Fix STLIterator postfix ++

It seems nothing actually requires std::output_iterator, probably bc of the
bogus postfix ++ requirement. So drop it.

* ra/ply.hh (STLIterator): Return void on postfix ++.
* test/big-0.cc: Don't require std::output_iterator.
* test/stl-compat.cc: Test STLIterator of map as output.

ra/big.hh

@@ -741,7 +741,7 @@ ravel_free(View<T, RANK> const & a)
     int r = a.rank()-1;
     for (; r>=0 && a.len(r)==1; --r) {}
     ra::dim_t s = r<0 ? 1 : a.step(r);
-    return ra::View<T, 1>({{size(a), s}}, a.cp);
+    return ra::View<T, 1>({{ra::size(a), s}}, a.cp);
 }
 
 template <class T, rank_t RANK, class S>

ra/ply.hh

@@ -393,8 +393,8 @@ struct STLIterator
         }
         return *this;
     }
-// std::input_iterator allows void, but std::output_iterator doesn't (p0541). Avoid
-    STLIterator & operator++(int) { static_assert(always_false<A>); }
+// see p0541 and p2550. Or just avoid.
+    void operator++(int) { return ++(*this); }
 };
 
 template <class A> STLIterator(A &&) -> STLIterator<A>;

test/big-0.cc

@@ -27,7 +27,7 @@ int main(int argc, char * * argv)
         ra::View<int, 2> a;
         static_assert(ra::rank_s<decltype(a().iter<0>())>()==ra::rank_s<decltype(a().iter())>());
         static_assert(std::input_iterator<decltype(a.begin())>);
-        static_assert(std::output_iterator<decltype(a.begin()), int>);
+        // static_assert(std::weak_output_iterator<decltype(a.begin()), int>); // p2550 when ready c++
     }
     tr.section("constructors");
     {

test/stl-compat.cc

@@ -106,6 +106,14 @@ int main()
         std::ranges::copy(std::ranges::subrange(ra::STLIterator(a+1), std::default_sentinel), b.begin());
         tr.test_eq(ra::ravel_free(a) + 1, b);
     }
+    tr.section("STLIterator as output");
+    {
+        using complex = std::complex<double>;
+        ra::Big<complex, 3> a({4, 2, 3}, ra::_0 - ra::_1 + ra::_2);
+        ra::Big<double, 1> b(4*2*3, real_part(ra::ravel_free(a)));
+        std::ranges::copy(std::ranges::subrange(b), ra::STLIterator(imag_part(a)));
+        tr.test_eq((ra::_0 - ra::_1 + ra::_2)*1.*complex(1, 1), a);
+    }
 #if __cpp_lib_span >= 202002L
     tr.section("std::span");
     {