Fix crash-on-valid with consteval temporary construction through list initialization

Clang currently crashes when lowering a consteval list initialization
of a temporary. This is partially working around an issue in the
template instantiation code (TreeTransform::TransformCXXTemporaryObjectExpr())
that does not yet know how to handle list initialization of temporaries
in all cases. However, it's also helping reduce fragility by ensuring
we always have a valid QualType when trying to emit a constant
expression during IR generation.

Fixes #55871

Differential Revision: https://reviews.llvm.org/D131194

clang/docs/ReleaseNotes.rst

@@ -65,7 +65,9 @@ Bug Fixes
 - Fix a crash when evaluating a multi-dimensional array's array filler
   expression is element-dependent. This fixes
   `Issue 50601 <https://github.com/llvm/llvm-project/issues/56016>`_.
-
+- Fixed a crash-on-valid with consteval evaluation of a list-initialized
+  constructor for a temporary object. This fixes
+  `Issue 55871 <https://github.com/llvm/llvm-project/issues/55871>`_.
 
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/CodeGen/CGExprConstant.cpp

@@ -1395,15 +1395,12 @@ ConstantEmitter::tryEmitAbstract(const APValue &value, QualType destType) {
 llvm::Constant *ConstantEmitter::tryEmitConstantExpr(const ConstantExpr *CE) {
   if (!CE->hasAPValueResult())
     return nullptr;
-  const Expr *Inner = CE->getSubExpr()->IgnoreImplicit();
-  QualType RetType;
-  if (auto *Call = dyn_cast<CallExpr>(Inner))
-    RetType = Call->getCallReturnType(CGM.getContext());
-  else if (auto *Ctor = dyn_cast<CXXConstructExpr>(Inner))
-    RetType = Ctor->getType();
-  llvm::Constant *Res =
-      emitAbstract(CE->getBeginLoc(), CE->getAPValueResult(), RetType);
-  return Res;
+
+  QualType RetType = CE->getType();
+  if (CE->isGLValue())
+    RetType = CGM.getContext().getLValueReferenceType(RetType);
+
+  return emitAbstract(CE->getBeginLoc(), CE->getAPValueResult(), RetType);
 }
 
 llvm::Constant *

clang/test/CodeGenCXX/cxx20-consteval-crash.cpp

@@ -56,3 +56,18 @@ int foo() {
 // CHECK: define{{.*}} signext i8 @_ZN10Issue545782f2IcEEcT_(
 // CHECK: ret i8 4
 }
+
+namespace Issue55871 {
+struct Item {
+  consteval Item(char c) :_char{c}{}
+  char _char;
+};
+
+int function(const Item& item1, const Item& item2) {
+  return 0;
+}
+
+int foo() {
+  return function(Item{'a'}, Item{'a'});
+}
+} // namespace Issue58871