libstdc++: LWG 3301 transform_view::iterator has incorrect iterator_category

libstdc++-v3/ChangeLog:

	LWG 3301 transform_view::_Iterator has incorrect iterator_category
	* include/std/ranges (transform_view::_Iterator::_S_iter_cat): Adjust
	determination of iterator_category as per LWG 3301.
	* testsuite/std/ranges/adaptors/transform.cc: Augment test.

libstdc++-v3/ChangeLog

 2020-02-25  Patrick Palka  <ppalka@redhat.com>
 
+	LWG 3301 transform_view::_Iterator has incorrect iterator_category
+	* include/std/ranges (transform_view::_Iterator::_S_iter_cat): Adjust
+	determination of iterator_category as per LWG 3301.
+	* testsuite/std/ranges/adaptors/transform.cc: Augment test.
+
 	LWG 3292 iota_view is under-constrained
 	* include/std/ranges (iota_view): Require that _Winc models semiregular
 	  as per LWG 3292.

libstdc++-v3/include/std/ranges

@@ -1570,6 +1570,9 @@ namespace views
 	  static constexpr auto
 	  _S_iter_cat()
 	  {
+	    using _Res = invoke_result_t<_Fp&, range_reference_t<_Base>>;
+	    if constexpr (is_lvalue_reference_v<_Res>)
+	      {
 		using _Cat
 		  = typename iterator_traits<_Base_iter>::iterator_category;
 		if constexpr (derived_from<_Cat, contiguous_iterator_tag>)
@@ -1577,6 +1580,9 @@ namespace views
 		else
 		  return _Cat{};
 	      }
+	    else
+	      return input_iterator_tag{};
+	  }
 
 	  static constexpr decltype(auto)
 	  __iter_move(const _Iterator& __i = {})

libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc

@@ -77,10 +77,34 @@ test03()
   VERIFY( ranges::equal(v, (int[]){1,2,3,4,5}) );
 }
 
+void
+test04()
+{
+  // LWG 3301
+    {
+      auto f = [] (int x) { return x; };
+      int x[] = {1,2,3,4,5};
+      auto v = x | views::transform(f);
+      auto i = v.begin();
+      using Cat = decltype(i)::iterator_category;
+      static_assert(std::same_as<Cat, std::input_iterator_tag>);
+    }
+
+    {
+      auto f = [] (int &x) -> int& { return x; };
+      int x[] = {1,2,3,4,5};
+      auto v = x | views::transform(f);
+      auto i = v.begin();
+      using Cat = decltype(i)::iterator_category;
+      static_assert(std::derived_from<Cat, std::forward_iterator_tag>);
+    }
+}
+
 int
 main()
 {
   test01();
   test02();
   test03();
+  test04();
 }