Put Variadic Generics Behind a Flag

include/swift/Basic/LangOptions.h

@@ -352,6 +352,9 @@ namespace swift {
     /// Enable experimental pairwise `buildBlock` for result builders.
     bool EnableExperimentalPairwiseBuildBlock = false;
 
+    /// Enable variadic generics.
+    bool EnableExperimentalVariadicGenerics = false;
+
     /// Disable the implicit import of the _Concurrency module.
     bool DisableImplicitConcurrencyModuleImport =
         !SWIFT_IMPLICIT_CONCURRENCY_IMPORT;

include/swift/Option/FrontendOptions.td

@@ -546,6 +546,10 @@ def enable_experimental_string_processing :
   Flag<["-"], "enable-experimental-string-processing">,
   HelpText<"Enable experimental string processing">;
 
+def enable_experimental_variadic_generics :
+  Flag<["-"], "enable-experimental-variadic-generics">,
+  HelpText<"Enable experimental support for variadic generic types">;
+
 def disable_availability_checking : Flag<["-"],
   "disable-availability-checking">,
   HelpText<"Disable checking for potentially unavailable APIs">;

lib/Frontend/CompilerInvocation.cpp

@@ -456,6 +456,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
                  OPT_disable_experimental_opened_existential_types,
                  false);
 
+  Opts.EnableExperimentalVariadicGenerics |=
+    Args.hasArg(OPT_enable_experimental_variadic_generics);
+
   // SwiftOnoneSupport produces different symbols when opening existentials,
   // so disable it.
   if (FrontendOpts.ModuleName == SWIFT_ONONE_SUPPORT)

lib/Parse/ParseDecl.cpp

@@ -2802,8 +2802,12 @@ bool Parser::parseNewDeclAttribute(DeclAttributes &Attributes, SourceLoc AtLoc,
     break;
   }
   case DAK_TypeSequence: {
-    auto range = SourceRange(Loc, Tok.getRange().getStart());
-    Attributes.add(TypeSequenceAttr::create(Context, AtLoc, range));
+    if (Context.LangOpts.EnableExperimentalVariadicGenerics) {
+      auto range = SourceRange(Loc, Tok.getRange().getStart());
+      Attributes.add(TypeSequenceAttr::create(Context, AtLoc, range));
+    } else {
+      DiscardAttribute = true;
+    }
     break;
   }
 

lib/Sema/CSApply.cpp

@@ -5500,7 +5500,8 @@ ArgumentList *ExprRewriter::coerceCallArguments(
     auto paramLabel = param.getLabel();
 
     // Handle variadic generic parameters.
-    if (paramInfo.isVariadicGenericParameter(paramIdx)) {
+    if (ctx.LangOpts.EnableExperimentalVariadicGenerics &&
+        paramInfo.isVariadicGenericParameter(paramIdx)) {
       assert(param.isVariadic());
       assert(!param.isInOut());
 

lib/Sema/CSSimplify.cpp

@@ -1614,7 +1614,8 @@ static ConstraintSystem::TypeMatchResult matchCallArguments(
     // We pull these out special because variadic parameters ban lots of
     // the more interesting typing constructs called out below like
     // inout and @autoclosure.
-    if (paramInfo.isVariadicGenericParameter(paramIdx)) {
+    if (cs.getASTContext().LangOpts.EnableExperimentalVariadicGenerics &&
+        paramInfo.isVariadicGenericParameter(paramIdx)) {
       auto *PET = paramTy->castTo<PackExpansionType>();
       OpenTypeSequenceElements openTypeSequence{cs, PET};
       for (auto argIdx : parameterBindings[paramIdx]) {

lib/Sema/TypeCheckType.cpp

@@ -3772,7 +3772,8 @@ NeverNullType TypeResolver::resolveTupleType(TupleTypeRepr *repr,
 
   bool complained = false;
   if (repr->hasEllipsis()) {
-    if (repr->getNumElements() == 1 && !repr->hasElementNames()) {
+    if (getASTContext().LangOpts.EnableExperimentalVariadicGenerics &&
+        repr->getNumElements() == 1 && !repr->hasElementNames()) {
       // This is probably a pack expansion. Try to resolve the pattern type.
       auto patternTy = resolveType(repr->getElementType(0), elementOptions);
       if (patternTy->hasError())

test/Constraints/type_sequence.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift
+// RUN: %target-typecheck-verify-swift -enable-experimental-variadic-generics
 
 struct TupleStruct<First, @_typeSequence Rest> {
   var first: First

test/decl/func/vararg.swift

@@ -1,7 +1,7 @@
 // RUN: %target-typecheck-verify-swift
 
-var t1a: (Int...) = (1) // expected-error{{cannot create expansion with non-variadic type 'Int'}}
-// expected-error@-1 {{cannot convert value of type 'Int' to specified type '(Int...)'}}
+var t1a: (Int...) = (1)
+// expected-error@-1 {{cannot create a variadic tuple}}
 var t2d: (Double = 0.0) = 1 // expected-error {{default argument not permitted in a tuple type}} {{18-23=}}
 
 func f1(_ a: Int...) { for _ in a {} }