Soften @impl warnings only for early adopters

Adopting @implementation turns all of the warnings into errors.

Author: beccadax

docs/ReferenceGuides/UnderscoredAttributes.md

@@ -779,6 +779,11 @@ of a header as non-`Sendable` so that you can make spot exceptions with
 
 ## `@_objcImplementation(CategoryName)`
 
+A pre-stable form of `@implementation`. The main difference between them is that
+many things that are errors with `@implementation` are warnings with
+`@_objcImplementation`, which permitted workarounds for compiler bugs and 
+changes in compiler behavior.
+
 Declares an extension that defines an implementation for the Objective-C
 category `CategoryName` on the class in question, or for the main `@interface`
 if the argument list is omitted.
@@ -841,12 +846,6 @@ Notes:
 
 * We don't currently plan to support ObjC generics.
 
-* Eventually, we want the main `@_objcImplementation` extension to be able to
-  declare stored properties that aren't in the interface. We also want
-  `final` stored properties to be allowed to be resilent Swift types, but
-  it's not clear how to achieve that without boxing them in `__SwiftValue`
-  (which we might do as a stopgap).
-     
 * We should think about ObjC "direct" members, but that would probably
   require a way to spell this in Swift. 
 

lib/Sema/TypeCheckDeclObjC.cpp

@@ -2810,9 +2810,11 @@ class ObjCImplementationChecker {
                         typename detail::PassArgument<ArgTypes>::type... Args) {
     auto diag = diags.diagnose(loc, diagID, std::forward<ArgTypes>(Args)...);
 
-    // WORKAROUND (5.9): Soften newly-introduced errors to make things easier
-    // for early adopters.
-    if (diags.declaredDiagnosticKindFor(diagID.ID) == DiagnosticKind::Error)
+    // Early adopters using the '@_objcImplementation' syntax may have had the
+    // ObjCImplementationChecker evolve out from under them. Soften their errors
+    // to warnings so we don't break their projects.
+    if (isEarlyAdopter
+         && diags.declaredDiagnosticKindFor(diagID.ID) == DiagnosticKind::Error)
       diag.wrapIn(diag::wrap_objc_implementation_will_become_error);
 
     return diag;
@@ -2823,12 +2825,18 @@ class ObjCImplementationChecker {
   /// Candidates with their explicit ObjC names, if any.
   llvm::SmallDenseMap<ValueDecl *, ObjCSelector, 16> unmatchedCandidates;
 
+  bool isEarlyAdopter;
+
 public:
   ObjCImplementationChecker(Decl *D)
       : diags(D->getASTContext().Diags)
   {
     assert(!D->hasClangNode() && "passed interface, not impl, to checker");
 
+    isEarlyAdopter = D->getAttrs()
+        .getAttribute<ObjCImplementationAttr>(/*AllowInvalid=*/true)
+        ->isEarlyAdopter();
+
     if (auto func = dyn_cast<AbstractFunctionDecl>(D)) {
       addCandidate(D);
       addRequirement(D->getImplementedObjCDecl());

test/decl/ext/cdecl_implementation_features.swift

@@ -1,11 +1,13 @@
 // RUN: not %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource) -typecheck %s -import-objc-header %S/Inputs/objc_implementation.h 2>&1 | %FileCheck --check-prefixes NO,CHECK %s
 // RUN: not %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource) -typecheck %s -import-objc-header %S/Inputs/objc_implementation.h -enable-experimental-feature CImplementation 2>&1 | %FileCheck --check-prefixes YES,C,CHECK %s
 
+// YES-DAG: cdecl_implementation_features.swift:[[@LINE+4]]:{{[0-9]+}}: warning: global function 'CImplFunc1' of type '(Double) -> ()' does not match type '(Int32) -> Void' declared by the header; this will become an error after adopting '@implementation'
 // NO-DAG: cdecl_implementation_features.swift:[[@LINE+3]]:{{[0-9]+}}: error: '_objcImplementation' attribute is only valid when experimental feature CImplementation is enabled
 // YES-NOT: cdecl_implementation_features.swift:[[@LINE+2]]:{{[0-9]+}}: warning: warning-only '@_objcImplementation' spelling is deprecated; use '@implementation' instead
 // TODO: When @implementation @_cdecl stabilizes, YES-NOT on the line above will become YES-DAG
 @_objcImplementation @_cdecl("CImplFunc1") func CImplFunc1(_: Double) {}
 
+// YES-DAG: cdecl_implementation_features.swift:[[@LINE+3]]:{{[0-9]+}}: error: global function 'CImplFunc2' of type '(Double) -> ()' does not match type '(Int32) -> Void' declared by the header{{$}}
 // NO-DAG: cdecl_implementation_features.swift:[[@LINE+2]]:{{[0-9]+}}: error: 'implementation' attribute is only valid when experimental feature CImplementation is enabled
 // YES-NOT: cdecl_implementation_features.swift:[[@LINE+1]]:{{[0-9]+}}: error: 'implementation' attribute is only valid when experimental feature CImplementation is enabled
 @implementation @_cdecl("CImplFunc2") func CImplFunc2(_: Double) {}

test/decl/ext/objc_implementation_features.swift

@@ -3,10 +3,12 @@
 // RUN: not %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource) -typecheck %s -import-objc-header %S/Inputs/objc_implementation.h 2>&1 | %FileCheck --check-prefixes NO,CHECK %s
 // RUN: not %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource) -typecheck %s -import-objc-header %S/Inputs/objc_implementation.h -enable-experimental-feature ObjCImplementation 2>&1 | %FileCheck --check-prefixes YES,CHECK %s
 
+// CHECK-DAG: objc_implementation_features.swift:[[@LINE+3]]:{{[0-9]+}}: warning: extension for main class interface should provide implementation for instance method 'subclassMethod(fromHeader1:)'; this will become an error after adopting '@implementation'
 // NO-NOT: objc_implementation_features.swift:[[@LINE+2]]:{{[0-9]+}}: warning: warning-only '@_objcImplementation' spelling is deprecated; use '@implementation' instead
 // YES-DAG: objc_implementation_features.swift:[[@LINE+1]]:{{[0-9]+}}: warning: warning-only '@_objcImplementation' spelling is deprecated; use '@implementation' instead
 @_objcImplementation extension ObjCSubclass {}
 
+// CHECK-DAG: objc_implementation_features.swift:[[@LINE+3]]:{{[0-9]+}}: error: extension for main class interface should provide implementation for initializer 'init()'{{$}}
 // NO-DAG: objc_implementation_features.swift:[[@LINE+2]]:{{[0-9]+}}: error: 'implementation' attribute is only valid when experimental feature ObjCImplementation is enabled
 // YES-NOT: objc_implementation_features.swift:[[@LINE+1]]:{{[0-9]+}}: error: 'implementation' attribute is only valid when experimental feature ObjCImplementation is enabled
 @implementation extension ObjCBasicInitClass {}