SILGen: Fix invalid SIL emitted when protocol requirement witnessed by base class convenience init [4.2]

lib/SILGen/SILGenPoly.cpp

@@ -3346,11 +3346,20 @@ static WitnessDispatchKind getWitnessDispatchKind(SILDeclRef witness) {
   // A natively ObjC method witness referenced this way will end up going
   // through its native thunk, which will redispatch the method after doing
   // bridging just like we want.
-  if (isFinal || isExtension || witness.isForeignToNativeThunk()
-      // Hack--We emit a static thunk for ObjC allocating constructors.
-      || (decl->hasClangNode() && witness.kind == SILDeclRef::Kind::Allocator))
+  if (isFinal || isExtension || witness.isForeignToNativeThunk())
     return WitnessDispatchKind::Static;
 
+  if (witness.kind == SILDeclRef::Kind::Allocator) {
+    // Non-required initializers can witness a protocol requirement if the class
+    // is final, so we can statically dispatch to them.
+    if (!cast<ConstructorDecl>(decl)->isRequired())
+      return WitnessDispatchKind::Static;
+
+    // We emit a static thunk for ObjC allocating constructors.
+    if (decl->hasClangNode())
+      return WitnessDispatchKind::Static;
+  }
+
   // Otherwise emit a class method.
   return WitnessDispatchKind::Class;
 }

test/SILGen/witness-init-requirement-with-base-class-init.swift

@@ -20,3 +20,32 @@ class Dog: Animal, BestFriend {}
 // CHECK-LABEL: sil private [transparent] [thunk] @$S4main3DogCAA10BestFriendA2aDP6createxyFZTW
 // CHECK:         [[SELF:%.*]] = apply
 // CHECK:         unchecked_ref_cast [[SELF]] : $Animal to $Dog
+
+class Base {
+  init() {}
+
+  convenience init(x: Int) {
+    self.init()
+  }
+}
+
+protocol Initable {
+  init(x: Int)
+}
+
+final class Derived : Base, Initable {}
+
+// CHECK-LABEL: sil hidden @$S4main4BaseC1xACSi_tcfC : $@convention(method) (Int, @thick Base.Type) -> @owned Base
+// CHECK:         [[SELF:%.*]] = alloc_ref_dynamic %1 : $@thick Base.Type, $Base
+// CHECK:         [[METHOD:%.*]] = function_ref @$S4main4BaseC1xACSi_tcfc
+// CHECK-NEXT:    [[RESULT:%.*]] = apply [[METHOD]](%0, [[SELF]])
+// CHECK-NEXT:    return [[RESULT]]
+
+// CHECK-LABEL: sil private [transparent] [thunk] @$S4main7DerivedCAA8InitableA2aDP1xxSi_tcfCTW : $@convention(witness_method: Initable) (Int, @thick Derived.Type) -> @out Derived
+// CHECK:         [[SELF:%.*]] = upcast %2 : $@thick Derived.Type to $@thick Base.Type
+// CHECK:         [[METHOD:%.*]] = function_ref @$S4main4BaseC1xACSi_tcfC
+// CHECK-NEXT:    [[RESULT:%.*]] = apply [[METHOD]](%1, [[SELF]])
+// CHECK-NEXT:    [[NEW_SELF:%.*]] = unchecked_ref_cast [[RESULT]] : $Base to $Derived
+// CHECK-NEXT:    store [[NEW_SELF]] to [init] %0 : $*Derived
+// CHECK-NEXT:    [[TUPLE:%.*]] = tuple ()
+// CHECK-NEXT:    return [[TUPLE]]