Work around GCC 9 rejecting a legitimate pointer comparison in a constexpr context

Author: gennaroprota

include/boost/static_string/config.hpp

@@ -286,4 +286,11 @@ using basic_string_view =
 #define BOOST_STATIC_STRING_GCC_NESTED_CLASS_WORKAROUND
 #endif
 
+// GCC 9 incorrectly rejects the pointer equality comparison in
+// ptr_in_range() in constant expressions. GCC 10 and later handle
+// it correctly.
+#if defined(__GNUC__) && !defined(__clang__) && (__GNUC__ == 9)
+#define BOOST_STATIC_STRING_CONSTEXPR_PTR_CMP_BROKEN
+#endif
+
 #endif

test/constexpr_tests.hpp

@@ -255,7 +255,8 @@ testConstantEvaluation()
   a.replace(a.begin(), a.end(), a.begin(), a.end());
   a.replace(a.begin(), a.end(), {'a'});
 
-#ifdef BOOST_STATIC_STRING_IS_CONST_EVAL
+#if defined(BOOST_STATIC_STRING_IS_CONST_EVAL) \
+        && !defined(BOOST_STATIC_STRING_CONSTEXPR_PTR_CMP_BROKEN)
   a.clear();
   a.replace(a.begin(), a.end(), "a");
   a.replace(a.begin(), a.end(), "a", 1);
@@ -443,7 +444,8 @@ testConstantEvaluation()
   a.replace(a.begin(), a.end(), a.begin(), a.end());
   a.replace(a.begin(), a.end(), {'a'});
 
-#ifdef BOOST_STATIC_STRING_IS_CONST_EVAL
+#if defined(BOOST_STATIC_STRING_IS_CONST_EVAL) \
+        && !defined(BOOST_STATIC_STRING_CONSTEXPR_PTR_CMP_BROKEN)
   a.clear();
   a.replace(a.begin(), a.end(), "a");
   a.replace(a.begin(), a.end(), "a", 1);