[SE-0280] Enum cases as protocol witnesses (swiftlang#28916)

* [Typechecker] Allow enum cases without payload to witness a static get-only property with Self type protocol requirement

* [SIL] Add support for payload cases as well

* [SILGen] Clean up comment

* [Typechecker] Re-enable some previously disabled witness matching code

Also properly handle the matching in some cases

* [Test] Update typechecker tests with payload enum test cases

* [Test] Update SILGen test

* [SIL] Add two FIXME's to address soon

* [SIL] Emit the enum case constructor unconditionally when an enum case is used as a witness

Also, tweak SILDeclRef::getLinkage to update the 'limit' to 'OnDemand' if we have an enum declaration

* [SILGen] Properly handle a enum witness in addMethodImplementation

Also remove a FIXME and code added to workaround the original bug

* [TBDGen] Handle enum case witness

* [Typechecker] Fix conflicts

* [Test] Fix tests

* [AST] Fix indentation in diagnostics def file

Author: theblixguy

include/swift/AST/DiagnosticsSema.def

@@ -1889,8 +1889,9 @@ ERROR(requirement_restricts_self,none,
       "'Self'",
       (DescriptiveDeclKind, DeclName, StringRef, unsigned, StringRef))
 ERROR(witness_argument_name_mismatch,none,
-      "%select{method|initializer}0 %1 has different argument labels from those "
-      "required by protocol %2 (%3)", (bool, DeclName, Type, DeclName))
+      "%0 %1 has different argument labels "
+      "from those required by protocol %2 (%3)",
+      (DescriptiveDeclKind, DeclName, Type, DeclName))
 ERROR(witness_initializer_not_required,none,
       "initializer requirement %0 can only be satisfied by a 'required' "
       "initializer in%select{| the definition of}1 non-final class %2",
@@ -2116,6 +2117,9 @@ NOTE(protocol_witness_throws_conflict,none,
      "candidate throws, but protocol does not allow it", ())
 NOTE(protocol_witness_not_objc,none,
      "candidate is explicitly '@nonobjc'", ())
+NOTE(protocol_witness_enum_case_payload, none,
+     "candidate is an enum case with associated values, "
+     "but protocol does not allow it", ())
 
 NOTE(protocol_witness_type,none,
      "possibly intended match", ())

include/swift/SILOptimizer/Utils/InstOptUtils.h

@@ -499,6 +499,10 @@ bool calleesAreStaticallyKnowable(SILModule &module, SILDeclRef decl);
 /// be reached by calling the function represented by Decl?
 bool calleesAreStaticallyKnowable(SILModule &module, AbstractFunctionDecl *afd);
 
+/// Do we have enough information to determine all callees that could
+/// be reached by calling the function represented by Decl?
+bool calleesAreStaticallyKnowable(SILModule &module, EnumElementDecl *eed);
+
 // Attempt to get the instance for , whose static type is the same as
 // its exact dynamic type, returning a null SILValue() if we cannot find it.
 // The information that a static type is the same as the exact dynamic,

lib/SIL/SILDeclRef.cpp

@@ -334,7 +334,7 @@ SILLinkage SILDeclRef::getLinkage(ForDefinition_t forDefinition) const {
       limit = Limit::OnDemand;
     }
   }
-  
+
   auto effectiveAccess = d->getEffectiveAccess();
 
   // Private setter implementations for an internal storage declaration should

lib/SILGen/SILGenType.cpp

@@ -387,11 +387,25 @@ template<typename T> class SILGenWitnessTable : public SILWitnessVisitor<T> {
 
 public:
   void addMethod(SILDeclRef requirementRef) {
-    auto reqAccessor = dyn_cast<AccessorDecl>(requirementRef.getDecl());
+    auto reqDecl = requirementRef.getDecl();
+
+    // Static functions can be witnessed by enum cases with payload
+    if (!(isa<AccessorDecl>(reqDecl) || isa<ConstructorDecl>(reqDecl))) {
+      auto FD = cast<FuncDecl>(reqDecl);
+      if (auto witness = asDerived().getWitness(FD)) {
+        if (auto EED = dyn_cast<EnumElementDecl>(witness.getDecl())) {
+          return addMethodImplementation(
+              requirementRef, SILDeclRef(EED, SILDeclRef::Kind::EnumElement),
+              witness);
+        }
+      }
+    }
+
+    auto reqAccessor = dyn_cast<AccessorDecl>(reqDecl);
 
