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

[Oleksandr T. via cfe-commits]

https://github.com/a-tarasyuk created https://github.com/llvm/llvm-project/pull/123667

Fixes #123524

>From 00b8b64879ad3ae35a0a671da563ac876ffaf228 Mon Sep 17 00:00:00 2001
From: Oleksandr T <oleksandr.tarasiuk at outlook.com>
Date: Mon, 20 Jan 2025 22:51:00 +0200
Subject: [PATCH] [Clang] use constant evaluation context for constexpr if
 conditions

---
 clang/docs/ReleaseNotes.rst         |  2 +-
 clang/lib/Parse/ParseExprCXX.cpp    |  6 ++++++
 clang/test/SemaCXX/constexpr-if.cpp | 10 ++++++++++
 3 files changed, 17 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/SemaCXX/constexpr-if.cpp

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index f6d5c346021d60..cd16ce13a4e6b6 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/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
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Parse/ParseExprCXX.cpp b/clang/lib/Parse/ParseExprCXX.cpp
index 33a90e0cb8a42a..e174d9a24e7440 100644
--- a/clang/lib/Parse/ParseExprCXX.cpp
+++ b/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())
diff --git a/clang/test/SemaCXX/constexpr-if.cpp b/clang/test/SemaCXX/constexpr-if.cpp
new file mode 100644
index 00000000000000..1832086fee42d4
--- /dev/null
+++ b/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) { }
+}
+}