SIL: Pick default ARC convention for enum case witness thunks based on the kind of the original decl.

Enum constructors take their arguments +1 by default, but they can now be used to satisfy protocol static member
requirements, which take arguments +0 by default. Type lowering would accidentally use the kind of the witness
to determine the conventions for the witness thunk, leading to a miscompile when the requirement is called through
the protocol. This differs from the fix on main in that the behavior change is contained to only cases where the
witness is an enum constructor, to minimize behavior change. rdar://73855052

Author: jckarter

lib/SIL/IR/SILFunctionType.cpp

@@ -2485,7 +2485,22 @@ static CanSILFunctionType getNativeSILFunctionType(
   case SILFunctionType::Representation::Method:
   case SILFunctionType::Representation::Closure:
   case SILFunctionType::Representation::WitnessMethod: {
-    switch (constant ? constant->kind : SILDeclRef::Kind::Func) {
+    // FIXME: We should use origConstant unconditionally here, since in the
+    // case of protocol witness thunk type lowering, we want to lower the
+    // witness's calling convention according to the convention of the protocol
+    // requirement, even if the witness has a different kind. Previously we
+    // would base this on the witness `constant`, and this worked because
+    // witnesses had always been the same decl kind as the requirement, but
+    // enum cases as static witnesses broke this invariant.
+    //
+    // To minimize the change in behavior for Swift 5.4, we only consider
+    // origConstant when constant is an EnumElement, to avoid affecting other
+    // potential corner cases I'm not thinking of in the moment.
+    auto constantToTest = constant;
+    if (constant && constant->kind == SILDeclRef::Kind::EnumElement) {
+      constantToTest = origConstant;
+    }
+    switch (constantToTest ? constantToTest->kind : SILDeclRef::Kind::Func) {
     case SILDeclRef::Kind::Initializer:
     case SILDeclRef::Kind::EnumElement:
       return getSILFunctionTypeForConventions(DefaultInitializerConventions());

test/SILGen/enum_witness_thunks.swift

@@ -0,0 +1,15 @@
+// RUN: %target-swift-emit-silgen %s | %FileCheck %s
+
+class C {}
+
+protocol P {
+    static func c(_: C) -> Self
+}
+
+enum E: P {
+    case c(C)
+}
+
+// CHECK-LABEL: sil {{.*}} @$s19enum_witness_thunks1EOAA1PA2aDP1cyxAA1CCFZTW : $@convention(witness_method: P) (@guaranteed C, @thick E.Type) -> @out E
+// CHECK:         [[COPY:%.*]] = copy_value
+// CHECK:         apply {{.*}}([[COPY]]