     // If it's not an accessor, just look for the witness.
     if (!reqAccessor) {
-      if (auto witness = asDerived().getWitness(requirementRef.getDecl())) {
+      if (auto witness = asDerived().getWitness(reqDecl)) {
         return addMethodImplementation(
             requirementRef, requirementRef.withDecl(witness.getDecl()),
             witness);
@@ -406,6 +420,13 @@ template<typename T> class SILGenWitnessTable : public SILWitnessVisitor<T> {
     if (!witness)
       return asDerived().addMissingMethod(requirementRef);
 
+    // Static properties can be witnessed by enum cases without payload
+    if (auto EED = dyn_cast<EnumElementDecl>(witness.getDecl())) {
+      return addMethodImplementation(
+          requirementRef, SILDeclRef(EED, SILDeclRef::Kind::EnumElement),
+          witness);
+    }
+
     auto witnessStorage = cast<AbstractStorageDecl>(witness.getDecl());
     if (reqAccessor->isSetter() && !witnessStorage->supportsMutation())
       return asDerived().addMissingMethod(requirementRef);
@@ -566,6 +587,10 @@ class SILGenConformance : public SILGenWitnessTable<SILGenConformance> {
         witnessLinkage = SILLinkage::Shared;
     }
 
+    if (isa<EnumElementDecl>(witnessRef.getDecl())) {
+      assert(witnessRef.isEnumElement() && "Witness decl, but different kind?");
+    }
+
     SILFunction *witnessFn = SGM.emitProtocolWitness(
         ProtocolConformanceRef(Conformance), witnessLinkage, witnessSerialized,
         requirementRef, witnessRef, isFree, witness);

lib/SILOptimizer/Utils/InstOptUtils.cpp

@@ -1797,6 +1797,11 @@ bool swift::calleesAreStaticallyKnowable(SILModule &module, SILDeclRef decl) {
   if (decl.isForeign)
     return false;
 
+  if (decl.isEnumElement()) {
+    return calleesAreStaticallyKnowable(module,
+                                        cast<EnumElementDecl>(decl.getDecl()));
+  }
+
   auto *afd = decl.getAbstractFunctionDecl();
   assert(afd && "Expected abstract function decl!");
   return calleesAreStaticallyKnowable(module, afd);
@@ -1845,6 +1850,41 @@ bool swift::calleesAreStaticallyKnowable(SILModule &module,
   llvm_unreachable("Unhandled access level in switch.");
 }
 
+/// Are the callees that could be called through Decl statically
+/// knowable based on the Decl and the compilation mode?
+// FIXME: Merge this with calleesAreStaticallyKnowable above
+bool swift::calleesAreStaticallyKnowable(SILModule &module,
+                                         EnumElementDecl *eed) {
+  const DeclContext *assocDC = module.getAssociatedContext();
+  if (!assocDC)
+    return false;
+
+  // Only handle members defined within the SILModule's associated context.
+  if (!eed->isChildContextOf(assocDC))
+    return false;
+
+  if (eed->isDynamic()) {
+    return false;
+  }
+
+  if (!eed->hasAccess())
+    return false;
+
+  // Only consider 'private' members, unless we are in whole-module compilation.
+  switch (eed->getEffectiveAccess()) {
+  case AccessLevel::Open:
+    return false;
+  case AccessLevel::Public:
+  case AccessLevel::Internal:
+    return module.isWholeModule();
+  case AccessLevel::FilePrivate:
+  case AccessLevel::Private:
+    return true;
+  }
+
+  llvm_unreachable("Unhandled access level in switch.");
+}
+
 Optional<FindLocalApplySitesResult>
 swift::findLocalApplySites(FunctionRefBaseInst *fri) {
   SmallVector<Operand *, 32> worklist(fri->use_begin(), fri->use_end());

lib/Sema/TypeCheckProtocol.cpp

@@ -484,8 +484,20 @@ swift::matchWitness(
   assert(!req->isInvalid() && "Cannot have an invalid requirement here");
 
