[committed] libstdc++: Implement LWG 3404 for C++20 subrange [PR 100044]

[Jonathan Wakely <jwakely@redhat.com>]

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These deduction guides became useless with LWG 3282 (implemented in
commit r10-6741) and so were removed by LWG 3404.

libstdc++-v3/ChangeLog:

	PR libstdc++/100044
	* include/bits/ranges_util.h (__detail::__iterator_sentinel_pair):
	Remove helper concept.
	(subrange(_Pr), subrange(Pr, __make_unsigned_like<...>)): Remove
	deduction guides, as per LWG 3404.
	* testsuite/std/ranges/subrange/lwg3282_neg.cc: Check that class
	template argument deduction fails.

Tested powerpc64le-linux. Committed to trunk.

I'll backport this to gcc-10 too.


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commit 7569ce583f540ae22c585cc5159e3b23deedd987
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Apr 12 12:49:17 2021

    libstdc++: Implement LWG 3404 for C++20 subrange [PR 100044]
    
    These deduction guides became useless with LWG 3282 (implemented in
    commit r10-6741) and so were removed by LWG 3404.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/100044
            * include/bits/ranges_util.h (__detail::__iterator_sentinel_pair):
            Remove helper concept.
            (subrange(_Pr), subrange(Pr, __make_unsigned_like<...>)): Remove
            deduction guides, as per LWG 3404.
            * testsuite/std/ranges/subrange/lwg3282_neg.cc: Check that class
            template argument deduction fails.

diff --git a/libstdc++-v3/include/bits/ranges_util.h b/libstdc++-v3/include/bits/ranges_util.h
index ae53afd97bd..9d51e1b874e 100644
--- a/libstdc++-v3/include/bits/ranges_util.h
+++ b/libstdc++-v3/include/bits/ranges_util.h
@@ -186,11 +186,6 @@ namespace ranges
 	&& __convertible_to_non_slicing<_Up, tuple_element_t<0, _Tp>>
 	&& convertible_to<_Vp, tuple_element_t<1, _Tp>>;
 
-    template<typename _Tp>
-      concept __iterator_sentinel_pair
-	= !range<_Tp> && __pair_like<_Tp>
-	&& sentinel_for<tuple_element_t<1, _Tp>, tuple_element_t<0, _Tp>>;
-
   } // namespace __detail
 
   enum class subrange_kind : bool { unsized, sized };
@@ -352,16 +347,6 @@ namespace ranges
 	     __detail::__make_unsigned_like_t<iter_difference_t<_It>>)
       -> subrange<_It, _Sent, subrange_kind::sized>;
 
-  template<__detail::__iterator_sentinel_pair _Pr>
-    subrange(_Pr)
-      -> subrange<tuple_element_t<0, _Pr>, tuple_element_t<1, _Pr>>;
-
-  template<__detail::__iterator_sentinel_pair _Pr>
-    subrange(_Pr, __detail::__make_unsigned_like_t<iter_difference_t<
-						     tuple_element_t<0, _Pr>>>)
-      -> subrange<tuple_element_t<0, _Pr>, tuple_element_t<1, _Pr>,
-		  subrange_kind::sized>;
-
   template<borrowed_range _Rng>
     subrange(_Rng&&)
       -> subrange<iterator_t<_Rng>, sentinel_t<_Rng>,
diff --git a/libstdc++-v3/testsuite/std/ranges/subrange/lwg3282_neg.cc b/libstdc++-v3/testsuite/std/ranges/subrange/lwg3282_neg.cc
index d9894e4f137..170f4d2962b 100644
--- a/libstdc++-v3/testsuite/std/ranges/subrange/lwg3282_neg.cc
+++ b/libstdc++-v3/testsuite/std/ranges/subrange/lwg3282_neg.cc
@@ -29,3 +29,16 @@ struct Base {};
 struct Derived : Base {};
 subrange<Derived*> sd;
 subrange<Base*> sb = sd; // { dg-error "conversion" }
+
+void
+test_lwg3404()
+{
+  // LWG 3404. Finish removing subrange's conversions from pair-like
+  std::pair<char*, char*> p;
+  subrange sb1(p);			// { dg-error "no matching function" }
+  // { dg-error "class template argument deduction" "" { target *-*-* } 38 }
+  subrange sb2(p, p.second - p.first);	// { dg-error "no matching function" }
+  // { dg-error "class template argument deduction" "" { target *-*-* } 40 }
+
+  // { dg-prune-output "in requirements with" }
+}