https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121529
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
So maybe:
--- a/libstdc++-v3/include/bits/valarray_after.h
+++ b/libstdc++-v3/include/bits/valarray_after.h
@@ -346,6 +346,7 @@ namespace __detail
operator _Op(const _Expr<_Dom1, typename _Dom1::value_type>& __v, \
const _Expr<_Dom2, typename _Dom2::value_type>& __w) \
{ \
+ __glibcxx_assert(__v.size() == __w.size()); \
typedef typename _Dom1::value_type _Arg; \
typedef typename __fun<_Name, _Arg>::result_type _Value; \
typedef _BinClos<_Name, _Expr, _Expr, _Dom1, _Dom2> _Closure; \
@@ -385,6 +386,7 @@ namespace __detail
operator _Op(const _Expr<_Dom,typename _Dom::value_type>& __e, \
const valarray<typename _Dom::value_type>& __v) \
{ \
+ __glibcxx_assert(__v.size() == __e.size()); \
typedef typename _Dom::value_type _Arg; \
typedef typename __fun<_Name, _Arg>::result_type _Value; \
typedef _BinClos<_Name, _Expr, _ValArray, _Dom, _Arg> _Closure; \
@@ -398,6 +400,7 @@ namespace __detail
operator _Op(const valarray<typename _Dom::value_type>& __v, \
const _Expr<_Dom, typename _Dom::value_type>& __e) \
{ \
+ __glibcxx_assert(__v.size() == __e.size()); \
typedef typename _Dom::value_type _Tp; \
typedef typename __fun<_Name, _Tp>::result_type _Value; \
typedef _BinClos<_Name, _ValArray, _Expr, _Tp, _Dom> _Closure; \
