fix: some fixes revealed during clang compilation

test/unit_test/static/si_test.cpp

@@ -56,8 +56,8 @@ static_assert(1 * Rm == 1'000'000'000'000'000'000 * Gm);
 static_assert(1 * Qm == 1'000'000'000'000'000'000 * Tm);
 
 // check for invalid prefixes
-template<template<auto U> typename prefix, Unit auto V1>
-concept can_not_be_prefixed = !requires { typename prefix<V1>; };
+template<template<auto U> typename prefix, auto V1>
+concept can_not_be_prefixed = Unit<std::remove_const_t<decltype(V1)>> && !requires { typename prefix<V1>; };
 
 static_assert(can_not_be_prefixed<si::milli_, si::degree_Celsius>);
 static_assert(can_not_be_prefixed<si::milli_, si::minute>);