add missing deduction guide for inplace_stop_callback

[stopcallback.inplace.general] declares a deduction guide so that
`inplace_stop_callback cb{token, fn};` deduces the callback type.
stdexec did not provide it, so class template argument deduction
failed. Add the guide and a test.

Fixes #1420

Author: kwy404

include/stdexec/stop_token.hpp

@@ -259,6 +259,9 @@ STDEXEC_P2300_NAMESPACE_BEGIN()
     STDEXEC_ATTRIBUTE(no_unique_address) _Fun __fun_;
   };
 
+  template <class _Fun>
+  inplace_stop_callback(inplace_stop_token, _Fun) -> inplace_stop_callback<_Fun>;
+
   inline inplace_stop_source::~inplace_stop_source()
   {
     STDEXEC_ASSERT((__state_.load(STDEXEC::__std::memory_order_relaxed) & __locked_flag_) == 0);

test/stdexec/concepts/test_concepts_stop_tokens.cpp

@@ -50,4 +50,11 @@ namespace
                                   std::stop_callback<on_stop_request>>);
 #endif
   }
+
+  TEST_CASE("inplace_stop_callback supports class template argument deduction", "[stop_token]")
+  {
+    ::STDEXEC::inplace_stop_source   source;
+    ::STDEXEC::inplace_stop_callback cb{source.get_token(), on_stop_request{}};
+    STATIC_REQUIRE(std::is_same_v<decltype(cb), ::STDEXEC::inplace_stop_callback<on_stop_request>>);
+  }
 }  // namespace