Add experimental feature for bound generic extensions

Author: DougGregor

include/swift/Basic/Features.def

@@ -101,6 +101,13 @@ EXPERIMENTAL_FEATURE(MoveOnly)
 EXPERIMENTAL_FEATURE(OneWayClosureParameters)
 EXPERIMENTAL_FEATURE(TypeWitnessSystemInference)
 
+/// Enable extensions of (sugared) bound generic types
+///
+/// \code
+/// extension [Int] { /**/ }
+/// \endcode
+EXPERIMENTAL_FEATURE(BoundGenericExtensions)
+
 #undef EXPERIMENTAL_FEATURE
 #undef FUTURE_FEATURE
 #undef SUPPRESSIBLE_LANGUAGE_FEATURE

include/swift/Basic/LangOptions.h

@@ -462,13 +462,6 @@ namespace swift {
     // FrontendOptions.
     bool AllowModuleWithCompilerErrors = false;
 
-    /// Enable extensions of (sugared) bound generic types
-    ///
-    /// \code
-    /// extension [Int] { /**/ }
-    /// \endcode
-    bool EnableExperimentalBoundGenericExtensions = false;
-
     /// A helper enum to represent whether or not we customized the default
     /// ASTVerifier behavior via a frontend flag. By default, we do not
     /// customize.

lib/AST/ASTPrinter.cpp

@@ -3035,6 +3035,10 @@ static bool usesFeatureTypeWitnessSystemInference(Decl *decl) {
   return false;
 }
 
+static bool usesFeatureBoundGenericExtensions(Decl *decl) {
+  return false;
+}
+
 static void
 suppressingFeatureNoAsyncAvailability(PrintOptions &options,
                                       llvm::function_ref<void()> action) {

lib/Frontend/CompilerInvocation.cpp

@@ -486,9 +486,6 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
       = A->getOption().matches(OPT_enable_deserialization_recovery);
   }
 
-  Opts.EnableExperimentalBoundGenericExtensions |=
-    Args.hasArg(OPT_enable_experimental_bound_generic_extensions);
-
   Opts.DisableAvailabilityChecking |=
       Args.hasArg(OPT_disable_availability_checking);
   Opts.CheckAPIAvailabilityOnly |=
@@ -666,6 +663,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     Opts.Features.insert(Feature::OneWayClosureParameters);
   if (Args.hasArg(OPT_enable_experimental_associated_type_inference))
     Opts.Features.insert(Feature::TypeWitnessSystemInference);
+  if (Args.hasArg(OPT_enable_experimental_bound_generic_extensions))
+    Opts.Features.insert(Feature::BoundGenericExtensions);
 
   Opts.EnableAppExtensionRestrictions |= Args.hasArg(OPT_enable_app_extension);
 

lib/Sema/TypeCheckDecl.cpp

@@ -2848,7 +2848,8 @@ ExtendedTypeRequest::evaluate(Evaluator &eval, ExtensionDecl *ext) const {
 
   // By default, the user cannot extend a bound generic type, unless it's
   // referenced via a non-generic typealias type.
-  if (!ext->getASTContext().LangOpts.EnableExperimentalBoundGenericExtensions &&
+  if (!ext->getASTContext().LangOpts.hasFeature(
+          Feature::BoundGenericExtensions) &&
       extendedType->isSpecialized() &&
       !isNonGenericTypeAliasType(extendedType)) {
     diags.diagnose(ext->getLoc(), diag::extension_specialization,