[clang][Interp] Fix initializing zero-size arrays

Author: tbaederr

clang/lib/AST/Interp/Pointer.h

@@ -556,12 +556,16 @@ class Pointer {
     if (!asBlockPointer().Pointee)
       return false;
 
-    return isElementPastEnd() || getSize() == getOffset();
+    return isElementPastEnd() ||
+           (getSize() == getOffset() && !isZeroSizeArray());
   }
 
   /// Checks if the pointer is an out-of-bounds element pointer.
   bool isElementPastEnd() const { return Offset == PastEndMark; }
 
+  /// Checks if the pointer is pointing to a zero-size array.
+  bool isZeroSizeArray() const { return getFieldDesc()->isZeroSizeArray(); }
+
   /// Dereferences the pointer, if it's live.
   template <typename T> T &deref() const {
     assert(isLive() && "Invalid pointer");

clang/test/AST/Interp/arrays.cpp

@@ -26,6 +26,7 @@ static_assert(foo[2][2] == nullptr, "");
 static_assert(foo[2][3] == &m, "");
 static_assert(foo[2][4] == nullptr, "");
 
+constexpr int ZeroSizeArray[] = {};
 
 constexpr int SomeInt[] = {1};
 constexpr int getSomeInt() { return *SomeInt; }

clang/unittests/AST/Interp/Descriptor.cpp

@@ -22,9 +22,10 @@ TEST(Descriptor, Primitives) {
       "  char s[4];\n"
       "  A a[3];\n"
       "  short l[3][3];\n"
+      "  int EmptyA[0];\n"
       "};\n"
       "constexpr S d = {0.0, \"foo\", {{true, false}, {false, true}, {false, false}},\n"
-      "  {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}};\n";
+      "  {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}, {}};\n";
 
   auto AST = tooling::buildASTFromCodeWithArgs(
       Code, {"-fexperimental-new-constant-interpreter"});
@@ -64,7 +65,7 @@ TEST(Descriptor, Primitives) {
   // Test the Record for the struct S.
   const Record *SRecord = GlobalDesc->ElemRecord;
   ASSERT_TRUE(SRecord);
-  ASSERT_TRUE(SRecord->getNumFields() == 4);
+  ASSERT_TRUE(SRecord->getNumFields() == 5);
   ASSERT_TRUE(SRecord->getNumBases() == 0);
   ASSERT_FALSE(SRecord->getDestructor());
 
@@ -113,6 +114,16 @@ TEST(Descriptor, Primitives) {
   ASSERT_TRUE(F4->Desc->getElemSize() > 0);
   ASSERT_TRUE(F4->Desc->ElemDesc->isPrimitiveArray());
 
+  // Fifth field. Zero-size array.
+  const Record::Field *F5 = SRecord->getField(4u);
+  ASSERT_TRUE(F5);
+  ASSERT_FALSE(F5->isBitField());
+  ASSERT_TRUE(F5->Desc->isArray());
+  ASSERT_FALSE(F5->Desc->isCompositeArray());
+  ASSERT_TRUE(F5->Desc->isPrimitiveArray());
+  ASSERT_FALSE(F5->Desc->isPrimitive());
+  ASSERT_EQ(F5->Desc->getNumElems(), 0u);
+
   // Check pointer stuff.
   // Global variables have an inline descriptor.
   ASSERT_TRUE(GlobalPtr.isRoot());
@@ -382,4 +393,13 @@ TEST(Descriptor, Primitives) {
     ASSERT_EQ(PE3.getArray(), NE3);
     ASSERT_EQ(PE3.getIndex(), 2u);
   }
+
+  // Zero-size array.
+  {
+    const Pointer &PF5 = GlobalPtr.atField(F5->Offset);
+
+    ASSERT_TRUE(PF5.isZeroSizeArray());
+    ASSERT_FALSE(PF5.isOnePastEnd());
+    ASSERT_FALSE(PF5.isElementPastEnd());
+  }
 }