[Clang] use constant evaluation context for constexpr if conditions

clang/docs/ReleaseNotes.rst

@@ -963,7 +963,7 @@ Bug Fixes to C++ Support
 - Fixed a crash caused by the incorrect construction of template arguments for CTAD alias guides when type
   constraints are applied. (#GH122134)
 - Fixed canonicalization of pack indexing types - Clang did not always recognized identical pack indexing. (#GH123033)
-
+- Clang now permits the use of immediate-escalating expressions in ``constexpr`` if conditions. (#GH123524)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Parse/ParseExprCXX.cpp

@@ -2203,6 +2203,12 @@ Parser::ParseCXXCondition(StmtResult *InitStmt, SourceLocation Loc,
       return ParseCXXCondition(nullptr, Loc, CK, MissingOK);
     }
 
+    EnterExpressionEvaluationContext Eval(
+        Actions, Sema::ExpressionEvaluationContext::ConstantEvaluated,
+        /*LambdaContextDecl=*/nullptr,
+        /*ExprContext=*/Sema::ExpressionEvaluationContextRecord::EK_Other,
+        /*ShouldEnter=*/CK == Sema::ConditionKind::ConstexprIf);
+
     // Parse the expression.
     ExprResult Expr = ParseExpression(); // expression
     if (Expr.isInvalid())

clang/test/SemaCXX/constexpr-if.cpp

@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -std=c++26 -verify %s
+
+// expected-no-diagnostics
+
+namespace GH123524 {
+consteval void fn1() {}
+void fn2() {
+  if constexpr (&fn1 != nullptr) { }
+}
+}