Fix silent wrong results for custom number types

Squashed onto develop cd6b428ef (CI fixes from #5860). Original commits:

- Fix silent wrong results for custom number types
- Fix CI: build the out_of_range.407 message from string_t and fix a link
- Fix CI: diagnostics message, clang-tidy, and MinGW section limit
- Format the out_of_range.407 number without dump()
- Move the std::deque array test out of unit-regression3.cpp
- Fix clang-tidy and clang 3.4/3.5 failures in the custom number type changes
- Fix CI: cast the 128-bit unsigned value explicitly in the integer fast path

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/value_t.hpp

@@ -14,7 +14,6 @@
 #include <cstdint> // uint8_t
 #include <limits> // numeric_limits
 #include <string> // string
-#include <type_traits> // is_signed
 
 #include <nlohmann/detail/macro_scope.hpp>
 #if JSON_HAS_THREE_WAY_COMPARISON
@@ -175,13 +174,16 @@ FloatType compare_integer_with_float(const IntegerType i, const FloatType f) noe
 
     // values of IntegerType lie in [-bound, bound) when signed and in
     // [0, bound) when unsigned; digits excludes the sign bit, so bound is a
-    // power of two that the float represents exactly
-    const FloatType bound = std::ldexp(static_cast<FloatType>(1), std::numeric_limits<IntegerType>::digits);
+    // power of two that the float represents exactly; the signedness comes
+    // from numeric_limits as well, because std::is_signed is false for class
+    // types such as 128-bit or multiprecision integers
+    using limits = std::numeric_limits<IntegerType>;
+    const FloatType bound = std::ldexp(static_cast<FloatType>(1), limits::digits);
     if (f >= bound)
     {
         return ordered(-1);
     }
-    if (std::is_signed<IntegerType>::value ? (f < -bound) : (f < static_cast<FloatType>(0)))
+    if (limits::is_signed ? (f < -bound) : (f < static_cast<FloatType>(0)))
     {
         return ordered(1);
     }