policy/feerate.h: avoid constraint self-dependency

In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/format:48,
                 from /usr/lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/bits/chrono_io.h:39,
                 from /usr/lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/chrono:3362,
                 from ./util/time.h:9,
                 from ./primitives/block.h:12,
                 from ./blockencodings.h:8,
                 from blockencodings.cpp:5:
/usr/lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/type_traits: In substitution of 'template<class _Up>  requires !(is_same_v<std::optional<_Tp>, typename std::remove_cvref<_It2>::type>) && (is_constructible_v<_Tp, const _Up&>) && (__construct_from_contained_value<_Up, typename std::remove_cv< <template-parameter-1-1> >::type>) constexpr std::optional<CFeeRate>::optional(const std::optional<_Tp>&) [with _Up = CFeeRate]':
/usr/lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/type_traits:1140:25:   required by substitution of 'template<class _Tp, class ... _Args> using std::__is_constructible_impl = std::__bool_constant<__is_constructible(_Tp, _Args ...)> [with _Tp = CFeeRate; _Args = {std::optional<CFeeRate>&}]'
 1140 |       = __bool_constant<__is_constructible(_Tp, _Args...)>;
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/type_traits:1145:12:   required from 'struct std::is_constructible<CFeeRate, std::optional<CFeeRate>&>'
 1145 |     struct is_constructible
      |            ^~~~~~~~~~~~~~~~
/usr/lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/type_traits:178:35:   required by substitution of 'template<class ... _Bn> std::__detail::__first_t<std::integral_constant<bool, false>, typename std::enable_if<(!(bool)(_Bn::value)), void>::type ...> std::__detail::__or_fn(int) [with _Bn = {std::is_constructible<CFeeRate, std::optional<CFeeRate>&>, std::is_convertible<std::optional<CFeeRate>&, CFeeRate>, std::is_constructible<CFeeRate, std::optional<CFeeRate> >, std::is_convertible<std::optional<CFeeRate>, CFeeRate>, std::is_constructible<CFeeRate, const std::optional<CFeeRate>&>, std::is_convertible<const std::optional<CFeeRate>&, CFeeRate>, std::is_constructible<CFeeRate, const std::optional<CFeeRate> >, std::is_convertible<const std::optional<CFeeRate>, CFeeRate>}]'
  178 |                                      __enable_if_t<!bool(_Bn::value)>...>;
      |                                                               ^~~~~
/usr/lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/type_traits:196:41:   required from 'struct std::__or_<std::is_constructible<CFeeRate, std::optional<CFeeRate>&>, std::is_convertible<std::optional<CFeeRate>&, CFeeRate>, std::is_constructible<CFeeRate, std::optional<CFeeRate> >, std::is_convertible<std::optional<CFeeRate>, CFeeRate>, std::is_constructible<CFeeRate, const std::optional<CFeeRate>&>, std::is_convertible<const std::optional<CFeeRate>&, CFeeRate>, std::is_constructible<CFeeRate, const std::optional<CFeeRate> >, std::is_convertible<const std::optional<CFeeRate>, CFeeRate> >'
  196 |     : decltype(__detail::__or_fn<_Bn...>(0))
      |                ~~~~~~~~~~~~~~~~~~~~~~~~~^~~
/usr/lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/optional:824:45:   required from 'constexpr const bool std::optional<CFeeRate>::__construct_from_contained_value<CFeeRate, CFeeRate>'
  824 |           = !__converts_from_optional<_Tp, _From>::value;
      |                                                    ^~~~~
/usr/lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/optional:884:7:   required by substitution of 'template<class _Up>  requires !(is_same_v<std::optional<_Tp>, typename std::remove_cvref<_It2>::type>) && (is_constructible_v<_Tp, const _Up&>) && (__construct_from_contained_value<_Up, typename std::remove_cv< <template-parameter-1-1> >::type>) constexpr std::optional<CFeeRate>::optional(const std::optional<_Tp>&) [with _Up = CFeeRate]'
  884 |           && __construct_from_contained_value<_Up>
      |              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./validation.h:164:41:   required from here
  164 |         return MempoolAcceptResult(state);
      |                                         ^
/usr/lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/optional:886:2:   required by the constraints of 'template<class _Tp> template<class _Up>  requires !(is_same_v<std::optional<_Tp>, typename std::remove_cvref<_It2>::type>) && (is_constructible_v<_Tp, const _Up&>) && (__construct_from_contained_value<_Up, typename std::remove_cv< <template-parameter-1-1> >::type>) constexpr std::optional<_Tp>::optional(const std::optional<_From>&)'
/usr/lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/optional:884:14: error: satisfaction of atomic constraint '__construct_from_contained_value<_Up, typename std::remove_cv< <template-parameter-1-1> >::type> [with _Tp = _Tp; _Up = _Up]' depends on itself
  884 |           && __construct_from_contained_value<_Up>
      |              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

src/policy/feerate.h

@@ -38,10 +38,8 @@ class CFeeRate
 public:
     /** Fee rate of 0 satoshis per kvB */
     CFeeRate() : nSatoshisPerK(0) { }
-    template<typename I>
+    template<std::integral I> // Disallow silent float -> int conversion
     explicit CFeeRate(const I _nSatoshisPerK): nSatoshisPerK(_nSatoshisPerK) {
-        // We've previously had bugs creep in from silent double->int conversion...
-        static_assert(std::is_integral<I>::value, "CFeeRate should be used without floats");
     }
 
     /**