Fix dynamic named arg format spec handling (#4361)

When dealing with dynamic named format args, need to account for
nested named args when skipping the content of the replacement.

Fixes #4360

include/fmt/base.h

@@ -1718,7 +1718,19 @@ class format_string_checker {
       -> const Char* {
     context_.advance_to(begin);
     if (id >= 0 && id < NUM_ARGS) return parse_funcs_[id](context_);
-    while (begin != end && *begin != '}') ++begin;
+
+    // If id is out of range, it means we do not know the type and cannot parse
+    // the format at compile time. Instead, skip over content until we finish
+    // the format spec, accounting for any nested replacements.
+    auto bracket_count = 0;
+    while (begin != end && (bracket_count > 0 || *begin != '}')) {
+      if (*begin == '{')
+        ++bracket_count;
+      else if (*begin == '}')
+        --bracket_count;
+
+      ++begin;
+    }
     return begin;
   }