[clang] [clang][bytecode] Diagnose placement-new construction to inactive field (PR #114047)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Timm Baeder (tbaederr)

<details>
<summary>Changes</summary>

We can reuse CheckActive() for this.

---
Full diff: https://github.com/llvm/llvm-project/pull/114047.diff


2 Files Affected:

- (modified) clang/lib/AST/ByteCode/Interp.cpp (+5) 
- (modified) clang/test/AST/ByteCode/placement-new.cpp (+15-1) 


``````````diff
diff --git a/clang/lib/AST/ByteCode/Interp.cpp b/clang/lib/AST/ByteCode/Interp.cpp
index 6e45cfb7e8a20c..513d4512b45cff 100644
--- a/clang/lib/AST/ByteCode/Interp.cpp
+++ b/clang/lib/AST/ByteCode/Interp.cpp
@@ -1451,6 +1451,11 @@ bool CheckNewTypeMismatch(InterpState &S, CodePtr OpPC, const Expr *E,
         << StorageType << AllocType;
     return false;
   }
+
+  // Can't activate fields in a union, unless the direct base is the union.
+  if (Ptr.inUnion() && !Ptr.isActive() && !Ptr.getBase().getRecord()->isUnion())
+    return CheckActive(S, OpPC, Ptr, AK_Construct);
+
   return true;
 }
 
diff --git a/clang/test/AST/ByteCode/placement-new.cpp b/clang/test/AST/ByteCode/placement-new.cpp
index 5673b5cba3f700..56f54ff168f3e8 100644
--- a/clang/test/AST/ByteCode/placement-new.cpp
+++ b/clang/test/AST/ByteCode/placement-new.cpp
@@ -14,7 +14,9 @@ namespace std {
   template<typename T, typename ...Args>
   constexpr void construct_at(void *p, Args &&...args) {
     new (p) T((Args&&)args...); // both-note {{in call to}} \
-                                // both-note {{placement new would change type of storage from 'int' to 'float'}}
+                                // both-note {{placement new would change type of storage from 'int' to 'float'}} \
+                                // both-note {{construction of subobject of member 'x' of union with active member 'a' is not allowed in a constant expression}}
+
   }
 }
 
@@ -284,6 +286,18 @@ namespace ConstructAt {
   static_assert(bad_construct_at_type()); // both-error {{not an integral constant expression}} \
                                           // both-note {{in call}}
 
+  constexpr bool bad_construct_at_subobject() {
+    struct X { int a, b; };
+    union A {
+      int a;
+      X x;
+    };
+    A a = {1};
+    std::construct_at<int>(&a.x.a, 1); // both-note {{in call}}
+    return true;
+  }
+  static_assert(bad_construct_at_subobject()); // both-error{{not an integral constant expression}} \
+                                               // both-note {{in call}}
 }
 
 namespace UsedToCrash {

``````````

</details>


https://github.com/llvm/llvm-project/pull/114047