Fix iterate subscribe exception specification (#2206)

* Fix iterate subscribe exception specification

* Format iterate subscribe exception specification

* Guard iterate exception test

Author: fallintoplace

include/exec/sequence/iterate.hpp

@@ -178,7 +178,12 @@ namespace experimental::execution
     struct __subscribe_fn
     {
       template <class _Range>
-      constexpr auto operator()(__ignore, _Range&& __range) noexcept
+      constexpr auto operator()(__ignore, _Range&& __range)
+        noexcept(noexcept(std::ranges::begin(static_cast<_Range&&>(__range)))
+                 && noexcept(std::ranges::end(static_cast<_Range&&>(__range)))
+                 && __nothrow_move_constructible<std::ranges::iterator_t<_Range>,
+                                                 std::ranges::sentinel_t<_Range>,
+                                                 _Receiver>)
       {
         return __operation{std::ranges::begin(static_cast<_Range&&>(__range)),
                            std::ranges::end(static_cast<_Range&&>(__range)),

test/exec/sequence/test_iterate.cpp

@@ -25,6 +25,28 @@
 namespace
 {
 
+#if !STDEXEC_NO_STDCPP_EXCEPTIONS()
+  struct begin_error
+  {};
+
+  struct begin_throws_range
+  {
+    using iterator = std::array<int, 1>::iterator;
+
+    iterator begin()
+    {
+      throw begin_error{};
+    }
+
+    iterator end() noexcept
+    {
+      return values.end();
+    }
+
+    std::array<int, 1> values{0};
+  };
+#endif  // !STDEXEC_NO_STDCPP_EXCEPTIONS()
+
   template <class Receiver>
   struct sum_item_rcvr
   {
@@ -140,4 +162,15 @@ namespace
     CHECK(sum == (42 + 43 + 44 + 1));
   }
 
+#if !STDEXEC_NO_STDCPP_EXCEPTIONS()
+  TEST_CASE("iterate - subscribe propagates begin exceptions", "[sequence_senders][iterate]")
+  {
+    auto iterate = exec::iterate(begin_throws_range{});
+    int  sum     = 0;
+
+    STATIC_REQUIRE_FALSE(noexcept(exec::subscribe(iterate, sum_receiver<>{.sum_ = sum})));
+    CHECK_THROWS_AS(exec::subscribe(iterate, sum_receiver<>{.sum_ = sum}), begin_error);
+  }
+#endif  // !STDEXEC_NO_STDCPP_EXCEPTIONS()
+
 }  // namespace