[PATCH] libstdc++: reduce std::variant template instantiation depth

[Patrick Palka]

Tested on x86_64-pc-linux-gnu, does this look OK for trunk?

-- >8 --

The recursively defined constraints on _Variadic_union's user-defined
destructor (necessary for maintaining trivial destructibility of the
variant iff all of its alternatives are) effectively require a template
instantiation depth of 3x the number of variants, with the instantiation
stack looking like

  ...
  _Variadic_union<B, C, ...>
  std::is_trivially_destructible_v<_Variadic_union<B, C, ...>>
  _Variadic_union<A, B, C, ...>::~_Variadic_union()
  _Variadic_union<A, B, C, ...>
  ...

Ideally the template depth should be ~equal to the number of variants
(plus a constant).  Luckily it seems we don't need to compute trivial
destructibility of the alternatives at all from _Variadic_union, since
its only user _Variant_storage already has that information.  To that
end this patch removes these recursive constraints and instead passes
this information down from _Variant_storage.  After this patch, the
template instantiation depth for 87619.cc is ~270 instead of ~780.

libstdc++-v3/ChangeLog:

        * include/std/variant (__detail::__variant::_Variadic_union):
        Add bool __trivially_destructible template parameter.
        (__detail::__variant::_Variadic_union::~_Variadic_union):
        Use __trivially_destructible in constraints instead.
        (_Variant_storage): Pass __trivially_destructible value to
        _Variadic_union.
---
 libstdc++-v3/include/std/variant | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index 20a76c8aa87..4b9002e0917 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -392,7 +392,7 @@ namespace __variant
     };
 
   // Defines members and ctors.
-  template<typename... _Types>
+  template<bool __trivially_destructible, typename... _Types>
     union _Variadic_union
     {
       _Variadic_union() = default;
@@ -401,8 +401,8 @@ namespace __variant
        _Variadic_union(in_place_index_t<_Np>, _Args&&...) = delete;
     };
 
-  template<typename _First, typename... _Rest>
-    union _Variadic_union<_First, _Rest...>
+  template<bool __trivially_destructible, typename _First, typename... _Rest>
+    union _Variadic_union<__trivially_destructible, _First, _Rest...>
     {
       constexpr _Variadic_union() : _M_rest() { }
 
@@ -427,13 +427,12 @@ namespace __variant
       ~_Variadic_union() = default;
 
       constexpr ~_Variadic_union()
-       requires (!is_trivially_destructible_v<_First>)
-             || (!is_trivially_destructible_v<_Variadic_union<_Rest...>>)
+       requires (!__trivially_destructible)
       { }
 #endif
 
       _Uninitialized<_First> _M_first;
-      _Variadic_union<_Rest...> _M_rest;
+      _Variadic_union<__trivially_destructible, _Rest...> _M_rest;
     };
 
   // _Never_valueless_alt is true for variant alternatives that can
@@ -514,7 +513,7 @@ namespace __variant
        return this->_M_index != __index_type(variant_npos);
       }
 
-      _Variadic_union<_Types...> _M_u;
+      _Variadic_union<false, _Types...> _M_u;
       using __index_type = __select_index<_Types...>;
       __index_type _M_index;
     };
@@ -552,7 +551,7 @@ namespace __variant
        return this->_M_index != static_cast<__index_type>(variant_npos);
       }
 
-      _Variadic_union<_Types...> _M_u;
+      _Variadic_union<true, _Types...> _M_u;
       using __index_type = __select_index<_Types...>;
       __index_type _M_index;
     };
-- 
2.43.0.254.ga26002b628