Sema: Relax check around implied conditional conformance

Fixes rdar://123933687.

Author: slavapestov

lib/Sema/TypeCheckProtocol.cpp

@@ -2424,8 +2424,16 @@ checkIndividualConformance(NormalProtocolConformance *conformance) {
       implyingConf = implyingConf->getImplyingConformance();
     }
 
+    // If the conditional requirements all have the form `T : Copyable`, then
+    // we accept the implied conformance with the same conditional requirements.
     auto implyingCondReqs = implyingConf->getConditionalRequirements();
-    if (!implyingCondReqs.empty()) {
+    bool allCondReqsInvertible = llvm::all_of(implyingCondReqs,
+        [&](Requirement req) {
+          return (req.getKind() == RequirementKind::Conformance &&
+                  req.getProtocolDecl()->getInvertibleProtocolKind());
+        });
+
+    if (!allCondReqsInvertible) {
       // FIXME:
       // We shouldn't suggest including witnesses for the conformance, because
       // those suggestions will go in the current DeclContext, but really they

test/Sema/conditionally_copyable.swift

@@ -0,0 +1,22 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature NoncopyableGenerics
+
+struct G<T: ~Copyable>: ~Copyable {}
+
+extension G: Copyable {}
+
+protocol Base {}
+protocol Derived: Base {}
+
+// Normally we would require the conditional conformance 'G: Base' to be
+// explicitly declared, but that would break source compatibility if G
+// used to be unconditionally Copyable.
+
+extension G: Derived {} // expected-note {{requirement from conditional conformance of 'G<NotCopyable>' to 'Base'}}
+
+struct NotCopyable: ~Copyable {}
+struct IsCopyable {}
+
+func f<T: Base>(_: T.Type) {}
+
+f(G<NotCopyable>.self)  // expected-error {{global function 'f' requires that 'NotCopyable' conform to 'Copyable'}}
+f(G<IsCopyable>.self)  // accepted