Incorrect expansion of ,##__VA_ARGS__ in macro

[wmacevoy]

Basic Infos

• [ X] This issue complies with the issue POLICY doc.
• [X ] I have read the documentation at readthedocs and the issue is not addressed there.
• [ ?] I have tested that the issue is present in current master branch (aka latest git).
  Latest build from README docs build failed (Mac OS 10.12.6) on python get.py with:
  > cd esp8266/tools
  > python get.py
  IOError: [Errno socket error] [SSL: TLSV1_ALERT_PROTOCOL_VERSION] tlsv1 alert protocol version (_ssl.c:590)
• [ X] I have searched the issue tracker for a similar issue.
• [ X] If there is a stack dump, I have decoded it.
• [X ] I have filled out all fields below.

Platform

• Hardware: Generic ESP8266 device
• Core Version: 2.4.1 - could not build from git
• Development Env: Arduino IDE 1.8.5
• Operating System: MacOS

Settings in IDE

• Module: Generic ESP8266 Module
• Flash Mode: qio
• Flash Size: 512MB
• lwip Variant: v2mss536
• Reset Method: ck
• Flash Frequency: 40Mhz
• CPU Frequency: 80Mhz
• Upload Using: na compile time error
• Upload Speed: na compile time error

Problem Description

g++ supports variadic macros which cleanly reduces to 0 arguments with the notation

#define va_macro(...) va(begin, ##__VA_ARGS__, end)

The ,##__VA_ARGS__ notation expands to nothing if no arguments are given (the leading comma is removed), while the comma remains for 1 or more arguments.

This works correctly on uno, mega and esp32 boards, but fails to expand correctly on esp8266. This notation is important for variable parameter macros. Here is a sketch that compiles under uno, mega, esp32, but has a compile-time-error for esp8266, with log files for uno, mega, esp32 (correct) and esp8266 (compile time error):

https://github.com/mmurdoch/arduinounit/tree/pr65/va_args

#define va_macro(...) va_sub(-9,##__VA_ARGS__,9)

// Unimportant, just to have something the va_macro can compile to
void va_sub(int first, int last) { }
void va_sub(int first, int va1, int last) { }
void va_sub(int first, int va1, int va2, int last) {}

void setup() {
   va_macro(1,2);  // ok: expands to va_sub(-9,1,2,9);
   va_macro(1); // ok: expands to va_sub(-9,1,9); 
   va_macro(); // fail: expands to va_sub(-9,,9) on ESP 8266, but should be va_sub(-9,9);
}

void loop() {

}

Debug Messages

None.  Does not compile.

[igrr]

IIRc, this is a GNU extension. In this project C++ code is compiled with -std=c++11, so GNU extensions are disabled.

[earlephilhower]

Yup, that'd be the cause. Here is G++ 5.4 under Linux, same behavior.

earle@server:~/Arduino/ss1$ g++ -c /tmp/a.c 
<no error>

earle@server:~/Arduino/ss1$ g++ -c --std=c++11 /tmp/a.c 
/tmp/a.c: In function ‘void setup()’:
/tmp/a.c:1:46: error: expected primary-expression before ‘,’ token
 #define va_macro(...) va_sub(-9,##__VA_ARGS__,9)
                                              ^
/tmp/a.c:14:4: note: in expansion of macro ‘va_macro’
    va_macro(); // compile time error just on ESP 8266
    ^

[wmacevoy]

It is a GNU extension, but AVR and ESP32 boards are compiled with those extensions enabled. And I can find no resolution to this without those options. This breaks ArduinoUnit on the ESP 8266. c++20 seems to finally have a fix; but until then the standard is broken. (see VA_OPT in https://en.cppreference.com/w/cpp/compiler_support). How horrible is it to compile with -std=gnu++11, which is what the other systems ArduinoUnit supports does? See https://github.com/mmurdoch/arduinounit/blob/pr65/README.md#known-bugs