A type-erased handle to a format argument specifically used as an argument to FormatUntyped().
Declared in <absl/strings/str_format.h>
using FormatArg = str_format_internal::FormatArgImpl;
You may construct FormatArg by passing reference-to-const of any printable type. FormatArg is both copyable and assignable. The source data must outlive the FormatArg instance. See example below.