[Clang] Always constant-evaluate operands of comparisons to nullptr

even if we know what the result is going to be.
There may be side effects we ought not to ignore,

Fixes #64923

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D158601

clang/docs/ReleaseNotes.rst

@@ -198,6 +198,11 @@ Bug Fixes to C++ Support
 - Update ``FunctionDeclBitfields.NumFunctionDeclBits``. This fixes:
   (`#64171 <https://github.com/llvm/llvm-project/issues/64171>`_).
 
+- Expressions producing ``nullptr`` are correctly evaluated
+  by the constant interpreter when appearing as the operand
+  of a binary comparision.
+  (`#64923 <https://github.com/llvm/llvm-project/issues/64923>_``)
+
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 - Fixed an import failure of recursive friend class template.

clang/lib/AST/ExprConstant.cpp

@@ -13310,6 +13310,10 @@ EvaluateComparisonBinaryOperator(EvalInfo &Info, const BinaryOperator *E,
     // C++11 [expr.rel]p4, [expr.eq]p3: If two operands of type std::nullptr_t
     // are compared, the result is true of the operator is <=, >= or ==, and
     // false otherwise.
+    LValue Res;
+    if (!EvaluatePointer(E->getLHS(), Res, Info) ||
+        !EvaluatePointer(E->getRHS(), Res, Info))
+      return false;
     return Success(CmpResult::Equal, E);
   }
 

clang/test/SemaCXX/compare-cxx2a.cpp

@@ -461,3 +461,21 @@ namespace PR52537 {
   template<typename T> bool f6() { return 0 < y6; } // expected-note {{instantiation of}}
   void g6() { f6<int>(); } // expected-note {{instantiation of}}
 }
+
+
+namespace GH64923 {
+using nullptr_t = decltype(nullptr);
+struct MyTask{};
+constexpr MyTask DoAnotherThing() {
+    return {};
+}
+
+constexpr nullptr_t operator++(MyTask &&T); // expected-note 2{{declared here}}
+void DoSomething() {
+  if constexpr (++DoAnotherThing() != nullptr) {} // expected-error {{constexpr if condition is not a constant expression}} \
+                                                  // expected-note  {{undefined function 'operator++' cannot be used in a constant expression}}
+
+  if constexpr (nullptr == ++DoAnotherThing()) {} // expected-error {{constexpr if condition is not a constant expression}} \
+                                                  // expected-note  {{undefined function 'operator++' cannot be used in a constant expression}}
+}
+}