   /// Make sure the witness is of the same kind as the requirement.
-  if (req->getKind() != witness->getKind())
-    return RequirementMatch(witness, MatchKind::KindConflict);
+  if (req->getKind() != witness->getKind()) {
+    // An enum case can witness:
+    // 1. A static get-only property requirement, as long as the property's
+    //    type is `Self` or it matches the type of the enum explicitly.
+    // 2. A static function requirement, if the enum case has a payload
+    //    and the payload types and labels match the function and the
+    //    function returns `Self` or the type of the enum.
+    //
+    // If there are any discrepencies, we'll diagnose it later. For now,
+    // let's assume the match is valid.
+    if (!((isa<VarDecl>(req) || isa<FuncDecl>(req)) &&
+          isa<EnumElementDecl>(witness)))
+      return RequirementMatch(witness, MatchKind::KindConflict);
+  }
 
   // If we're currently validating the witness, bail out.
   if (witness->isRecursiveValidation())
@@ -502,7 +514,8 @@ swift::matchWitness(
   // Perform basic matching of the requirement and witness.
   bool decomposeFunctionType = false;
   bool ignoreReturnType = false;
-  if (auto funcReq = dyn_cast<FuncDecl>(req)) {
+  if (isa<FuncDecl>(req) && isa<FuncDecl>(witness)) {
+    auto funcReq = cast<FuncDecl>(req);
     auto funcWitness = cast<FuncDecl>(witness);
 
     // Either both must be 'static' or neither.
@@ -564,6 +577,18 @@ swift::matchWitness(
   } else if (isa<ConstructorDecl>(witness)) {
     decomposeFunctionType = true;
     ignoreReturnType = true;
+  } else if (isa<EnumElementDecl>(witness)) {
+    auto enumCase = cast<EnumElementDecl>(witness);
+    if (enumCase->hasAssociatedValues() && isa<VarDecl>(req))
+      return RequirementMatch(witness, MatchKind::EnumCaseWithAssociatedValues);
+    auto isValid = isa<VarDecl>(req) || isa<FuncDecl>(req);
+    if (!isValid)
+      return RequirementMatch(witness, MatchKind::KindConflict);
+    if (!cast<ValueDecl>(req)->isStatic())
+      return RequirementMatch(witness, MatchKind::StaticNonStaticConflict);
+    if (isa<VarDecl>(req) &&
+        cast<VarDecl>(req)->getParsedAccessor(AccessorKind::Set))
+      return RequirementMatch(witness, MatchKind::SettableConflict);
   }
 
   // If the requirement is @objc, the witness must not be marked with @nonobjc.
@@ -2182,7 +2207,8 @@ diagnoseMatch(ModuleDecl *module, NormalProtocolConformance *conformance,
   if (match.Kind != MatchKind::RenamedMatch &&
       !match.Witness->getAttrs().hasAttribute<ImplementsAttr>() &&
       match.Witness->getFullName() &&
-      req->getFullName() != match.Witness->getFullName())
+      req->getFullName() != match.Witness->getFullName() &&
+      !isa<EnumElementDecl>(match.Witness))
     return;
 
   // Form a string describing the associated type deductions.
@@ -2234,7 +2260,7 @@ diagnoseMatch(ModuleDecl *module, NormalProtocolConformance *conformance,
     break;
 
   case MatchKind::TypeConflict: {
-    if (!isa<TypeDecl>(req)) {
+    if (!isa<TypeDecl>(req) && !isa<EnumElementDecl>(match.Witness)) {
       computeFixitsForOverridenDeclaration(match.Witness, req, [&](bool){
         return diags.diagnose(match.Witness,
                               diag::protocol_witness_type_conflict,
@@ -2278,6 +2304,8 @@ diagnoseMatch(ModuleDecl *module, NormalProtocolConformance *conformance,
     auto witness = match.Witness;
     auto diag = diags.diagnose(witness, diag::protocol_witness_static_conflict,
                                !req->isInstanceMember());
+    if (isa<EnumElementDecl>(witness))
+      break;
     if (req->isInstanceMember()) {
       SourceLoc loc;
       if (auto FD = dyn_cast<FuncDecl>(witness)) {
@@ -2403,6 +2431,9 @@ diagnoseMatch(ModuleDecl *module, NormalProtocolConformance *conformance,
     }
     break;
   }
+  case MatchKind::EnumCaseWithAssociatedValues:
+    diags.diagnose(match.Witness, diag::protocol_witness_enum_case_payload);
+    break;
   }
 }
 
