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Can't use sizeof... to initialize struct template members

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
cpp
Domain
embedded-iot

Research direction

Start with the minimal InfoArray template in C:\Users\vhtmf\Documents\Arduino\sketch_nov5a\sketch_nov5a.ino and reproduce the reported compiler errors. Then inspect the Arduino SAM compilation toolchain's handling of C++ parameter packs; done means identifying the cause and documenting or resolving whether sizeof...(Ts) can initialize the member.

Written by the indexing model from the issue text.

Description

#include<stdint.h>
template<typename... Ts>
struct InfoArray {
  uint8_t Number = sizeof...(Ts);
};

The code above does not compile.

C:\Users\vhtmf\Documents\Arduino\sketch_nov5a\sketch_nov5a.ino:4:20: error: expected ';' at end of member declaration
   uint8_t Number = sizeof...(Ts);
                    ^
C:\Users\vhtmf\Documents\Arduino\sketch_nov5a\sketch_nov5a.ino:4:26: error: expected unqualified-id before '...' token
   uint8_t Number = sizeof...(Ts);
                          ^
C:\Users\vhtmf\Documents\Arduino\sketch_nov5a\sketch_nov5a.ino:4:20: error: expected primary-expression at end of input
   uint8_t Number = sizeof...(Ts);
                    ^

exit status 1

Compilation error: expected ';' at end of member declaration
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