From 77362661171c77b1a5ab27698f3c2d7c6a11c4c9 Mon Sep 17 00:00:00 2001
From: Ole Erik Peistorpet <OleErikPeistorpet@users.noreply.github.com>
Date: Sun, 26 May 2024 12:12:05 +0200
Subject: [PATCH] transform_iterator merely input_iterator when not const
 callable

---
 unit_test/view_gtest.cpp  |  2 +-
 view/transform_iterator.h | 12 +++++++-----
 2 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/unit_test/view_gtest.cpp b/unit_test/view_gtest.cpp
index e829428f..ff327c83 100644
--- a/unit_test/view_gtest.cpp
+++ b/unit_test/view_gtest.cpp
@@ -212,7 +212,7 @@ TEST(viewTest, viewTransformMutableLambda)
 	auto v = view::transform(dummy, iota);
 	using I = decltype(v.begin());
 
-	static_assert(std::is_same_v<I::iterator_category, std::bidirectional_iterator_tag>);
+	static_assert(std::is_same_v<I::iterator_category, std::input_iterator_tag>);
 #if OEL_STD_RANGES
 	static_assert(std::input_or_output_iterator<I>);
 	static_assert(std::ranges::range<decltype(v)>);
diff --git a/view/transform_iterator.h b/view/transform_iterator.h
index 3b34f5b5..d97dc955 100644
--- a/view/transform_iterator.h
+++ b/view/transform_iterator.h
@@ -26,11 +26,13 @@ class transform_iterator
 {
 	_detail::TightPair< Iterator, typename _detail::AssignableWrap<UnaryFunc>::Type > _m;
 
-	static constexpr bool _isBidirectional = iter_is_bidirectional<Iterator>;
+	inline static constexpr auto _isBidirectional = iter_is_bidirectional<Iterator>;
+	inline static constexpr auto _isConstCallable = std::is_invocable_v< UnaryFunc const, decltype(*_m.first) >;
 
 public:
-	using iterator_category = std::conditional_t<
-			std::is_copy_constructible_v<UnaryFunc>,
+	using iterator_category =
+		std::conditional_t<
+			std::is_copy_constructible_v<UnaryFunc> and _isConstCallable,
 			std::conditional_t<
 				_isBidirectional,
 				std::bidirectional_iterator_tag,
@@ -50,7 +52,7 @@ class transform_iterator
 	constexpr const Iterator & base() const & noexcept  OEL_ALWAYS_INLINE { return _m.first; }
 
 	constexpr reference operator*() const
-		OEL_REQUIRES(std::invocable< UnaryFunc const, decltype(*_m.first) >)
+		OEL_REQUIRES(_isConstCallable)
 		{
 			const UnaryFunc & f = _m.second();
 			return f(*_m.first);
@@ -100,7 +102,7 @@ class transform_iterator
 		(const transform_iterator & left, sentinel_wrapper<S> right)   { return left._m.first - right._s; }
 
 	constexpr bool operator==(const transform_iterator & right) const   { return _m.first == right._m.first; }
-
+	// These are not hidden friends because MSC 2017 gives error C3615
 	constexpr bool operator!=(const transform_iterator & right) const   { return _m.first != right._m.first; }
 
 	template< typename S >