[clang] [clang][bytecode] Allow operations on volatile objects in ctors (PR #174425)

[Timm Baeder via cfe-commits]

https://github.com/tbaederr created https://github.com/llvm/llvm-project/pull/174425

None

>From 7ca97b7a57638d722737d31332a9e6b7e9caef75 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timm=20B=C3=A4der?= <tbaeder at redhat.com>
Date: Mon, 5 Jan 2026 16:42:37 +0100
Subject: [PATCH] [clang][bytecode] Allow operations on volatile objects in
 ctors

---
 clang/lib/AST/ByteCode/Interp.cpp |  4 ++++
 clang/test/AST/ByteCode/cxx23.cpp | 20 +++++++++++++++-----
 2 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/clang/lib/AST/ByteCode/Interp.cpp b/clang/lib/AST/ByteCode/Interp.cpp
index 889ac1e1a9a7e..966c9d9b5bf66 100644
--- a/clang/lib/AST/ByteCode/Interp.cpp
+++ b/clang/lib/AST/ByteCode/Interp.cpp
@@ -626,6 +626,10 @@ static bool CheckVolatile(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
   if (!S.getLangOpts().CPlusPlus)
     return Invalid(S, OpPC);
 
+  // Volatile object can be written-to and read if they are being constructed.
+  if (llvm::is_contained(S.InitializingBlocks, Ptr.block()))
+    return true;
+
   // The reason why Ptr is volatile might be further up the hierarchy.
   // Find that pointer.
   Pointer P = Ptr;
diff --git a/clang/test/AST/ByteCode/cxx23.cpp b/clang/test/AST/ByteCode/cxx23.cpp
index 2a8b061d7671a..4a4ff4b48e8b5 100644
--- a/clang/test/AST/ByteCode/cxx23.cpp
+++ b/clang/test/AST/ByteCode/cxx23.cpp
@@ -395,7 +395,7 @@ namespace UnionMemberCallDiags {
 }
 #endif
 
-namespace VolatileWrites {
+namespace Volatile {
   constexpr void test1() {// all20-error {{never produces a constant expression}}
     int k;
     volatile int &m = k;
@@ -448,10 +448,8 @@ namespace VolatileWrites {
   }
   static_assert(test7(12)); // all-error {{not an integral constant expression}} \
                             // all-note {{in call to}}
-}
 
-namespace VolatileReads {
-  constexpr int test1(bool b) {
+  constexpr int test8(bool b) {
     if (!b)
       return -1;
     struct C {
@@ -460,8 +458,20 @@ namespace VolatileReads {
     volatile C c{12}; // all-note {{volatile object declared here}}
     return ((C&)(c)).a; // all-note {{read of volatile object}}
   }
-  static_assert(test1(true) == 0); // all-error {{not an integral constant expression}} \
+  static_assert(test8(true) == 0); // all-error {{not an integral constant expression}} \
                                    // all-note {{in call to}}
+
+  struct C {
+    int n;
+    constexpr C() : n(1) { n = 2; int b= n;}
+  };
+  constexpr int f(bool get) {
+    if (get)
+      return -1;
+    volatile C c;
+    return 2;
+  }
+  static_assert(f(false) == 2, "");
 }
 
 namespace AIEWithIndex0Narrows {