Fix case calculation for SPE with zero-sized payload

Author: tbkka

stdlib/public/RemoteInspection/TypeLowering.cpp

@@ -557,8 +557,9 @@ class SinglePayloadEnumTypeInfo: public EnumTypeInfo {
   //
   // The above logic generalizes the following important cases:
   // * A payload with XIs will generally have enough to
-  //   encode all payloadcases.  It will have no discriminator
-  //   case, so everything is effectively on "page zero."
+  //   encode all payload cases.  If so, then it will have
+  //   no discriminator allocated, so the discriminator is
+  //   always treated as zero.
   // * If the payload has no XIs but is not zero-sized, then
   //   we'll need a page one.  That page will usually be
   //   large enough to encode all non-payload cases.
@@ -586,13 +587,14 @@ class SinglePayloadEnumTypeInfo: public EnumTypeInfo {
     unsigned nonPayloadCasesUsingXIs = PayloadCase.TI.getNumExtraInhabitants();
     int ComputedCase = 0;
     if (discriminator == 0) {
-      // Discriminator is for a page that encodes payload (and maybe tag data too)
+      // This is Page 0, which encodes payload case and some additional cases in Xis
       int XITag;
       if (!PayloadCase.TI.readExtraInhabitantIndex(reader, address, &XITag)) {
         return false;
       }
       ComputedCase = XITag < 0 ? 0 : XITag + 1;
     } else {
+      // This is some other page, so the entire payload area is just a case index
       unsigned payloadTag;
       if (!reader.readInteger(address, PayloadSize, &payloadTag)) {
         return false;
@@ -602,10 +604,9 @@ class SinglePayloadEnumTypeInfo: public EnumTypeInfo {
          ? ValueWitnessFlags::MaxNumExtraInhabitants
          : (1UL << (PayloadSize * 8UL));
       ComputedCase =
-        1
-        + nonPayloadCasesUsingXIs
-        + (discriminator - 1) * casesPerNonPayloadPage
-        + payloadTag;
+        1 + nonPayloadCasesUsingXIs // Cases on page 0
+        + (discriminator - 1) * casesPerNonPayloadPage // Cases on other pages
+        + payloadTag; // Cases on this page
     }
     if (static_cast<unsigned>(ComputedCase) < getNumCases()) {
       *CaseIndex = ComputedCase;

validation-test/Reflection/reflect_nested_generic.swift

@@ -237,6 +237,29 @@ reflect(enum: Outer2.E<S2>.Inner.F<S3>??.none)
 //CHECK: Enum value:
 //CHECK-NEXT: (enum_value name=none index=1)
 
+struct S4: P { var a: Bool = true }
+
+reflect(enum: Outer2.E<S3>.Inner.F<S4>.b)
+
+//CHECK: Reflecting an enum.
+//CHECK: Type reference:
+//CHECK: Type info:
+//CHECK-NEXT: (single_payload_enum size=1 alignment=1 stride=1 num_extra_inhabitants=252 bitwise_takable=1
+//CHECK-NEXT:   (case name=u index=0 offset=0
+//CHECK-NEXT:     (struct size=1 alignment=1 stride=1 num_extra_inhabitants=254 bitwise_takable=1
+//CHECK-NEXT:       (field name=a offset=0
+//CHECK-NEXT:         (struct size=1 alignment=1 stride=1 num_extra_inhabitants=254 bitwise_takable=1
+//CHECK-NEXT:           (field name=_value offset=0
+//CHECK-NEXT:             (builtin size=1 alignment=1 stride=1 num_extra_inhabitants=254 bitwise_takable=1))))))
+//CHECK-NEXT:   (case name=a index=1)
+//CHECK-NEXT:   (case name=b index=2))
+
+//CHECK: Mangled name: $s22reflect_nested_generic6Outer2V1EO1FOy_AA2S3V_AA2S4VG
+//CHECK: Demangled name: reflect_nested_generic.Outer2.E<reflect_nested_generic.S3>.F<reflect_nested_generic.S4>
+
+//CHECK: Enum value:
+//CHECK-NEXT: (enum_value name=b index=2)
+
 reflect(enum: Outer2.E<S1>.Inner.F<S2>.Innerer?.none)
 
 //CHECK: Reflecting an enum.