diff --git a/include/boost/static_string/config.hpp b/include/boost/static_string/config.hpp
index 3c91f26..3d12242 100644
--- a/include/boost/static_string/config.hpp
+++ b/include/boost/static_string/config.hpp
@@ -35,12 +35,10 @@
 // Can we use is_constant_evaluated?
 // Standard version
 #if __cpp_lib_is_constant_evaluated >= 201811L
-#define BOOST_STATIC_STRING_USE_IS_CONST_EVAL
 #define BOOST_STATIC_STRING_IS_CONST_EVAL std::is_constant_evaluated()
-#elif defined(__clang__) && (__clang_major__ > 8)
+#elif defined(__clang__) && __has_builtin(__builtin_is_constant_evaluated)
 // If we have clang version 9+, we can use the intrinsic
 // While gcc also has this, we don't need it
-#define BOOST_STATIC_STRING_USE_IS_CONST_EVAL
 #define BOOST_STATIC_STRING_IS_CONST_EVAL __builtin_is_constant_evaluated()
 #endif
 
@@ -158,12 +156,13 @@ defined(__clang__) && \
 #define BOOST_STATIC_STRING_CPP14_CONSTEXPR
 #endif
 
-// These are for compiler/library configurations
+// This is for compiler/library configurations
 // that cannot use the library comparison function
 // objects at all in constant expresssions. In these
 // cases, we use whatever will make more constexpr work.
-#if defined(__clang__) && defined(__GLIBCXX__) && \
-((__GLIBCXX__ > 20181206) && (__GLIBCXX__ < 20190812))
+#if defined(__clang__) && defined(__GLIBCXX__) && ((__GLIBCXX__ > 20181206 && __GLIBCXX__ <= 20190812))
+#define BOOST_STATIC_STRING_NO_PTR_COMP_FUNCTIONS
+#elif __has_builtin(__builtin_constant_p) || defined(BOOST_STATIC_STRING_IS_CONST_EVAL)
 #define BOOST_STATIC_STRING_NO_PTR_COMP_FUNCTIONS
 #endif
 
diff --git a/include/boost/static_string/static_string.hpp b/include/boost/static_string/static_string.hpp
index a4b6f4f..ad152a6 100644
--- a/include/boost/static_string/static_string.hpp
+++ b/include/boost/static_string/static_string.hpp
@@ -596,8 +596,9 @@ find_first_of(
   return last;
 }
 
-// Check if a pointer lies within a range (inclusive) without unspecified behavior,
-// allowing it to be used in a constant evaluation
+// Check if a pointer lies within the range [src_first, src_last) 
+// without unspecified behavior, allowing it to be used
+// in a constant evaluation.
 template<typename T>
 BOOST_STATIC_STRING_CPP14_CONSTEXPR
 inline
@@ -615,7 +616,7 @@ is_inside(
   // no applications in constant expressions.
 #ifdef BOOST_STATIC_STRING_CPP14
 #ifdef BOOST_STATIC_STRING_NO_PTR_COMP_FUNCTIONS 
-#ifdef BOOST_STATIC_STRING_USE_IS_CONST_EVAL
+#ifdef BOOST_STATIC_STRING_IS_CONST_EVAL
   // Our second best option is to use is_constant_evaluated
   // and a loop that checks for equality, since equality for 
   // pointer to object types is never unspecified in this case.
@@ -626,18 +627,27 @@ is_inside(
         return true;
     return false;
   }
+#elif defined(__clang__) && __has_builtin(__builtin_constant_p)
+  // We can also try using __builtin_constant_p
+  if (__builtin_constant_p((ptr, src_first, src_last)))
+  {
+    for (; src_first != src_last; ++src_first)
+      if (src_first == ptr)
+        return true;
+    return false;
+  }
 #else
   // If library comparison functions don't work, and
-  // we cannot use is_constant_evaluated, we can use
-  // the builtin comparison operators instead.
-  return ptr >= src_first && ptr <= src_last;
+  // we cannot use any of the above, we can use
+  // try builtin comparison operators instead.
+  return ptr >= src_first && ptr < src_last;
 #endif
 #endif
 #endif
   // Use the library comparison functions if we can't use 
   // is_constant_evaluated or if we don't need to.
   return std::greater_equal<const T*>()(ptr, src_first) &&
-    std::less_equal<const T*>()(ptr, src_last);
+    std::less<const T*>()(ptr, src_last);
 }
 } // detail
 #endif