[Clang] use constant evaluation context for constexpr if conditions (#123667)

Fixes #123524

---

This PR addresses the issue of immediate function expressions not
properly evaluated in `constexpr` if conditions. Adding the
`ConstantEvaluated` context for expressions in `constexpr` if statements
ensures that these expressions are treated as manifestly
constant-evaluated and parsed correctly.

Author: a-tarasyuk

clang/docs/ReleaseNotes.rst

@@ -294,6 +294,7 @@ Bug Fixes to C++ Support
   direct-list-initialized from an array is corrected to direct-initialization.
 - Clang no longer crashes when a coroutine is declared ``[[noreturn]]``. (#GH127327)
 - Clang now uses the parameter location for abbreviated function templates in ``extern "C"``. (#GH46386)
+- Clang now correctly parses ``if constexpr`` expressions in immediate function context. (#GH123524)
 
 Improvements to C++ diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Parse/ParseExprCXX.cpp

@@ -2203,8 +2203,16 @@ Parser::ParseCXXCondition(StmtResult *InitStmt, SourceLocation Loc,
       return ParseCXXCondition(nullptr, Loc, CK, MissingOK);
     }
 
-    // Parse the expression.
-    ExprResult Expr = ParseExpression(); // expression
+    ExprResult Expr = [&] {
+      EnterExpressionEvaluationContext Eval(
+          Actions, Sema::ExpressionEvaluationContext::ConstantEvaluated,
+          /*LambdaContextDecl=*/nullptr,
+          /*ExprContext=*/Sema::ExpressionEvaluationContextRecord::EK_Other,
+          /*ShouldEnter=*/CK == Sema::ConditionKind::ConstexprIf);
+      // Parse the expression.
+      return ParseExpression(); // expression
+    }();
+
     if (Expr.isInvalid())
       return Sema::ConditionError();
 

clang/test/SemaCXX/constexpr-if.cpp

@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -std=c++20 -verify=cxx20,expected %s
+// RUN: %clang_cc1 -std=c++23 -verify=cxx23,expected %s
+// RUN: %clang_cc1 -std=c++26 -verify=cxx26,expected %s
+
+// expected-no-diagnostics
+
+namespace GH123524 {
+consteval void fn1() {}
+void fn2() {
+  if constexpr (&fn1 != nullptr) { }
+}
+}