Add upcoming feature ImplicitOpenExistentials for SE-0352

To maintain source compatibility, SE-0352 does not open existentials
with "self-conforming" type, such as `any Error` or existentials based
on `@objc` protocols. The proposal specified that this behavior would
change in Swift 6. Implement that behavior change, which can be
enabled prior to Swift 6 with the upcoming feature
`ImplicitOpenExistentials` (as documented in SE-0362).

Fixes #70873 / rdar://120902975.

CHANGELOG.md

@@ -4,6 +4,22 @@
 > This is in reverse chronological order, so newer entries are added to the top.
 
 ## Swift 6.0
+* [SE-0352][]:
+  The Swift 6 language mode will open existential values with
+  "self-conforming" types (such as `any Error` or `@objc` protocols)
+  passed to generic functions. For example:
+
+  ```swift
+  func takeError<E: Error>(_ error: E) { }
+
+  func passError(error: any Error) {
+    takeError(error)  // Swift 5 does not open `any Error`, Swift 6 does
+  }
+  ```
+
+  This behavior can be enabled prior to the Swift 6 language mode
+  using the upcoming language feature `ImplicitOpenExistentials`.
+
 * [SE-0422][]:
   Non-built-in expression macros can now be used as default arguments that
   expand at each call site. For example, a custom `#CurrentFile` macro used as

include/swift/Basic/Features.def

@@ -129,6 +129,7 @@ SUPPRESSIBLE_LANGUAGE_FEATURE(Extern, 0, "@_extern")
 LANGUAGE_FEATURE(ExpressionMacroDefaultArguments, 422, "Expression macro as caller-side default argument")
 LANGUAGE_FEATURE(BuiltinStoreRaw, 0, "Builtin.storeRaw")
 
+// Swift 6
 UPCOMING_FEATURE(ConciseMagicFile, 274, 6)
 UPCOMING_FEATURE(ForwardTrailingClosures, 286, 6)
 UPCOMING_FEATURE(StrictConcurrency, 0337, 6)
@@ -140,9 +141,11 @@ UPCOMING_FEATURE(InternalImportsByDefault, 409, 6)
 UPCOMING_FEATURE(IsolatedDefaultValues, 411, 6)
 UPCOMING_FEATURE(GlobalConcurrency, 412, 6)
 UPCOMING_FEATURE(FullTypedThrows, 413, 6)
-UPCOMING_FEATURE(ExistentialAny, 335, 7)
 UPCOMING_FEATURE(InferSendableFromCaptures, 418, 6)
+UPCOMING_FEATURE(ImplicitOpenExistentials, 352, 6)
 
+// Swift 7
+UPCOMING_FEATURE(ExistentialAny, 335, 7)
 
 EXPERIMENTAL_FEATURE(StaticAssert, false)
 EXPERIMENTAL_FEATURE(NamedOpaqueTypes, false)

lib/AST/FeatureSet.cpp

@@ -439,6 +439,7 @@ UNINTERESTING_FEATURE(GlobalConcurrency)
 UNINTERESTING_FEATURE(FullTypedThrows)
 UNINTERESTING_FEATURE(ExistentialAny)
 UNINTERESTING_FEATURE(InferSendableFromCaptures)
+UNINTERESTING_FEATURE(ImplicitOpenExistentials)
 
 // ----------------------------------------------------------------------------
 // MARK: - Experimental Features

lib/ASTGen/Sources/ASTGen/SourceManager.swift

@@ -37,6 +37,7 @@ class SourceManager {
 }
 
 /// MARK: Source file management
+
 extension SourceManager {
   /// Inserts a new source file into the source manager.
   ///

lib/Sema/CSSimplify.cpp

@@ -1578,10 +1578,13 @@ shouldOpenExistentialCallArgument(ValueDecl *callee, unsigned paramIdx,
     return std::nullopt;
 
   // If the existential argument conforms to all of protocol requirements on
-  // the formal parameter's type, don't open.
+  // the formal parameter's type, don't open unless ImplicitOpenExistentials is
+  // enabled.
+
   // If all of the conformance requirements on the formal parameter's type
   // are self-conforming, don't open.
-  {
+  ASTContext &ctx = argTy->getASTContext();
+  if (!ctx.LangOpts.hasFeature(Feature::ImplicitOpenExistentials)) {
     Type existentialObjectType;
     if (auto existentialMetaTy = argTy->getAs<ExistentialMetatypeType>())
       existentialObjectType = existentialMetaTy->getInstanceType();

test/Constraints/opened_existentials_feature.swift

@@ -0,0 +1,20 @@
+// RUN: %target-typecheck-verify-swift -enable-upcoming-feature ImplicitOpenExistentials
+// RUN: %target-typecheck-verify-swift -swift-version 6
+
+#if _runtime(_ObjC)
+@objc
+protocol X {}
+
+func foo<T: X>(_ val: T.Type) {}
+
+func bar(_ val: X.Type) {
+  // Only succeeds when we're allowed to open an @objc existential.
+  foo(val)
+}
+#endif
+
+func takeError<E: Error>(_ error: E) { }
+
+func passError(error: any Error) {
+  takeError(error)  // okay
+}