[gcc r10-10929] libstdc++: Remove precondition checks from ranges::subrange

[Jonathan Wakely]

https://gcc.gnu.org/g:45a31caac6dce9c08fcbdb2436d8ec6dca4e5f3c

commit r10-10929-g45a31caac6dce9c08fcbdb2436d8ec6dca4e5f3c
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Jun 15 15:07:25 2021 +0100

    libstdc++: Remove precondition checks from ranges::subrange
    
    The assertion in the subrange constructor causes semantic changes,
    because the call to ranges::distance performs additional operations that
    are not part of the constructor's specification. That will fail to
    compile if the iterator is move-only, because the argument to
    ranges::distance is passed by value. It will modify the subrange if the
    iterator is not a forward iterator, because incrementing the copy also
    affects the _M_begin member. Those problems could be prevented by using
    if-constexpr to only do the assertion for copyable forward iterators,
    but the call to ranges::distance can also prevent the constructor being
    usable in constant expressions. If the member initializers are usable in
    constant expressions, but iterator increments of equality comparisons
    are not, then the checks done by __glibcxx_assert might
    make constant evaluation fail.
    
    This change removes the assertion.
    
    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
    
    libstdc++-v3/ChangeLog:
    
            * include/std/ranges (subrange(i, s, n)): Remove assertion.
            * testsuite/std/ranges/subrange/constexpr.cc: New test.
    
    (cherry picked from commit a88fc03ba7e52d9a072f25d42bb9619fedb7892e)

Diff:
---
 libstdc++-v3/include/std/ranges                    |  2 --
 .../testsuite/std/ranges/subrange/constexpr.cc     | 26 ++++++++++++++++++++++
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index faa55ebceb7..2ff8dbd53ed 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -264,8 +264,6 @@ namespace ranges
 	requires (_Kind == subrange_kind::sized)
       : _M_begin(std::move(__i)), _M_end(__s)
       {
-	using __detail::__to_unsigned_like;
-	__glibcxx_assert(__n == __to_unsigned_like(ranges::distance(__i, __s)));
 	if constexpr (_S_store_size)
 	  _M_size._M_size = __n;
       }
diff --git a/libstdc++-v3/testsuite/std/ranges/subrange/constexpr.cc b/libstdc++-v3/testsuite/std/ranges/subrange/constexpr.cc
new file mode 100644
index 00000000000..f5bc52bef84
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/ranges/subrange/constexpr.cc
@@ -0,0 +1,26 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do compile { target c++20 } }
+
+#include <ranges>
+
+struct iterator
+{
+  using difference_type = int;
+
+  int i;
+
+  int operator*() const { return i; }
+
+  // These are intentionally not constexpr:
+  iterator& operator++() { ++i; return *this; }
+  iterator operator++(int) { return {i++}; }
+  bool operator==(const iterator& it) const { return i == it.i; }
+};
+
+constexpr iterator begin(1), end(2);
+
+using std::ranges::subrange;
+using std::ranges::subrange_kind;
+
+// This used to fail due to using operator++ and operator== in an assertion:
+constexpr subrange<iterator, iterator, subrange_kind::sized> s(begin, end, 1);