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

Author: dinomight

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;
   }
 

test/format-test.cc

@@ -582,6 +582,8 @@ TEST(format_test, named_arg) {
   EXPECT_EQ("1/a/A", fmt::format("{_1}/{a_}/{A_}", fmt::arg("a_", 'a'),
                                  fmt::arg("A_", "A"), fmt::arg("_1", 1)));
   EXPECT_EQ(fmt::format("{0:{width}}", -42, fmt::arg("width", 4)), " -42");
+  EXPECT_EQ(fmt::format("{value:{width}}", fmt::arg("value", -42),
+      fmt::arg("width", 4)), " -42");
   EXPECT_EQ("st",
             fmt::format("{0:.{precision}}", "str", fmt::arg("precision", 2)));
   EXPECT_EQ(fmt::format("{} {two}", 1, fmt::arg("two", 2)), "1 2");