[clang][Interp] Fix lifetime diagnostics for dead records

This used to crash the interpreter, either because we ran into the
assertion in CheckMutable() or because we accessed a Descriptor* pointer
preceding the field of a record. Those are preceded by an
InlineDescriptor though.

Differential Revision: https://reviews.llvm.org/D152132

clang/lib/AST/Interp/Descriptor.cpp

@@ -170,9 +170,8 @@ static void moveRecord(Block *B, const std::byte *Src, std::byte *Dst,
                        const Descriptor *D) {
   for (const auto &F : D->ElemRecord->fields()) {
     auto FieldOff = F.Offset;
-    auto FieldDesc = F.Desc;
+    auto *FieldDesc = F.Desc;
 
-    *(reinterpret_cast<Descriptor **>(Dst + FieldOff) - 1) = FieldDesc;
     if (auto Fn = FieldDesc->MoveFn)
       Fn(B, Src + FieldOff, Dst + FieldOff, FieldDesc);
   }

clang/lib/AST/Interp/InterpBlock.h

@@ -156,6 +156,7 @@ class DeadBlock final {
 
   /// Returns a pointer to the stored data.
   std::byte *data() { return B.data(); }
+  std::byte *rawData() { return B.rawData(); }
 
 private:
   friend class Block;

clang/lib/AST/Interp/InterpState.cpp

@@ -58,9 +58,13 @@ void InterpState::deallocate(Block *B) {
         reinterpret_cast<char *>(std::malloc(sizeof(DeadBlock) + Size));
     auto *D = new (Memory) DeadBlock(DeadBlocks, B);
 
-    // Move data from one block to another.
-    if (Desc->MoveFn)
+    // Move data and metadata from the old block to the new (dead)block.
+    if (Desc->MoveFn) {
       Desc->MoveFn(B, B->data(), D->data(), Desc);
+      if (Desc->getMetadataSize() > 0)
+        std::memcpy(D->rawData(), B->rawData(), Desc->getMetadataSize());
+    }
+
   } else {
     // Free storage, if necessary.
     if (Desc->DtorFn)

clang/test/AST/Interp/lifetimes.cpp

@@ -0,0 +1,24 @@
+// RUN: %clang_cc1 -fexperimental-new-constant-interpreter -verify %s
+// RUN: %clang_cc1 -verify=ref %s
+
+struct Foo {
+  int a;
+};
+
+constexpr int dead1() { // expected-error {{never produces a constant expression}}
+
+  Foo *F2 = nullptr;
+  {
+    Foo F{12}; // expected-note 2{{declared here}}
+    F2 = &F;
+  } // Ends lifetime of F.
+
+  return F2->a; // expected-note 2{{read of variable whose lifetime has ended}} \
+                // ref-note {{read of object outside its lifetime is not allowed in a constant expression}}
+}
+static_assert(dead1() == 1, ""); // expected-error {{not an integral constant expression}} \
+                                 // expected-note {{in call to}} \
+                                 // ref-error {{not an integral constant expression}} \
+                                 // ref-note {{in call to}} \
+
+