[clang] Placement new error when modifying consts

[mariusdr]

Closes #131432

Raise an error when placement new is used to modify a const-qualified variable in a constexpr function.

Note that diag::note_constexpr_modify_const_type is used since the it is also raised when the sample program is compiled with '-fexperimental-new-constant-interpreter'.

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[llvmbot]

@llvm/pr-subscribers-clang

Author: marius doerner (mariusdr)

Changes

Closes #131432

Raise an error when placement new is used to modify a const-qualified variable in a constexpr function.

Note that diag::note_constexpr_modify_const_type is used since the it is also raised when the sample program is compiled with '-fexperimental-new-constant-interpreter'.

---

Full diff: https://github.com/llvm/llvm-project/pull/132460.diff

2 Files Affected:

• (modified) clang/lib/AST/ExprConstant.cpp (+9)
• (modified) clang/test/AST/ByteCode/placement-new.cpp (+39)

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 92a28897cf3ee..b4fc3d4471064 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -10415,7 +10415,16 @@ bool PointerExprEvaluator::VisitCXXNewExpr(const CXXNewExpr *E) {
 
       typedef bool result_type;
       bool failed() { return false; }
+      bool checkConst(QualType QT) {
+        if (QT.isConstQualified()) {
+          Info.FFDiag(E, diag::note_constexpr_modify_const_type) << QT;
+          return false;
+        }
+        return true;
+      }
       bool found(APValue &Subobj, QualType SubobjType) {
+        if (!checkConst(SubobjType))
+          return false;
         // FIXME: Reject the cases where [basic.life]p8 would not permit the
         // old name of the object to be used to name the new object.
         unsigned SubobjectSize = 1;
diff --git a/clang/test/AST/ByteCode/placement-new.cpp b/clang/test/AST/ByteCode/placement-new.cpp
index c353162a7aab0..9b12c9f2b1714 100644
--- a/clang/test/AST/ByteCode/placement-new.cpp
+++ b/clang/test/AST/ByteCode/placement-new.cpp
@@ -376,3 +376,42 @@ constexpr int N = [] // expected-error {{must be initialized by a constant expre
     return s.a[0];
 }();
 #endif
+
+constexpr int modify_const_variable() {
+  const int a = 10;
+  new ((int *)&a) int(12); // both-note {{modification of object of const-qualified type 'const int' is not allowed in a constant expression}}
+  return a;
+}
+static_assert(modify_const_variable()); // both-error {{not an integral constant expression}} \
+                                        // both-note {{in call to}}
+
+typedef const int T0;
+typedef T0 T1;
+constexpr T1 modify_const_variable_td() {
+  T1 a = 10;
+  new ((int *)&a) int(12); // both-note {{modification of object of const-qualified type 'T1' (aka 'const int') is not allowed in a constant expression}}
+  return a;
+}
+static_assert(modify_const_variable_td()); // both-error {{not an integral constant expression}} \
+                                           // both-note {{in call to}}
+
+template<typename T>
+constexpr T modify_const_variable_tmpl() {
+  T a = 10;
+  new ((int *)&a) int(12); // both-note {{modification of object of const-qualified type 'const int' is not allowed in a constant expression}}
+  return a;
+}
+static_assert(modify_const_variable_tmpl<const int>()); // both-error {{not an integral constant expression}} \
+                                                        // both-note {{in call to}}
+
+namespace ModifyMutableMember {
+  struct S {
+    mutable int a {10};
+  };
+  constexpr int modify_mutable_member() {
+    const S s;
+    new ((int *)&s.a) int(12);
+    return s.a;
+  }
+  static_assert(modify_mutable_member() == 12);
+}

[tbaederr]

The tests should go into a file not under ByteCode/ since they aren't specific to the bytecode interpreter. There should be existing placement new tests in SemaCXX/.

[mariusdr]

The tests should go into a file not under ByteCode/ since they aren't specific to the bytecode interpreter. There should be existing placement new tests in SemaCXX/.

Thanks for the hint, moved them to clang/test/SemaCXX/cxx2c-constexpr-placement-new.cpp

[cor3ntin]

This change needs a release note.
Please add an entry to clang/docs/ReleaseNotes.rst in the section the most adapted to the change, and referencing any Github issue this change fixes. Thanks!

[mariusdr]

Added release notes under "Bug Fixes to C++ Support". Thanks for the review.

[cor3ntin]

Thanks.
Will you need me to merge that for you?

[mariusdr]

Thanks. Will you need me to merge that for you?

Yes, please

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