Introduce feature ForwardTrailingClosures for SE-0286.

Replace the "Swift version 6" checks for forward-scanning trailing
closure matching with checks for this new feature.

Author: DougGregor

include/swift/Basic/Features.def

@@ -90,6 +90,7 @@ SUPPRESSIBLE_LANGUAGE_FEATURE(UnavailableFromAsync, 0, "@_unavailableFromAsync",
 SUPPRESSIBLE_LANGUAGE_FEATURE(NoAsyncAvailability, 340, "@available(*, noasync)", true)
 
 FUTURE_FEATURE(ConciseMagicFile, 274, 6)
+FUTURE_FEATURE(ForwardTrailingClosures, 286, 6)
 
 EXPERIMENTAL_FEATURE(StaticAssert)
 EXPERIMENTAL_FEATURE(VariadicGenerics)

lib/AST/ASTPrinter.cpp

@@ -3027,6 +3027,10 @@ static bool usesFeatureTypeWitnessSystemInference(Decl *decl) {
   return false;
 }
 
+static bool usesFeatureForwardTrailingClosures(Decl *decl) {
+  return false;
+}
+
 static void
 suppressingFeatureNoAsyncAvailability(PrintOptions &options,
                                       llvm::function_ref<void()> action) {

lib/Sema/CSSimplify.cpp

@@ -472,7 +472,7 @@ static bool matchCallArgumentsImpl(
       // way to successfully match arguments to parameters.
       if (!parameterRequiresArgument(params, paramInfo, paramIdx) &&
           !param.getPlainType()->getASTContext().LangOpts
-              .isSwiftVersionAtLeast(6) &&
+              .hasFeature(Feature::ForwardTrailingClosures) &&
           anyParameterRequiresArgument(
               params, paramInfo, paramIdx + 1,
               nextArgIdx + 1 < numArgs
@@ -934,7 +934,7 @@ static bool requiresBothTrailingClosureDirections(
 
   // If backward matching is disabled, only scan forward.
   ASTContext &ctx = params.front().getPlainType()->getASTContext();
-  if (ctx.LangOpts.isSwiftVersionAtLeast(6))
+  if (ctx.LangOpts.hasFeature(Feature::ForwardTrailingClosures))
     return false;
 
   // If there are at least two parameters that meet the backward scan's