[CSGen] Suppress favoring in presence of non-disfavored variadic generic overloads

Since this type of early favoring checks number of arguments and matches
labels it would always favor non-variadic overloads which is incorrect.

(cherry picked from commit 59908ee)

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -6169,8 +6169,13 @@ Type isPlaceholderVar(PatternBindingDecl *PB);
 /// Dump an anchor node for a constraint locator or contextual type.
 void dumpAnchor(ASTNode anchor, SourceManager *SM, raw_ostream &out);
 
+bool isPackExpansionType(Type type);
+
 bool isSingleUnlabeledPackExpansionTuple(Type type);
 
+bool containsPackExpansionType(ArrayRef<AnyFunctionType::Param> params);
+bool containsPackExpansionType(TupleType *tuple);
+
 /// \returns null if \c type is not a single unlabeled pack expansion tuple.
 Type getPatternTypeOfSingleUnlabeledPackExpansionTuple(Type type);
 

lib/Sema/CSGen.cpp

@@ -592,7 +592,25 @@ namespace {
     // Find the argument type.
     size_t nArgs = expr->getArgs()->size();
     auto fnExpr = expr->getFn();
-    
+
+    auto mustConsiderVariadicGenericOverloads = [&](ValueDecl *overload) {
+      if (overload->getAttrs().hasAttribute<DisfavoredOverloadAttr>())
+        return false;
+
+      auto genericContext = overload->getAsGenericContext();
+      if (!genericContext)
+        return false;
+
+      auto *GPL = genericContext->getGenericParams();
+      if (!GPL)
+        return false;
+
+      return llvm::any_of(GPL->getParams(),
+                          [&](const GenericTypeParamDecl *GP) {
+                            return GP->isParameterPack();
+                          });
+    };
+
     // Check to ensure that we have an OverloadedDeclRef, and that we're not
     // favoring multiple overload constraints. (Otherwise, in this case
     // favoring is useless.
@@ -630,8 +648,9 @@ namespace {
         return nArgs == paramCount.first ||
                nArgs == paramCount.second;
       };
-      
-      favorCallOverloads(expr, CS, isFavoredDecl);
+
+      favorCallOverloads(expr, CS, isFavoredDecl,
+                         mustConsiderVariadicGenericOverloads);
     }
 
     // We only currently perform favoring for unary args.
@@ -655,7 +674,8 @@ namespace {
       // inside an extension context, since any archetypes in the parameter
       // list could match exactly.
       auto mustConsider = [&](ValueDecl *value) -> bool {
-        return isa<ProtocolDecl>(value->getDeclContext());
+        return isa<ProtocolDecl>(value->getDeclContext()) ||
+               mustConsiderVariadicGenericOverloads(value);
       };
 
       favorCallOverloads(expr, CS, isFavoredDecl, mustConsider);

lib/Sema/CSSimplify.cpp

@@ -117,7 +117,7 @@ static llvm::Optional<unsigned> scoreParamAndArgNameTypo(StringRef paramName,
   return dist;
 }
 
-static bool isPackExpansionType(Type type) {
+bool constraints::isPackExpansionType(Type type) {
   if (type->is<PackExpansionType>())
     return true;
 
@@ -143,13 +143,13 @@ Type constraints::getPatternTypeOfSingleUnlabeledPackExpansionTuple(Type type) {
   return expansion->getPatternType();
 }
 
-static bool containsPackExpansionType(ArrayRef<AnyFunctionType::Param> params) {
+bool constraints::containsPackExpansionType(ArrayRef<AnyFunctionType::Param> params) {
   return llvm::any_of(params, [&](const auto &param) {
     return isPackExpansionType(param.getPlainType());
   });
 }
 
-static bool containsPackExpansionType(TupleType *tuple) {
+bool constraints::containsPackExpansionType(TupleType *tuple) {
   return llvm::any_of(tuple->getElements(), [&](const auto &elt) {
     return isPackExpansionType(elt.getType());
   });