[CSSimplify] Modify logic on match representations to allow ONLY thick to thin and thick to thick representations on subtype context

Author: LucianoPAlmeida

lib/Sema/CSSimplify.cpp

@@ -1353,16 +1353,41 @@ ConstraintSystem::matchTupleTypes(TupleType *tuple1, TupleType *tuple2,
 // corresponding function type representations and the given match kind.
 static bool matchFunctionRepresentations(FunctionTypeRepresentation rep1,
                                          FunctionTypeRepresentation rep2,
-                                         ConstraintKind kind) {
+                                         ConstraintKind kind,
+                                         ConstraintLocator *locator) {
   switch (kind) {
   case ConstraintKind::Bind:
   case ConstraintKind::BindParam:
   case ConstraintKind::BindToPointerType:
   case ConstraintKind::Equal:
     return rep1 != rep2;
+      
+  case ConstraintKind::Subtype: {
+    if (!locator->isLastElement<LocatorPathElt::FunctionArgument>()) 
+      return false;
+    
+    auto isThin = [](FunctionTypeRepresentation rep) {
+      return rep == FunctionTypeRepresentation::CFunctionPointer ||
+          rep == FunctionTypeRepresentation::Thin;
+    };
+    
+    auto isThick = [](FunctionTypeRepresentation rep) {
+      return rep == FunctionTypeRepresentation::Swift ||
+          rep == swift::FunctionTypeRepresentation::Block;
+    };
+
+    // Allowing conventions "thin" (c, thin) to "thick" (swift, block)
+    if (isThin(rep1) && isThick(rep2))
+      return false;
+    
+    // Allowing conventions "thick" (swift, block) to "thick" (swift, block)
+    if (isThick(rep1) && isThick(rep2))
+      return false;
+    
+    return rep1 != rep2;
+  }
 
   case ConstraintKind::OpaqueUnderlyingType:
-  case ConstraintKind::Subtype:
   case ConstraintKind::Conversion:
   case ConstraintKind::BridgingConversion:
   case ConstraintKind::ArgumentConversion:
@@ -1657,7 +1682,7 @@ ConstraintSystem::matchFunctionTypes(FunctionType *func1, FunctionType *func2,
 
   if (matchFunctionRepresentations(func1->getExtInfo().getRepresentation(),
                                    func2->getExtInfo().getRepresentation(),
-                                   kind)) {
+                                   kind, getConstraintLocator(locator))) {
     return getTypeMatchFailure(locator);
   }