[Clang] use constant evaluation context for constexpr if conditions

clang/docs/ReleaseNotes.rst

@@ -977,6 +977,7 @@ Bug Fixes to C++ Support
 - Fix immediate escalation not propagating through inherited constructors.  (#GH112677)
 - Fixed assertions or false compiler diagnostics in the case of C++ modules for
   lambda functions or inline friend functions defined inside templates (#GH122493).
+- 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 +1,12 @@
+// 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) { }
+}
+}