Hi all; I'm using GCC 8.1.0 on my GNU/Linux system to build C++. I'm introducing precompiled header support into my builds and it's going fine but I did discover one issue. I'm using the fmt library ( https://github.com/fmtlib/fmt --note I'm using the latest _released_ version 5.3.0 not the git master HEAD) and one feature of this library is the ability to get compile-time errors for incorrect formatting strings, using a macro around the format string. I don't want to get into the guts of the implementation (and certainly don't understand it fully) but the macro looks like this: #define FMT_STRING(s) [] { \ typedef typename std::remove_cv<std::remove_pointer< \ typename std::decay<decltype(s)>::type>::type>::type ct; \ struct str : fmt::compile_string { \ typedef ct char_type; \ FMT_CONSTEXPR operator fmt::basic_string_view<ct>() const { \ return {s, sizeof(s) / sizeof(ct) - 1}; \ } \ }; \ return str{}; \ }() If I build my code normally without precompiled headers, all is well. But if I build my code WITH precompiled headers, I get this warning: /Foo.cpp: In lambda function: /src/fmt/include/fmt/format.h:3517:18: error: typedef 'Foo::myMethod()::<lambda()>::str::char_type' locally defined but not used [-Werror=unused-local-typedefs] typedef ct char_type; \ I can do something like add __attribute__((unused)) here but the thing that I'm wondering about is why I get the warning only for precompiled headers, but not if I don't use them? Is that a bug in GCC's precompiled headers? Or an unavoidable side-effect? Or...?