@@ -3325,12 +3356,10 @@ ConformanceChecker::resolveWitnessViaLookup(ValueDecl *requirement) {
           auto &diags = proto->getASTContext().Diags;
           {
             SourceLoc diagLoc = getLocForDiagnosingWitness(conformance,witness);
-            auto diag = diags.diagnose(diagLoc,
-                                       diag::witness_argument_name_mismatch,
-                                       isa<ConstructorDecl>(witness),
-                                       witness->getFullName(),
-                                       proto->getDeclaredType(),
-                                       requirement->getFullName());
+            auto diag = diags.diagnose(
+                diagLoc, diag::witness_argument_name_mismatch,
+                witness->getDescriptiveKind(), witness->getFullName(),
+                proto->getDeclaredType(), requirement->getFullName());
             if (diagLoc == witness->getLoc()) {
               fixDeclarationName(diag, witness, requirement->getFullName());
             } else {

lib/Sema/TypeCheckProtocol.h

@@ -211,6 +211,10 @@ enum class MatchKind : uint8_t {
 
   /// The witness is missing a `@differentiable` attribute from the requirement.
   MissingDifferentiableAttr,
+  
+  /// The witness did not match because it is an enum case with
+  /// associated values.
+  EnumCaseWithAssociatedValues,
 };
 
 /// Describes the kind of optional adjustment performed when
@@ -437,6 +441,7 @@ struct RequirementMatch {
     case MatchKind::ThrowsConflict:
     case MatchKind::NonObjC:
     case MatchKind::MissingDifferentiableAttr:
+    case MatchKind::EnumCaseWithAssociatedValues:
       return false;
     }
 
@@ -467,6 +472,7 @@ struct RequirementMatch {
     case MatchKind::ThrowsConflict:
     case MatchKind::NonObjC:
     case MatchKind::MissingDifferentiableAttr:
+    case MatchKind::EnumCaseWithAssociatedValues:
       return false;
     }
 

lib/TBDGen/TBDGen.cpp

@@ -482,21 +482,23 @@ void TBDGenVisitor::addConformances(DeclContext *DC) {
       }
     };
 
-    rootConformance->forEachValueWitness(
-        [&](ValueDecl *valueReq, Witness witness) {
-          auto witnessDecl = witness.getDecl();
-          if (isa<AbstractFunctionDecl>(valueReq)) {
-            addSymbolIfNecessary(valueReq, witnessDecl);
-          } else if (auto *storage = dyn_cast<AbstractStorageDecl>(valueReq)) {
-            auto witnessStorage = cast<AbstractStorageDecl>(witnessDecl);
-            storage->visitOpaqueAccessors([&](AccessorDecl *reqtAccessor) {
-              auto witnessAccessor =
-                witnessStorage->getSynthesizedAccessor(
-                  reqtAccessor->getAccessorKind());
-              addSymbolIfNecessary(reqtAccessor, witnessAccessor);
-            });
-          }
-        });
+    rootConformance->forEachValueWitness([&](ValueDecl *valueReq,
+                                             Witness witness) {
+      auto witnessDecl = witness.getDecl();
+      if (isa<AbstractFunctionDecl>(valueReq)) {
+        addSymbolIfNecessary(valueReq, witnessDecl);
+      } else if (auto *storage = dyn_cast<AbstractStorageDecl>(valueReq)) {
+        if (auto witnessStorage = dyn_cast<AbstractStorageDecl>(witnessDecl)) {
+          storage->visitOpaqueAccessors([&](AccessorDecl *reqtAccessor) {
+            auto witnessAccessor = witnessStorage->getSynthesizedAccessor(
+                reqtAccessor->getAccessorKind());
+            addSymbolIfNecessary(reqtAccessor, witnessAccessor);
+          });
+        } else if (isa<EnumElementDecl>(witnessDecl)) {
+          addSymbolIfNecessary(valueReq, witnessDecl);
+        }
+      }
+    });
   }
 }
 

