[Clang] Disallow non-lvalue values in constant expressions to prevent invalid pointer offset computation (llvm#95479)

Fixes llvm#95366

Author: a-tarasyuk

clang/docs/ReleaseNotes.rst

@@ -859,6 +859,8 @@ Bug Fixes to C++ Support
   (#GH88081), (#GH89496), (#GH90669) and (#GH91633).
 - Fixed handling of brace ellison when building deduction guides. (#GH64625), (#GH83368).
 - Clang now instantiates local constexpr functions eagerly for constant evaluators. (#GH35052), (#GH94849)
+- Fixed a failed assertion when attempting to convert an integer representing the difference
+  between the addresses of two labels (a GNU extension) to a pointer within a constant expression. (#GH95366).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/AST/ExprConstant.cpp

@@ -9325,6 +9325,13 @@ bool PointerExprEvaluator::VisitCastExpr(const CastExpr *E) {
       Result.IsNullPtr = false;
       return true;
     } else {
+      // In rare instances, the value isn't an lvalue.
+      // For example, when the value is the difference between the addresses of
+      // two labels. We reject that as a constant expression because we can't
+      // compute a valid offset to convert into a pointer.
+      if (!Value.isLValue())
+        return false;
+
       // Cast is of an lvalue, no need to change value.
       Result.setFrom(Info.Ctx, Value);
       return true;

clang/test/Sema/integral-to-ptr.c

@@ -0,0 +1,3 @@
+// RUN: %clang_cc1 %s -verify -fsyntax-only -std=c11
+
+int x(void) { e: b: ; return &&e - &&b < x; } // expected-warning {{ordered comparison between pointer and integer}}