Parameter pack compilation error

[jputcu]

Using etl-20.38.0, I get the following errors on arduino/avr-gcc 7.3

include\etl\parameter_pack.h:141:74: error: template definition of non-template 'constexpr const size_t etl::parameter_pack<TTypes>::index_of_type<T>::value'
   constexpr size_t parameter_pack<TTypes...>::template index_of_type<T>::value;
                                                                          ^~~~~

[jwellbelove]

Does it compile if you delete the template before index_of_type<T>::value?

[jwellbelove]

What C++ standard are you using?

[jputcu]

I'm using C++17, seems like the same issue as #704

[jputcu]

I'm currently on 20.37.2 which doesn't give the error.

[jwellbelove]

etl-18-32-11.patch

[jputcu]

I get the same error. It keeps taking line 141. If I hardcode the other path, it compiles. My arduino/avr-gcc compiler uses the following flags: avr-g++.exe -DETL_NO_CPP_NAN_SUPPORT -DETL_NO_HUGE_VAL_SUPPORT -DETL_NO_STL -DETL_TARGET_DEVICE_GENERIC -DETL_TARGET_OS_NONE -DF_CPU=14745600 -mmcu=atmega128 -g -funsigned-char -mrelax -ffunction-sections -fdata-sections -mcall-prologues -fno-rtti -fno-exceptions -fno-threadsafe-statics -Os -DNDEBUG -std=c++1z -Wall -Wextra.

I also noticed the following change related to the ETL_NO_CPP_NAN_SUPPORT

include/etl/profiles/determine_compiler_language_support.h:198:0: warning: "ETL_NO_CPP_NAN_SUPPORT" redefined
   #define ETL_NO_CPP_NAN_SUPPORT

[jputcu]

20.37.3 also has the same parameter_pack issue.

[jwellbelove]

I wonder if the -std=c++1z is the issue? The line in parameter_pack.h tests for full C++17 spec. Does the error go away if you use -std=c++17, if available?

[jputcu]

That flag is selected by conan to get C++17 on gcc 7.3. I think the compiler is not fully C++17 yet. Small note: both the official Microchip and Arduino compiler use the 7.3 version, so I think the issue I'm facing will probably also appear on your Arduino package.

[jwellbelove]

What is the value of __cplusplus for -std=c++1z?

[jputcu]

echo | ~/.conan2/p/b/micro197c0baa3ac02/p/bin/avr-g++  -dM -E -x c++ --std=c++1z - | grep cplusplus
#define __cplusplus 201703L

C++17 gives the same value.

[jwellbelove]

etl-756.patch

[jwellbelove]

Does the above patch work for you?

[jputcu]

I was planning to try it today. The problem is that the error only shows in my top level project, ETL is the lowest level conan package, so I will need to hack some to apply the patch. Would it be better in the future to let the CI build the unit tests using the AVR compiler, they don't need to run, to catch these compilation errors?

[jwellbelove]

All you would need to do is put together a minimal app that invokes the problem type definition.

[jputcu]

I've been able to apply the patch and all mentioned warnings are gone.

[jwellbelove]

I added a preprocessor condition that checks for GCC & versions before 8.x.x

[jwellbelove]

Fixed 20.38.2