IRGen: Fix reflection metadata for zero-sized enum cases

If an enum has a payload case with zero size, we treat it as an empty case
for ABI purposes. Unfortunately, this meant that reflection metadata was
incomplete for such cases, with a Mirror reporting that the enum value
had zero children.

Tweak the field type metadata emission slightly to preserve the payload
type for such enum cases.

Fixes <https://bugs.swift.org/browse/SR-12044> / <rdar://problem/58861157>.

lib/IRGen/GenReflection.cpp

@@ -673,7 +673,7 @@ private:
   const NominalTypeDecl *NTD;
 
   void addFieldDecl(const ValueDecl *value, Type type,
-                    GenericSignature genericSig, bool indirect=false) {
+                    bool indirect=false) {
     reflection::FieldRecordFlags flags;
     flags.setIsIndirectCase(indirect);
     if (auto var = dyn_cast<VarDecl>(value))
@@ -684,6 +684,8 @@ private:
     if (!type) {
       B.addInt32(0);
     } else {
+      auto genericSig = NTD->getGenericSignature();
+
       // The standard library's Mirror demangles metadata from field
       // descriptors, so use MangledTypeRefRole::Metadata to ensure
       // runtime metadata is available.
@@ -717,8 +719,7 @@ private:
     auto properties = NTD->getStoredProperties();
     B.addInt32(properties.size());
     for (auto property : properties)
-      addFieldDecl(property, property->getInterfaceType(),
+      addFieldDecl(property, property->getInterfaceType());
-                   NTD->getGenericSignature());
   }
 
   void layoutEnum() {
@@ -743,12 +744,11 @@ private:
       bool indirect = (enumCase.decl->isIndirect() ||
                        enumDecl->isIndirect());
       addFieldDecl(enumCase.decl, enumCase.decl->getArgumentInterfaceType(),
-                   enumDecl->getGenericSignature(),
                    indirect);
     }
 
     for (auto enumCase : strategy.getElementsWithNoPayload()) {
-      addFieldDecl(enumCase.decl, CanType(), nullptr);
+      addFieldDecl(enumCase.decl, enumCase.decl->getArgumentInterfaceType());
     }
   }
 

test/Reflection/typeref_lowering.swift

@@ -1041,7 +1041,11 @@
 // CHECK-64-NEXT:   (field name=noPayload offset=0
 // CHECK-64-NEXT:     (no_payload_enum size=1 alignment=1 stride=1 num_extra_inhabitants=252 bitwise_takable=1))
 // CHECK-64-NEXT:   (field name=sillyNoPayload offset=1
-// CHECK-64-NEXT:     (no_payload_enum size=1 alignment=1 stride=1 num_extra_inhabitants=252 bitwise_takable=1))
+// CHECK-64-NEXT:    (multi_payload_enum size=1 alignment=1 stride=1 num_extra_inhabitants=252 bitwise_takable=1
+// CHECK-64-NEXT:      (field name=A offset=0
+// CHECK-64-NEXT:        (no_payload_enum size=0 alignment=1 stride=1 num_extra_inhabitants=0 bitwise_takable=1))
+// CHECK-64-NEXT:      (field name=B offset=0
+// CHECK-64-NEXT:        (no_payload_enum size=0 alignment=1 stride=1 num_extra_inhabitants=0 bitwise_takable=1))))
 // CHECK-64-NEXT:   (field name=singleton offset=8
 // CHECK-64-NEXT:     (reference kind=strong refcounting=native))
 // CHECK-64-NEXT:   (field name=singlePayload offset=16

test/stdlib/Mirror.swift

@@ -1145,6 +1145,28 @@ mirrors.test("Enum/SingletonNonGeneric/DefaultMirror") {
   }
 }
 
+enum ZeroSizedEnumWithDefaultMirror {
+  case π
+}
+
+enum SingletonZeroSizedEnumWithDefaultMirror {
+  case wrap(ZeroSizedEnumWithDefaultMirror)
+}
+
+mirrors.test("Enum/SingletonZeroSizedEnumWithDefaultMirror/DefaultMirror") {
+  do {
+    let value = SingletonZeroSizedEnumWithDefaultMirror.wrap(.π)
+    var output = ""
+    dump(value, to: &output)
+
+    let expected =
+      "▿ Mirror.SingletonZeroSizedEnumWithDefaultMirror.wrap\n" +
+      "  - wrap: Mirror.ZeroSizedEnumWithDefaultMirror.π\n"
+
+    expectEqual(expected, output)
+  }
+}
+
 enum SingletonGenericEnumWithDefaultMirror<T> {
   case OnlyOne(T)
 }
@@ -2222,7 +2244,8 @@ if #available(macOS 10.15, iOS 13.0, tvOS 13.0, watchOS 6.0, *) {
     testSTSDump(STSContainer.Cases<Int>.a(.init()),
                 STSContainer℠.Cases<Int>.a(.init()),
                 """
-      - Mirror.STSContainer<Mirror.STSOuter>.Cases<Swift.Int>.a\n
+      ▿ Mirror.STSContainer<Mirror.STSOuter>.Cases<Swift.Int>.a
+        - a: Mirror.STSOuter\n
       """)
 
     testSTSDump(STSContainer.Cases<Int>.b(.init()),