[clang][bytecode] Loosen assertion This() for array elements

getRecord() returns null on array elements, even for composite arrays.
The assertion here was overly restrictive and having an array element
as instance pointer should be fine otherwise.

Author: tbaederr

clang/lib/AST/ByteCode/Interp.h

@@ -2431,9 +2431,12 @@ inline bool This(InterpState &S, CodePtr OpPC) {
   // Ensure the This pointer has been cast to the correct base.
   if (!This.isDummy()) {
     assert(isa<CXXMethodDecl>(S.Current->getFunction()->getDecl()));
-    assert(This.getRecord());
+    [[maybe_unused]] const Record *R = This.getRecord();
+    if (!R)
+      R = This.narrow().getRecord();
+    assert(R);
     assert(
-        This.getRecord()->getDecl() ==
+        R->getDecl() ==
         cast<CXXMethodDecl>(S.Current->getFunction()->getDecl())->getParent());
   }
 

clang/test/AST/ByteCode/placement-new.cpp

@@ -339,6 +339,30 @@ namespace PR48606 {
   static_assert(f());
 }
 
+/// This used to crash because of an assertion in the implementation
+/// of the This instruction.
+namespace ExplicitThisOnArrayElement {
+  struct S {
+    int a = 12;
+    constexpr S(int a) {
+      this->a = a;
+    }
+  };
+
+  template <class _Tp, class... _Args>
+  constexpr void construct_at(_Tp *__location, _Args &&...__args) {
+    new (__location) _Tp(__args...);
+  }
+
+  constexpr bool foo() {
+    auto *M = std::allocator<S>().allocate(13); // both-note {{allocation performed here was not deallocated}}
+    construct_at(M, 12);
+    return true;
+  }
+
+  static_assert(foo()); // both-error {{not an integral constant expression}}
+}
+
 #ifdef BYTECODE
 constexpr int N = [] // expected-error {{must be initialized by a constant expression}} \
                      // expected-note {{assignment to dereferenced one-past-the-end pointer is not allowed in a constant expression}} \