<format>: format("{:a<10}", "") fails STL internal check _First < _Last #4241
Description
Found by std/utilities/format/format.functions/vformat.pass.cpp in the upcoming libcxx update. I'm working on a fix.
C:\Temp>type meow.cpp
#include <cassert>
#include <format>
#include <string>
using namespace std;
int main() {
const string actual = format("{:a<10}", "");
assert(actual == "aaaaaaaaaa");
}C:\Temp>cl /EHsc /nologo /W4 /std:c++20 /MTd /Od meow.cpp && meow
meow.cpp
C:\Temp>cl /EHsc /nologo /W4 /std:c++20 /MTd /Od /D_ENABLE_STL_INTERNAL_CHECK meow.cpp && meow
meow.cpp
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Microsoft Visual C++ Runtime Library
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Debug Assertion Failed!
Program: C:\Temp\meow.exe
File: C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.39.33218\include\format
Line: 244
Expression: STL internal check: _First < _Last
For information on how your program can cause an assertion
failure, see the Visual C++ documentation on asserts.
(Press Retry to debug the application)
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Abort Retry Ignore
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