckConcurrency.cpp

@@ -6040,6 +6040,10 @@ bool swift::checkSendableConformance(
     }
   }
 
+  // In -swift-version 5 mode, a conditional conformance to a protocol can imply
+  // a Sendable conformance. The implied conformance is unconditional, so check
+  // the storage for sendability as if the conformance was declared on the nominal,
+  // and not some (possibly constrained) extension.
   if (conformance->getSourceKind() == ConformanceEntryKind::Implied)
     conformanceDC = nominal;
   return checkSendableInstanceStorage(nominal, conformanceDC, check);

lib/Sema/TypeCheckProtocol.cpp

@@ -2416,7 +2416,9 @@ checkIndividualConformance(NormalProtocolConformance *conformance) {
 
   bool allowImpliedConditionalConformance = false;
   if (Proto->isSpecificProtocol(KnownProtocolKind::Sendable)) {
-    if (Context.LangOpts.StrictConcurrencyLevel != StrictConcurrency::Complete)
+    // In -swift-version 5 mode, a conditional conformance to a protocol can imply
+    // a Sendable conformance.
+    if (!Context.LangOpts.isSwiftVersionAtLeast(6))
       allowImpliedConditionalConformance = true;
   } else if (Proto->isMarkerProtocol()) {
     allowImpliedConditionalConformance = true;

test/Concurrency/implied_sendable_conformance_swift5.swift

@@ -1,5 +1,5 @@
-// RUN: %target-typecheck-verify-swift -swift-version 5
-// RUN: %target-swift-emit-silgen %s -swift-version 5
+// RUN: %target-typecheck-verify-swift -swift-version 5 -strict-concurrency=complete
+// RUN: %target-swift-emit-silgen %s -swift-version 5 -strict-concurrency=complete
 
 protocol P: Sendable {}
 protocol Q: Sendable {}