<iomanip>: Case sensitivity when parsing months

[fsb4000]

Describe the bug
According to the C++ standard, month and day string parsing is meant to be case insensitive.

From https://isocpp.org/files/papers/N4860.pdf (c++ 20 draft):
or from http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3337.pdf (c++11 draft)

The next element of fmt is equal to’%’, optionally followed by a modifier character, followed by a conversion specifier character, format, together forming a conversion specification valid for the ISO/IEC 9945 function strptime

From https://pubs.opengroup.org/onlinepubs/9699919799/functions/strptime.html

Case is ignored when matching items in buf such as month or weekday names.

Clang with libc++ also parses fine: https://gcc.godbolt.org/z/ceb98o

Command-line test case

C:\Temp>type main.cpp
#include <iostream>
#include <sstream>
#include <iomanip>

int main()
{
    std::tm t = {};
    std::istringstream ss("2011-FEB-18");
    ss >> std::get_time(&t, "%Y-%b-%d");
    if (ss.fail()) {
        std::cout << "Parse failed\n";
    }
    else {
        std::cout << std::put_time(&t, "%c") << '\n';
    }
}

C:\Temp>cl /EHsc /W4 /WX main.cpp
Оптимизирующий компилятор Microsoft (R) C/C++ версии 19.27.29109 для x64
(C) Корпорация Майкрософт (Microsoft Corporation).  Все права защищены.

main.cpp
Microsoft (R) Incremental Linker Version 14.27.29109.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:main.exe
main.obj

C:\Temp>main.exe
Parse failed

Expected behavior
Expected output:

Sun Feb 18 00:00:00 2011

STL version
Microsoft Visual Studio Community 2019 Preview Version 16.7.0 Preview 6.0

Additional context
VSO-596742 / AB#596742 | DevCom-229445

[Arzaghi]

I'm trying to fix the issue and encountered another case that doesn't make sense.

#include <iostream>
#include <sstream>
#include <iomanip>

int main()
{
    std::tm t = {};
    std::istringstream ss("2011-D-18");
    ss >> std::get_time(&t, "%Y-%b-%d");
    if (ss.fail()) {
        std::cout << "Parse failed\n";
    }
    else {
        std::cout << std::put_time(&t, "%c") << '\n';
    }
}

MSVC output: 01/18/11 00:00:00
GCC output: parse failed

what should be the expected output for this code according to the Standard definitions?

[fsb4000]

gcc output is correct.

for "C" locale it should parse only:

Jan
January
Feb
February
Mar
March
Apr
April
May
May
Jun
June
Jul
July
Aug
August
Sep
September
Oct
October
Nov
November
Dec
December

plus case insensitive: dEc, DEC, etc...

Current implementation of std::get_time is buggy :(