test/SILGen/protocol_enum_witness.swift

@@ -0,0 +1,49 @@
+// RUN: %target-swift-emit-silgen %s | %FileCheck %s
+
+protocol Foo {
+  static var button: Self { get }
+}
+
+enum Bar: Foo {
+  case button
+}
+
+protocol AnotherFoo {
+  static func bar(arg: Int) -> Self
+}
+
+enum AnotherBar: AnotherFoo {
+  case bar(arg: Int)
+}
+
+// CHECK-LABEL: sil private [transparent] [thunk] [ossa] @$s21protocol_enum_witness3BarOAA3FooA2aDP6buttonxvgZTW : $@convention(witness_method: Foo) (@thick Bar.Type) -> @out Bar {
+// CHECK: bb0([[BAR:%.*]] : $*Bar, [[BAR_TYPE:%.*]] : $@thick Bar.Type):
+// CHECK-NEXT: [[META_TYPE:%.*]] = metatype $@thin Bar.Type
+// CHECK: [[REF:%.*]] = function_ref @$s21protocol_enum_witness3BarO6buttonyA2CmF : $@convention(method) (@thin Bar.Type) -> Bar
+// CHECK-NEXT: [[RESULT:%.*]] = apply [[REF]]([[META_TYPE]]) : $@convention(method) (@thin Bar.Type) -> Bar
+// CHECK-NEXT: store [[RESULT]] to [trivial] [[BAR]] : $*Bar
+// CHECK-NEXT: [[TUPLE:%.*]] = tuple ()
+// CHECK-NEXT: return [[TUPLE]] : $()
+// CHECK-END: }
+
+// CHECK-LABEL: sil hidden [transparent] [ossa] @$s21protocol_enum_witness3BarO6buttonyA2CmF : $@convention(method) (@thin Bar.Type) -> Bar {
+// CHECK: bb0({{%.*}} : $@thin Bar.Type):
+// CHECK-NEXT: [[CASE:%.*]] = enum $Bar, #Bar.button!enumelt
+// CHECK-NEXT: return [[CASE]] : $Bar
+// CHECK-END: }
+
+// CHECK-LABEL: sil private [transparent] [thunk] [ossa] @$s21protocol_enum_witness10AnotherBarOAA0D3FooA2aDP3bar3argxSi_tFZTW : $@convention(witness_method: AnotherFoo) (Int, @thick AnotherBar.Type) -> @out AnotherBar {
+// CHECK: bb0([[ANOTHER_BAR:%.*]] : $*AnotherBar, [[INT_ARG:%.*]] : $Int, [[ANOTHER_BAR_TYPE:%.*]] : $@thick AnotherBar.Type):
+// CHECK-NEXT: [[META_TYPE:%.*]] = metatype $@thin AnotherBar.Type
+// CHECK: [[REF:%.*]] = function_ref @$s21protocol_enum_witness10AnotherBarO3baryACSi_tcACmF : $@convention(method) (Int, @thin AnotherBar.Type) -> AnotherBar
+// CHECK-NEXT: [[RESULT:%.*]] = apply [[REF]]([[INT_ARG]], [[META_TYPE]]) : $@convention(method) (Int, @thin AnotherBar.Type) -> AnotherBar
+// CHECK-NEXT: store [[RESULT]] to [trivial] [[ANOTHER_BAR]] : $*AnotherBar
+// CHECK-NEXT: [[TUPLE:%.*]] = tuple ()
+// CHECK-NEXT: return [[TUPLE]] : $()
+// CHECK-END: }
+
+// CHECK-LABEL: sil_witness_table hidden Bar: Foo module protocol_enum_witness {
+// CHECK: method #Foo.button!getter: <Self where Self : Foo> (Self.Type) -> () -> Self : @$s21protocol_enum_witness3BarOAA3FooA2aDP6buttonxvgZTW
+
+// CHECK-LABEL: sil_witness_table hidden AnotherBar: AnotherFoo module protocol_enum_witness {
+// CHECK: method #AnotherFoo.bar: <Self where Self : AnotherFoo> (Self.Type) -> (Int) -> Self : @$s21protocol_enum_witness10AnotherBarOAA0D3FooA2aDP3bar3argxSi_tFZTW