[clang][ExprConst] Fix crash on uninitialized array subobject (#67817)

https://reviews.llvm.org/D146358 was assuming that all subobjects have
their own name (`SubobjectDecl`), but it was not true for array
elements.

Fixes #67317

Author: hazohelet

clang/docs/ReleaseNotes.rst

@@ -719,6 +719,8 @@ Bug Fixes in This Version
   points (i.e., uses function descriptor objects instead).
 - Fixes a ``clang-17`` regression where ``LLVM_UNREACHABLE_OPTIMIZE=OFF``
   cannot be used with ``Release`` mode builds. (`#68237 <https://github.com/llvm/llvm-project/issues/68237>`_).
+- Fix crash from constexpr evaluator evaluating uninitialized arrays as rvalue.
+  Fixes (`#67317 <https://github.com/llvm/llvm-project/issues/67317>`_)
 - No longer use C++ ``thread_local`` semantics in C23 when using
   ``thread_local`` instead of ``_Thread_local``.
   Fixes (`#70068 <https://github.com/llvm/llvm-project/issues/70068>`_) and

clang/include/clang/Basic/DiagnosticASTKinds.td

@@ -69,7 +69,7 @@ def note_consteval_address_accessible : Note<
   "%select{pointer|reference}0 to a consteval declaration "
   "is not a constant expression">;
 def note_constexpr_uninitialized : Note<
-  "subobject %0 is not initialized">;
+  "subobject %select{of type |}0%1 is not initialized">;
 def note_constexpr_uninitialized_base : Note<
   "constructor of base class %0 is not called">;
 def note_constexpr_static_local : Note<

clang/lib/AST/ExprConstant.cpp

@@ -2379,10 +2379,15 @@ static bool CheckEvaluationResult(CheckEvaluationResultKind CERK,
                                   const FieldDecl *SubobjectDecl,
                                   CheckedTemporaries &CheckedTemps) {
   if (!Value.hasValue()) {
-    assert(SubobjectDecl && "SubobjectDecl shall be non-null");
-    Info.FFDiag(DiagLoc, diag::note_constexpr_uninitialized) << SubobjectDecl;
-    Info.Note(SubobjectDecl->getLocation(),
-              diag::note_constexpr_subobject_declared_here);
+    if (SubobjectDecl) {
+      Info.FFDiag(DiagLoc, diag::note_constexpr_uninitialized)
+          << /*(name)*/ 1 << SubobjectDecl;
+      Info.Note(SubobjectDecl->getLocation(),
+                diag::note_constexpr_subobject_declared_here);
+    } else {
+      Info.FFDiag(DiagLoc, diag::note_constexpr_uninitialized)
+          << /*of type*/ 0 << Type;
+    }
     return false;
   }
 

clang/lib/AST/Interp/Interp.cpp

@@ -382,7 +382,8 @@ bool CheckPure(InterpState &S, CodePtr OpPC, const CXXMethodDecl *MD) {
 static void DiagnoseUninitializedSubobject(InterpState &S, const SourceInfo &SI,
                                            const FieldDecl *SubObjDecl) {
   assert(SubObjDecl && "Subobject declaration does not exist");
-  S.FFDiag(SI, diag::note_constexpr_uninitialized) << SubObjDecl;
+  S.FFDiag(SI, diag::note_constexpr_uninitialized)
+      << /*(name)*/ 1 << SubObjDecl;
   S.Note(SubObjDecl->getLocation(),
          diag::note_constexpr_subobject_declared_here);
 }

clang/test/SemaCXX/constant-expression-cxx2a.cpp

@@ -1492,3 +1492,9 @@ class B{
 class D : B{}; // expected-error {{deleted function '~D' cannot override a non-deleted function}}
 // expected-note@-1 {{destructor of 'D' is implicitly deleted because base class 'B' has an inaccessible destructor}}
 }
+
+namespace GH67317 {
+  constexpr unsigned char a = // expected-error {{constexpr variable 'a' must be initialized by a constant expression}} \
+                              // expected-note {{subobject of type 'const unsigned char' is not initialized}}
+    __builtin_bit_cast(unsigned char, *new char[3][1]);
+};

clang/test/SemaCXX/eval-crashes.cpp

@@ -54,3 +54,10 @@ namespace pr33140_10 {
   int a(const int &n = 0);
   bool b() { return a() == a(); }
 }
+
+namespace GH67317 {
+struct array {
+  int (&data)[2];
+  array() : data(*new int[1][2]) {}
+};
+}