[clang] bfdeb58 - [Clang] use constant evaluation context for constexpr if conditions (#123667)

[via cfe-commits]

Author: Oleksandr T.
Date: 2025-03-10T01:53:20+02:00
New Revision: bfdeb5873073ed94b63b1ef1f7e91845c1be88a1

URL: https://github.com/llvm/llvm-project/commit/bfdeb5873073ed94b63b1ef1f7e91845c1be88a1
DIFF: https://github.com/llvm/llvm-project/commit/bfdeb5873073ed94b63b1ef1f7e91845c1be88a1.diff

LOG: [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.

Added: 
    clang/test/SemaCXX/constexpr-if.cpp

Modified: 
    clang/docs/ReleaseNotes.rst
    clang/lib/Parse/ParseExprCXX.cpp

Removed: 
    


################################################################################
diff  --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 7859871b0493a..372a95c80717c 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/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
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

diff  --git a/clang/lib/Parse/ParseExprCXX.cpp b/clang/lib/Parse/ParseExprCXX.cpp
index 33a90e0cb8a42..26be78ee8ca15 100644
--- a/clang/lib/Parse/ParseExprCXX.cpp
+++ b/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();
 

diff  --git a/clang/test/SemaCXX/constexpr-if.cpp b/clang/test/SemaCXX/constexpr-if.cpp
new file mode 100644
index 0000000000000..494fc45c55c4e
--- /dev/null
+++ b/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) { }
+}
+}