Fix is_narrowing_v - closes hsutter#1257

The current implementation of `is_narrowing_v` does not handle type conversions correctly.
For example, `cpp2::impl::is_narrowing_v<std::size_t, int>` returns `false`,
indicating that it mistakenly perceives the conversion from a signed `int` to an unsigned `size_t`
as potentially non-narrowing.

Closes hsutter#1257

include/cpp2util.h

@@ -719,6 +719,9 @@ concept valid_custom_is_operator = predicate_member_fun<X, F, &F::op_is>
                         || brace_initializable_to<X, argument_of_op_is_t<F>> 
                       );
 
+template <typename T>
+concept arithmetic = std::is_arithmetic_v<std::remove_cvref_t<T>>;
+
 //-----------------------------------------------------------------------
 //
 //  General helpers
@@ -1736,20 +1739,7 @@ inline constexpr auto is( X const& x, bool (*value)(X const&) ) -> bool {
 //  If it's confusing, we can switch this to <From, To>
 
 template< typename To, typename From >
-inline constexpr auto is_narrowing_v =
-    // [dcl.init.list] 7.1
-    (std::is_floating_point_v<From> && std::is_integral_v<To>) ||
-    // [dcl.init.list] 7.2
-    (std::is_floating_point_v<From> && std::is_floating_point_v<To> && sizeof(From) > sizeof(To)) ||
-    // [dcl.init.list] 7.3
-    (std::is_integral_v<From> && std::is_floating_point_v<To>) ||
-    (std::is_enum_v<From> && std::is_floating_point_v<To>) ||
-    // [dcl.init.list] 7.4
-    (std::is_integral_v<From> && std::is_integral_v<To> && sizeof(From) > sizeof(To)) ||
-    (std::is_enum_v<From> && std::is_integral_v<To> && sizeof(From) > sizeof(To)) ||
-    // [dcl.init.list] 7.5
-    (std::is_pointer_v<From> && std::is_same_v<To, bool>)
-    ;
+concept is_narrowing_v = arithmetic<To> && arithmetic<From> && !brace_initializable_to<From,To>;
 
 template< typename To, typename From >
 inline constexpr auto is_unsafe_pointer_conversion_v =

source/reflect.h

@@ -3661,9 +3661,9 @@ parse_context_branch_reset_state::parse_context_branch_reset_state(){}
 #line 1993 "reflect.h2"
     [[nodiscard]] auto parse_context::grab_n(cpp2::impl::in<int> n, cpp2::impl::out<std::string> r) & -> bool
     {
-        if (cpp2::impl::cmp_less_eq(pos + cpp2::impl::as_<size_t>(n),regex.size())) {
-            r.construct(regex.substr(pos, cpp2::impl::as_<size_t>(n)));
-            pos += (cpp2::impl::as_<size_t>(n)) - 1;
+        if (cpp2::impl::cmp_less_eq(pos + cpp2::unsafe_narrow<size_t>(n),regex.size())) {
+            r.construct(regex.substr(pos, cpp2::unsafe_narrow<size_t>(n)));
+            pos += (cpp2::unsafe_narrow<size_t>(n)) - 1;
             return true; 
         }
         else {
@@ -3844,7 +3844,7 @@ parse_context_branch_reset_state::parse_context_branch_reset_state(){}
 
 #line 2178 "reflect.h2"
     auto generation_function_context::remove_tabs(cpp2::impl::in<int> c) & -> void{
-        tabs = tabs.substr(0, (cpp2::impl::as_<size_t>(c)) * 2);
+        tabs = tabs.substr(0, (cpp2::unsafe_narrow<size_t>(c)) * 2);
     }
 
     generation_function_context::generation_function_context(auto const& code_, auto const& tabs_)

source/reflect.h2

@@ -1992,9 +1992,9 @@ parse_context: type =
 
     grab_n: (inout this, in n: int, out r: std::string) -> bool = 
     {
-        if pos + n as size_t <= regex..size() {
-            r = regex..substr(pos, n as size_t);
-            pos += (n as size_t) - 1;
+        if pos + cpp2::unsafe_narrow<size_t>(n) <= regex..size() {
+            r = regex..substr(pos, cpp2::unsafe_narrow<size_t>(n));
+            pos += (cpp2::unsafe_narrow<size_t>(n)) - 1;
             return true;
         }
         else {
@@ -2176,7 +2176,7 @@ generation_function_context: @struct type = {
     }
 
     remove_tabs: (inout this, c: int) = {
-        tabs = tabs..substr(0, (c as size_t) * 2);
+        tabs = tabs..substr(0, (cpp2::unsafe_narrow<size_t>(c)) * 2);
     }
 }