gcc: fix assert usage in constexpr function

Author: spyridon97

include/scn/scan.h

@@ -3777,7 +3777,12 @@ class SCN_TRIVIAL_ABI scan_error {
                 return std::errc::result_out_of_range;
             case max_error:
             default:
-                assert(false);
+#if !SCN_GCC || SCN_GCC >= SCN_COMPILER(9, 0, 0)
+                // gcc 7 thinks we'll get here, even when we won't
+                // gcc 8 has a bug in debug mode,
+                // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86678
+                SCN_EXPECT(false);
+#endif
                 SCN_UNREACHABLE;
         }
     }
@@ -4155,8 +4160,10 @@ inline constexpr char32_t decode_utf8_code_point_exhaustive(
         return cp;
     }
 
-#if !SCN_GCC || SCN_GCC >= SCN_COMPILER(8, 0, 0)
+#if !SCN_GCC || SCN_GCC >= SCN_COMPILER(9, 0, 0)
     // gcc 7 thinks we'll get here, even when we won't
+    // gcc 8 has a bug in debug mode,
+    // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86678
     SCN_EXPECT(false);
 #endif
     SCN_UNREACHABLE;
@@ -4213,8 +4220,10 @@ inline constexpr char32_t decode_utf8_code_point_exhaustive_valid(
         return cp;
     }
 
-#if !SCN_GCC || SCN_GCC >= SCN_COMPILER(8, 0, 0)
+#if !SCN_GCC || SCN_GCC >= SCN_COMPILER(9, 0, 0)
     // gcc 7 thinks we'll get here, even when we won't
+    // gcc 8 has a bug in debug mode,
+    // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86678
     SCN_EXPECT(false);
 #endif
     SCN_UNREACHABLE;
@@ -5767,7 +5776,6 @@ constexpr size_t encode_types()
                (encode_types_impl<CharT, Ts...>() << packed_arg_bits);
     }
     else {
-        SCN_EXPECT(false);
         SCN_UNREACHABLE;
     }
 }
@@ -6991,7 +6999,12 @@ struct format_specs {
                 return 16;
 
             default:
+#if !SCN_GCC || SCN_GCC >= SCN_COMPILER(9, 0, 0)
+                // gcc 7 thinks we'll get here, even when we won't
+                // gcc 8 has a bug in debug mode,
+                // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86678
                 SCN_EXPECT(false);
+#endif
                 SCN_UNREACHABLE;
         }
         SCN_GCC_COMPAT_POP
@@ -7241,7 +7254,12 @@ constexpr presentation_type parse_presentation_type(CharT type)
         case '/':
             // Should be handled by parse_presentation_set and
             // parse_presentation_regex
+#if !SCN_GCC || SCN_GCC >= SCN_COMPILER(9, 0, 0)
+            // gcc 7 thinks we'll get here, even when we won't
+            // gcc 8 has a bug in debug mode,
+            // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86678
             SCN_EXPECT(false);
+#endif
             SCN_UNREACHABLE;
         default:
             return presentation_type::none;