[Clang] use constant evaluation context for constexpr if conditions

[a-tarasyuk]

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.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Oleksandr T. (a-tarasyuk)

Changes

Fixes #123524

---

Full diff: https://github.com/llvm/llvm-project/pull/123667.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+1-1)
• (modified) clang/lib/Parse/ParseExprCXX.cpp (+6)
• (added) clang/test/SemaCXX/constexpr-if.cpp (+10)

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) { }
+}
+}

[shafik]

Please add a more detailed summary to this PR. I have asked this on several PRs and I believe the confusion reflected in the comments is very much attributed to not explaining why you believe your fix works. It may be incorrect but a solid summary will allow the reviewers to quickly understand the assumptions you are making in your fix and should reduce back and forth and allow for more targeted feedback quicker.

[erichkeane]

This now lacks my concerns, as this is only applying to the singular expression. But parsing isn't my expertise, so @cor3ntin or @Endilll should probably think about this for a minute or two.