Compiler warning from std::chrono but is not being used - c++

Question

NOTE: This bug only occurs with x64 projects in both release and debug modes.
Odd warnings involving std::chrono appear on this piece of code using VC2019 at warning level 3. This is a stripped down piece of code that processes command line flags. I've removed most of the guts that aren't relevant to the problem.
#if 1 // enable bug
#include <chrono> // excluding this also eliminates chrono warnings
using CorrectedIntType=int;
#else
using CorrectedIntType=size_t;
#endif
#include <iostream>
#include <vector>
#include <string>
#include <type_traits>
using std::vector;
using std::string;
namespace {
void fixup(const std::string& argcmd, std::string& arg) { arg = argcmd; }
template<class T>
void procVal(std::vector<std::string>& arglist, CorrectedIntType idx, T& arg)
{
fixup(arglist[idx], arg);
arglist.erase(arglist.begin() + idx);
}
template<class T, class ...TA>
void procVal(std::vector<std::string>& arglist, CorrectedIntType idx, T& arg, TA&...argv)
{
procVal(arglist, idx, arg);
procVal(arglist, idx, argv...);
}
template<class T, class ...TA>
bool procFlag(const char* pc, std::vector<std::string>& arglist, T& arg1, TA&...argv)
{
std::string flag(pc);
for (size_t i = 0; i < arglist.size(); i++)
{
if (arglist[i] == flag)
{
arglist.erase(arglist.begin() + i);
procVal(arglist, i, arg1); // process one argument after flag
return true;
}
}
return false;
}
}
int main()
{
string outfile;
vector<string> test = { "test" };
procFlag("-o", test, outfile); // assigns test[0] to outfile and removes it
std::cout << outfile << '\n';
}
Warnings:
1>Source.cpp
1>C:\Users\mgray\Documents\Visual Studio 2017\Projects\CommandLineCPP\stackoverflow\Source.cpp(35,1): warning C4267: 'argument': conversion from 'size_t' to 'CorrectedIntType', possible loss of data
1>C:\Users\mgray\Documents\Visual Studio 2017\Projects\CommandLineCPP\stackoverflow\Source.cpp(54): message : see reference to function template instantiation 'bool `anonymous-namespace'::procFlag<std::string,>(const char *,std::vector<std::string,std::allocator<std::string>> &,T &)' being compiled
1> with
1> [
1> T=std::string
1> ]
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.24.28314\include\chrono(632): message : see reference to class template instantiation 'std::chrono::duration<double,std::ratio<1,1>>' being compiled
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.24.28314\include\chrono(178): message : see reference to class template instantiation 'std::chrono::duration<__int64,std::nano>' being compiled
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.24.28314\include\chrono(610): message : see reference to class template instantiation 'std::chrono::time_point<std::chrono::steady_clock,std::chrono::nanoseconds>' being compiled
While the code works, even with the int -<> size_t conversion issue which is a legitimate warning, all warnings go away when the macro at the top is set to 0. So somehow the size difference between size_t and int triggers chrono messages. It concerns me that the chrono warnings exist since it isn't involved. Is this a bug in VS2019? Any ideas as to why the chrono warning references are occurring?

Answer 1

This is a valid warning and it is not related to <chrono> but to your own code and CorrectedIntType type. Here is a simplified code without <chrono>: https://gcc.godbolt.org/z/qf9v8TEh7
In the definition of procVal, the second parameter is CorrectedIntType:
void procVal(std::vector<std::string>& arglist, CorrectedIntType idx, T& arg)
but it is called from procFlag with size_t value:
bool procFlag(const char* pc, std::vector<std::string>& arglist, T& arg1)
...
for (size_t i = 0; i < arglist.size(); i++)
...
procVal(arglist, i, arg1);
So one can fix the warning by changing i type to CorrectedIntType as well.