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

We can reuse CheckActive() for this.

Author: tbaederr

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;
 }
 

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 {