Sema: Re-introduce the hack for re-using generic signature of extende…

…d protocol

In Swift 5.10 if you wrote `extension Foo {}` for some protocol Foo,
the extension would always re-use the generic signature of Foo, which
is <Self where Self: Foo>. In Swift 6 this no longer works because Foo
might be ~Copyable, in which case `extension Foo {}` adds default
requirements, so we changed GenericSignatureRequest to just always
build a new signature if we're given an extension.

However, to avoid a request cycle with a code example that really should
have never worked at all, I'm re-introducing the hack for re-using the
signature.

Fixes rdar://problem/129540617.

lib/Sema/TypeCheckGeneric.cpp

@@ -784,6 +784,29 @@ GenericSignatureRequest::evaluate(Evaluator &evaluator,
 
     auto *extendedNominal = ext->getExtendedNominal();
 
+    collectAdditionalExtensionRequirements(ext->getExtendedType(), extraReqs);
+
+    // Avoid building a generic signature if we have an unconstrained protocol
+    // extension of a protocol that does not suppress conformance to ~Copyable
+    // or ~Escapable. This is just here to allow this silly edge case which
+    // should have been banned:
+    //
+    // protocol P where A == B { associatedtype A }
+    // extension P { typealias B = Int }
+    if (auto *proto = dyn_cast<ProtocolDecl>(extendedNominal)) {
+      if (extraReqs.empty() &&
+          !ext->getTrailingWhereClause()) {
+        InvertibleProtocolSet protos;
+        for (auto *inherited : proto->getAllInheritedProtocols()) {
+          if (auto kind = inherited->getInvertibleProtocolKind())
+            protos.insert(*kind);
+        }
+
+        if (protos == InvertibleProtocolSet::allKnown())
+          return extendedNominal->getGenericSignatureOfContext();
+      }
+    }
+
     if (isa<BuiltinTupleDecl>(extendedNominal)) {
       genericParams = ext->getGenericParams();
     } else {
@@ -798,8 +821,6 @@ GenericSignatureRequest::evaluate(Evaluator &evaluator,
     if (ext->hasInvertibleLocalConformance())
       inferInvertibleReqs = false;
 
-    collectAdditionalExtensionRequirements(ext->getExtendedType(), extraReqs);
-
   } else {
     llvm_unreachable("Unknown generic declaration kind");
   }