[stdlib] Reinstate original versions of metatype comparisons

Add a new language feature to avoid the stdlib’s swiftinterface becoming unintelligible to older compilers due to the generalization of Builtin.is_same_metatype.

Restore the original version of the `==` operator so that metatypes can continue to be compared even if the generalized version ends up getting omitted.

rdar://149396721

Author: lorentey

include/swift/Basic/Features.def

@@ -257,6 +257,7 @@ LANGUAGE_FEATURE(SendableCompletionHandlers, 463, "Objective-C completion handle
 LANGUAGE_FEATURE(AsyncExecutionBehaviorAttributes, 0, "@concurrent and nonisolated(nonsending)")
 LANGUAGE_FEATURE(IsolatedConformances, 407, "Global-actor isolated conformances")
 LANGUAGE_FEATURE(ValueGenericsNameLookup, 452, "Value generics appearing as static members for namelookup")
+LANGUAGE_FEATURE(GeneralizedIsSameMetaTypeBuiltin, 465, "Builtin.is_same_metatype with support for noncopyable/nonescapable types")
 
 // Swift 6
 UPCOMING_FEATURE(ConciseMagicFile, 274, 6)

stdlib/public/core/Builtin.swift

@@ -158,6 +158,7 @@ internal func != (lhs: Builtin.RawPointer, rhs: Builtin.RawPointer) -> Bool {
   return !(lhs == rhs)
 }
 
+#if compiler(>=5.3) && $GeneralizedIsSameMetaTypeBuiltin
 /// Returns a Boolean value indicating whether two types are identical.
 ///
 /// - Parameters:
@@ -196,13 +197,24 @@ public func != (
 ) -> Bool {
   !(t0 == t1)
 }
+#endif
 
 #if !$Embedded
 // Embedded Swift is unhappy about conversions from `Any.Type` to
 // `any (~Copyable & ~Escapable).Type` (rdar://145706221)
-@usableFromInline
-@_spi(SwiftStdlibLegacyABI) @available(swift, obsoleted: 1)
-internal func == (t0: Any.Type?, t1: Any.Type?) -> Bool {
+
+/// Returns a Boolean value indicating whether two types are identical.
+///
+/// - Parameters:
+///   - t0: A type to compare.
+///   - t1: Another type to compare.
+/// - Returns: `true` if both `t0` and `t1` are `nil` or if they represent the
+///   same type; otherwise, `false`.
+public func == (t0: Any.Type?, t1: Any.Type?) -> Bool {
+  // FIXME: This is a legacy ABI entry point that has been obsoleted by the
+  // generalized definition above. We need to mark it as SwiftStdlibLegacyABI
+  // once all supported compilers have the $GeneralizedIsSameMetaTypeBuiltin
+  // feature. (rdar://149396721)
   switch (t0, t1) {
   case (.none, .none): return true
   case let (.some(ty0), .some(ty1)):
@@ -211,9 +223,18 @@ internal func == (t0: Any.Type?, t1: Any.Type?) -> Bool {
   }
 }
 
-@usableFromInline
-@_spi(SwiftStdlibLegacyABI) @available(swift, obsoleted: 1)
-internal func != (t0: Any.Type?, t1: Any.Type?) -> Bool {
+/// Returns a Boolean value indicating whether two types are not identical.
+///
+/// - Parameters:
+///   - t0: A type to compare.
+///   - t1: Another type to compare.
+/// - Returns: `true` if one, but not both, of `t0` and `t1` are `nil`, or if
+///   they represent different types; otherwise, `false`.
+public func != (t0: Any.Type?, t1: Any.Type?) -> Bool {
+  // FIXME: This is a legacy ABI entry point that has been obsoleted by the
+  // generalized definition above. We need to mark it as SwiftStdlibLegacyABI
+  // once all supported compilers have the $GeneralizedIsSameMetaTypeBuiltin
+  // feature. (rdar://149396721)
   !(t0 == t1)
 }
 #endif