[Clang][ExprConstant] fix constant expression did not evaluate to integer (#97146)

fixes #96670

The cause is that we might return a lvalue here at


https://github.com/llvm/llvm-project/blob/3e53c97d33210db68188e731e93ee48dbaeeae32/clang/lib/AST/ExprConstant.cpp#L15861-L15865

This PR will make sure we return a rvalue in `FastEvaluateAsRValue`.

Author: Backl1ght

clang/docs/ReleaseNotes.rst

@@ -967,6 +967,8 @@ Bug Fixes to C++ Support
 - Fixed an assertion failure about invalid conversion when calling lambda. (#GH96205).
 - Fixed a bug where the first operand of binary ``operator&`` would be transformed as if it was the operand
   of the address of operator. (#GH97483).
+- Fixed an assertion failure about a constant expression which is a known integer but is not
+  evaluated to an integer. (#GH96670).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/AST/ExprConstant.cpp

@@ -15859,9 +15859,12 @@ static bool FastEvaluateAsRValue(const Expr *Exp, Expr::EvalResult &Result,
 
   if (const auto *CE = dyn_cast<ConstantExpr>(Exp)) {
     if (CE->hasAPValueResult()) {
-      Result.Val = CE->getAPValueResult();
-      IsConst = true;
-      return true;
+      APValue APV = CE->getAPValueResult();
+      if (!APV.isLValue()) {
+        Result.Val = std::move(APV);
+        IsConst = true;
+        return true;
+      }
     }
 
     // The SubExpr is usually just an IntegerLiteral.

clang/test/SemaCXX/eval-crashes.cpp

@@ -61,3 +61,13 @@ struct array {
   array() : data(*new int[1][2]) {}
 };
 }
+
+namespace GH96670 {
+inline constexpr long ullNil = -1;
+
+template<typename T = long, const T &Nil = ullNil>
+struct Test {};
+
+inline constexpr long lNil = -1;
+Test<long, lNil> c;
+}