Fix clang compilation error

changed milestone to %OCT23

requested review from @fweig

assigned to @fweig

huh, structured bindings are a c++ 17 feature: https://en.cppreference.com/w/cpp/language/structured_binding

We already use these in a number of places. Not sure why this one in particular is causing issues...

I'm kind of puzzled by this because structure bindings have been supported by clang since forever. Do you have the compiler version that crashed by any chance?

I don't have an issue to merge this for a one-time workaround. But a whole bunch of things in algo are designed to be used with structured bindings. So this issue could just keep popping up again and again.

@fweig,

when compiling the following code

#include <tuple>

std::tuple<int,int> x() {
        return {1,2};
}

void f() {
    auto [a, b] = x();
    auto l = [&]() {
        return a+b;
    };
}

int main() {
  f();
  return 1;
}

with clang 14 I get the following error

/opt/homebrew/Cellar/llvm@14/14.0.6/bin/clang++ -std=c++17 test.cxx 

test.cxx:10:16: error: reference to local binding 'a' declared in enclosing function 'f'
        return a+b;
               ^
test.cxx:8:11: note: 'a' declared here
    auto [a, b] = x();
          ^
test.cxx:10:18: error: reference to local binding 'b' declared in enclosing function 'f'
        return a+b;
                 ^
test.cxx:8:14: note: 'b' declared here
    auto [a, b] = x();
             ^
2 errors generated.

when compiling with clang 16 which supports the structured bindings I get the following warning

/opt/homebrew/Cellar/llvm@16/16.0.6/bin/clang++ -std=c++17 test.cxx 
test.cxx:10:16: warning: captured structured bindings are a C++20 extension [-Wc++20-extensions]
        return a+b;
               ^
test.cxx:8:11: note: 'a' declared here
    auto [a, b] = x();
          ^
test.cxx:10:18: warning: captured structured bindings are a C++20 extension [-Wc++20-extensions]
        return a+b;
                 ^
test.cxx:8:14: note: 'b' declared here
    auto [a, b] = x();
             ^
2 warnings generated.

I have no idea why clang and cppreference disagree and I also have no clue why the compilation crashes for me.

I am also fine if the code isn't changed and I patch my version only locally.

Ok, thanks. I can reproduce the errors in godbolt and gcc accepts this without error or warning. But I'm still confused: These errors are related to lambda-capture with structured bindings. We don't use that at all in the patched code?

@fweig,

as I said I don't understand the issue completely. The original error message was

/tmp/cbmroot/algo/unpack/Unpack.cxx:198:28: error: reference to local binding 'numMs' declared in enclosing function 'cbm::algo::Unpack::ParallelMsLoop<CbmStsDigi, cbm::algo::sts::Unpack, cbm::algo::sts::UnpackMonitorData>'
    for (size_t i = 0; i < numMs; i++) {
                           ^
/tmp/cbmroot/algo/unpack/Unpack.cxx:35:9: note: in instantiation of function template specialization 'cbm::algo::Unpack::ParallelMsLoop<CbmStsDigi, cbm::algo::sts::Unpack, cbm::algo::sts::UnpackMonitorData>' requested here
        ParallelMsLoop(Subsystem::STS, monitor, digiTs.fSts, monitor.fSts, *timeslice, fAlgoSts, 0x20);
        ^
/tmp/cbmroot/algo/unpack/Unpack.cxx:187:11: note: 'numMs' declared here
    auto [numMs, sizeBytes] =
          ^
/tmp/cbmroot/algo/unpack/Unpack.cxx:216:28: error: reference to local binding 'numMs' declared in enclosing function 'cbm::algo::Unpack::ParallelMsLoop<CbmStsDigi, cbm::algo::sts::Unpack, cbm::algo::sts::UnpackMonitorData>'
    for (size_t i = 0; i < numMs; i++) {
                           ^
/tmp/cbmroot/algo/unpack/Unpack.cxx:187:11: note: 'numMs' declared here
    auto [numMs, sizeBytes] =
          ^
/tmp/cbmroot/algo/unpack/Unpack.cxx:198:28: error: reference to local binding 'numMs' declared in enclosing function 'cbm::algo::Unpack::ParallelMsLoop<CbmTofDigi, cbm::algo::tof::Unpack, cbm::algo::tof::UnpackMonitorData>'
    for (size_t i = 0; i < numMs; i++) {
                           ^
/tmp/cbmroot/algo/unpack/Unpack.cxx:40:9: note: in instantiation of function template specialization 'cbm::algo::Unpack::ParallelMsLoop<CbmTofDigi, cbm::algo::tof::Unpack, cbm::algo::tof::UnpackMonitorData>' requested here
        ParallelMsLoop(Subsystem::TOF, monitor, digiTs.fTof, monitor.fTof, *timeslice, fAlgoTof, 0x00);
        ^
/tmp/cbmroot/algo/unpack/Unpack.cxx:187:11: note: 'numMs' declared here
    auto [numMs, sizeBytes] =
          ^
/tmp/cbmroot/algo/unpack/Unpack.cxx:216:28: error: reference to local binding 'numMs' declared in enclosing function 'cbm::algo::Unpack::ParallelMsLoop<CbmTofDigi, cbm::algo::tof::Unpack, cbm::algo::tof::UnpackMonitorData>'
    for (size_t i = 0; i < numMs; i++) {
                           ^
/tmp/cbmroot/algo/unpack/Unpack.cxx:187:11: note: 'numMs' declared here
    auto [numMs, sizeBytes] =
          ^

numMs is already used before without an error

    std::vector<std::vector<Digi>> msDigis(numMs);  // unpacked digis per microslice
    std::vector<Monitor> monitor(numMs);            // unpacking monitoring data per microslice

Whatever is different when using numMs in the loop.

The error is only raised when using numMs in the loop.

Thanks! Looks like a weird issue with the OpenMP section... Sorry for dragging this on. I wanted to understand the issue, because this seems like something we might encounter again.