[Clang][AST] Fix HandleLValueBase to deal with references (#140105)

Since P2280R4 Unknown references and pointers was implemented,
HandleLValueBase now has to deal with referneces:

D.MostDerivedType->getAsCXXRecordDecl()

will return a nullptr if D.MostDerivedType is a ReferenceType. The fix
is to use getNonReferenceType() to obtain the Pointee Type if we have a
reference.

Fixes: #139452

Author: shafik

clang/docs/ReleaseNotes.rst

@@ -615,6 +615,8 @@ Bug Fixes in This Version
   argument which contains a pragma. (#GH113722)
 - Fixed assertion failures when generating name lookup table in modules. (#GH61065, #GH134739)
 - Fixed an assertion failure in constant compound literal statements. (#GH139160)
+- Fix crash due to unknown references and pointer implementation and handling of
+  base classes. (GH139452)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/AST/ExprConstant.cpp

@@ -3314,7 +3314,11 @@ static bool HandleLValueBase(EvalInfo &Info, const Expr *E, LValue &Obj,
     return false;
 
   // Extract most-derived object and corresponding type.
-  DerivedDecl = D.MostDerivedType->getAsCXXRecordDecl();
+  // FIXME: After implementing P2280R4 it became possible to get references
+  // here. We do MostDerivedType->getAsCXXRecordDecl() in several other
+  // locations and if we see crashes in those locations in the future
+  // it may make more sense to move this fix into Lvalue::set.
+  DerivedDecl = D.MostDerivedType.getNonReferenceType()->getAsCXXRecordDecl();
   if (!CastToDerivedClass(Info, E, Obj, DerivedDecl, D.MostDerivedPathLength))
     return false;
 

clang/test/SemaCXX/constant-expression-p2280r4.cpp

@@ -178,3 +178,24 @@ namespace extern_reference_used_as_unknown {
   int y;
   constinit int& g = (x,y); // expected-warning {{left operand of comma operator has no effect}}
 }
+
+namespace GH139452 {
+struct Dummy {
+  explicit operator bool() const noexcept { return true; }
+};
+
+struct Base { int error; };
+struct Derived : virtual Base { };
+
+template <class R>
+constexpr R get_value() {
+    const auto& derived_val = Derived{};
+    if (derived_val.error != 0)
+        /* nothing */;
+    return R{};
+}
+
+int f() {
+    return !get_value<Dummy>(); // contextually convert the function call result to bool
+}
+}