NCGenerics: omit inverses in stub diagnostic

fixes rdar://121077877

Author: kavon

include/swift/AST/Requirement.h

@@ -28,6 +28,7 @@
 
 namespace swift {
 class GenericContext;
+class GenericTypeParamType;
 
 /// Return type of Requirement::checkRequirement().
 enum class CheckRequirementResult : uint8_t {
@@ -264,6 +265,9 @@ struct InverseRequirement {
   static void expandDefaults(ASTContext &ctx,
                              ArrayRef<Type> gps,
                              SmallVectorImpl<StructuralRequirement> &result);
+  static void expandDefaults(ASTContext &ctx,
+                             ArrayRef<GenericTypeParamType *> gps,
+                             SmallVectorImpl<Requirement> &result);
 };
 
 } // end namespace swift

lib/AST/Requirement.cpp

@@ -366,3 +366,17 @@ void InverseRequirement::expandDefaults(
     }
   }
 }
+
+void InverseRequirement::expandDefaults(ASTContext &ctx,
+                           ArrayRef<GenericTypeParamType *> gps,
+                           SmallVectorImpl<Requirement> &result) {
+  SmallVector<Type, 4> gpsAsType;
+  for (auto *gp : gps)
+    gpsAsType.push_back(gp);
+
+  SmallVector<StructuralRequirement, 8> structReqs;
+  expandDefaults(ctx, gpsAsType, structReqs);
+
+  for (auto sreq : structReqs)
+    result.push_back(sreq.req);
+}

lib/Sema/TypeCheckProtocol.cpp

@@ -2495,10 +2495,15 @@ static Type getTypeForDisplay(ModuleDecl *module, ValueDecl *decl) {
 
   GenericSignature sigWithoutReqts;
   if (auto genericFn = type->getAs<GenericFunctionType>()) {
-    // For generic functions, build a new generic function... but strip off
-    // the requirements. They don't add value.
+    // For generic functions, build a new generic function... but without
+    // any explicit requirements. They don't add value. We need to provide the
+    // implicit requirements so that inverses like ~Copyable do not appear.
+    auto genericParams = genericFn->getGenericParams();
+    SmallVector<Requirement> implicitReqs;
+    InverseRequirement::expandDefaults(genericFn->getASTContext(),
+                                       genericParams, implicitReqs);
     sigWithoutReqts
-      = GenericSignature::get(genericFn->getGenericParams(), {});
+      = GenericSignature::get(genericFn->getGenericParams(), implicitReqs);
   }
 
   // For functions, strip off the 'Self' parameter clause.

test/Generics/inverse_misc.swift

@@ -0,0 +1,14 @@
+// RUN: %target-typecheck-verify-swift \
+// RUN:   -parse-stdlib -module-name Swift \
+// RUN:   -enable-experimental-feature NoncopyableGenerics
+
+// NOTE: -parse-stdlib is a transitional workaround and should not be required.
+
+// rdar://121077877 (wrong generic signature in protocol stub diagnostic)
+protocol Subscriber {}
+protocol P {
+  func receive<T>(subscriber: T) where T : Subscriber
+  // expected-note@-1 {{protocol requires function 'receive(subscriber:)' with type '<T> (subscriber: T) -> ()'; add a stub for conformance}}
+}
+struct S: P { // expected-error {{type 'S' does not conform to protocol 'P'}}
+}