[gjs] arg-inl: Use more constexpr functions and constexpr checks

[Philip Chimento <pchimento src gnome org>]

commit b0ea57d30c14268a16932cdf7589900b00f34fee
Author: Marco Trevisan (Treviño) <mail 3v1n0 net>
Date:   Mon Aug 10 18:58:08 2020 +0200

    arg-inl: Use more constexpr functions and constexpr checks

 gi/arg-inl.h | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)
---
diff --git a/gi/arg-inl.h b/gi/arg-inl.h
index 952260c5..06330884 100644
--- a/gi/arg-inl.h
+++ b/gi/arg-inl.h
@@ -166,19 +166,15 @@ inline void gjs_arg_unset(GIArgument* arg) {
 // Implementation to store rounded (u)int64_t numbers into double
 
 template <typename BigT>
-[[nodiscard]] inline BigT max_safe_big_number() {
+[[nodiscard]] inline constexpr BigT max_safe_big_number() {
     return BigT(1) << std::numeric_limits<double>::digits;
 }
 
 template <typename BigT>
-[[nodiscard]] inline std::enable_if_t<std::is_signed_v<BigT>, BigT>
-min_safe_big_number() {
-    return -(max_safe_big_number<BigT>()) + 1;
-}
+[[nodiscard]] inline constexpr BigT min_safe_big_number() {
+    if constexpr (std::is_signed_v<BigT>)
+        return -(max_safe_big_number<BigT>()) + 1;
 
-template <typename BigT>
-[[nodiscard]] inline std::enable_if_t<std::is_unsigned_v<BigT>, BigT>
-min_safe_big_number() {
     return std::numeric_limits<BigT>::lowest();
 }