[Features] Add a dedicated experimental feature flag for tuple conformances.

Author: hborla

include/swift/AST/DiagnosticsCommon.def

@@ -228,6 +228,13 @@ ERROR(macro_experimental,none,
 ERROR(ambiguous_macro_reference,none,
       "ambiguous reference to macro %0", (DeclName))
 
+//------------------------------------------------------------------------------
+// MARK: tuple conformances
+//------------------------------------------------------------------------------
+ERROR(experimental_tuple_extension,none,
+      "tuple extensions are experimental",
+      ())
+
 //------------------------------------------------------------------------------
 // MARK: bridged diagnostics
 //------------------------------------------------------------------------------

include/swift/Basic/Features.def

@@ -114,6 +114,7 @@ EXPERIMENTAL_FEATURE(NamedOpaqueTypes, false)
 EXPERIMENTAL_FEATURE(FlowSensitiveConcurrencyCaptures, false)
 EXPERIMENTAL_FEATURE(FreestandingMacros, true)
 EXPERIMENTAL_FEATURE(CodeItemMacros, true)
+EXPERIMENTAL_FEATURE(TupleConformances, false)
 
 // FIXME: MoveOnlyClasses is not intended to be in production,
 // but our tests currently rely on it, and we want to run those

lib/AST/ASTPrinter.cpp

@@ -3208,6 +3208,10 @@ static bool usesFeatureVariadicGenerics(Decl *decl) {
   return false;
 }
 
+static bool usesFeatureTupleConformances(Decl *decl) {
+  return false;
+}
+
 static bool usesFeatureLayoutPrespecialization(Decl *decl) {
   auto &attrs = decl->getAttrs();
   return std::any_of(attrs.begin(), attrs.end(), [](auto *attr) {

lib/AST/NameLookup.cpp

@@ -2981,7 +2981,16 @@ ExtendedNominalRequest::evaluate(Evaluator &evaluator,
 
   // If there is more than 1 element, we will emit a warning or an error
   // elsewhere, so don't handle that case here.
-  return nominalTypes.empty() ? nullptr : nominalTypes[0];
+  if (nominalTypes.empty())
+    return nullptr;
+
+  // Diagnose experimental tuple extensions.
+  if (isa<BuiltinTupleDecl>(nominalTypes[0]) &&
+      !ctx.LangOpts.hasFeature(Feature::TupleConformances)) {
+    ext->diagnose(diag::experimental_tuple_extension);
+  }
+
+  return nominalTypes[0];
 }
 
 /// Whether there are only associated types in the set of declarations.

test/Generics/tuple-conformances.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift -enable-experimental-feature VariadicGenerics -parse-stdlib
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature TupleConformances -parse-stdlib
 
 // REQUIRES: asserts