[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
(cherry picked from commit 136f2ba)

# Conflicts:
#	clang/docs/ReleaseNotes.rst

Author: shafik

clang/docs/ReleaseNotes.rst

@@ -909,6 +909,8 @@ Bug Fixes in This Version
   being deleted has a potentially throwing destructor (#GH118660).
 - Clang now outputs correct values when #embed data contains bytes with negative
   signed char values (#GH102798).
+- 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

@@ -3311,7 +3311,11 @@ 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

@@ -179,3 +179,24 @@ 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
+}
+}