[PATCH] c++, v3: Implement C++26 P3074R7 and CWG3189 - trivial unions [PR119059]

[Jakub Jelinek]

On Thu, May 21, 2026 at 05:28:03PM +0200, Jakub Jelinek wrote:
> > Before C++26, a non-trivial variant member ctor makes t's deleted, so I
> > guess triviality never mattered?
> 
> For ctor with the exception of variant member with DMI, but then the default
> ctor is marked non-trivial because of
>               if (trivial_p)
>                 *trivial_p = false;
> in the DECL_INITIAL (field) walk_field_subobs handling.
> So shall I turn this
> > 
> > > +	  if (cxx_dialect < cxx26
> > > +	      || TREE_CODE (DECL_CONTEXT (field)) != UNION_TYPE)
> 
> into
> 	  if (TREE_CODE (DECL_CONTEXT (field)) != UNION_TYPE)
> ?

Tested incremental
--- gcc/cp/class.cc.jj	2026-05-20 22:41:11.372062501 +0200
+++ gcc/cp/class.cc	2026-05-21 17:35:59.834804002 +0200
@@ -3912,9 +3912,12 @@ check_field_decl (tree field,
 	    |= TYPE_HAS_COMPLEX_MOVE_ASSIGN (type);
 	  TYPE_HAS_COMPLEX_MOVE_CTOR (t) |= TYPE_HAS_COMPLEX_MOVE_CTOR (type);
 	  /* In C++26, triviality of default ctor or dtor of a variant member
-	     doesn't matter for triviality of the t's default ctor or dtor.  */
-	  if (cxx_dialect < cxx26
-	      || TREE_CODE (DECL_CONTEXT (field)) != UNION_TYPE)
+	     doesn't matter for triviality of the t's default ctor or dtor.
+	     Before C++26, non-trivial default ctor or dtor of a variant
+	     member makes it deleted with the exception of default ctor
+	     when DMI is present, but in that case default ctor is
+	     non-trivial.   */
+	  if (TREE_CODE (DECL_CONTEXT (field)) != UNION_TYPE)
 	    {
 	      TYPE_HAS_NONTRIVIAL_DESTRUCTOR (t)
 		|= TYPE_HAS_NONTRIVIAL_DESTRUCTOR (type);

and it didn't regress anything in
GXX_TESTSUITE_STDS=98,11,14,17,20,23,26 make -j32 -k check-g++

	Jakub