`<xloctime>`: `do_get_monthname` and `do_get_weekday` do not set `eofbit` in some cases, but `libcxx` tests expect them to do

[cpplearner]

(I'm filing this issue to record my analysis about a few libcxx test failures related to get_monthname and get_weekday. These functions are super old, rarely used, and it's unclear what the correct behavior is.)

These `libcxx` tests seem to be affected

``` std/localization/locale.categories/category.time/locale.time.get.byname/get_monthname.pass.cpp std/localization/locale.categories/category.time/locale.time.get.byname/get_monthname_wide.pass.cpp std/localization/locale.categories/category.time/locale.time.get.byname/get_weekday.pass.cpp std/localization/locale.categories/category.time/locale.time.get.byname/get_weekday_wide.pass.cpp std/localization/locale.categories/category.time/locale.time.get/locale.time.get.members/get_monthname_wide.pass.cpp std/localization/locale.categories/category.time/locale.time.get/locale.time.get.members/get_monthname.pass.cpp std/localization/locale.categories/category.time/locale.time.get/locale.time.get.members/get_weekday_wide.pass.cpp std/localization/locale.categories/category.time/locale.time.get/locale.time.get.members/get_weekday.pass.cpp ```

Describe the bug

When they reach the end of input after a successful match, time_get::do_get_monthname and time_get::do_get_weekday do not set eofbit, but libcxx tests expect them to do.

Command-line test case

D:\test>type test-time-get.cpp

#include <cassert>
#include <ctime>
#include <iostream>
#include <locale>

using I = const char*;

class my_facet : public std::time_get<char, I>
{
public:
    explicit my_facet(std::size_t refs = 0)
        : time_get(refs) {}
};

int main() {
    const my_facet f(1);
    std::ios str(nullptr);
    std::ios_base::iostate err;
    std::tm t;

    {
        std::cout << "Test 1: Get month name\n";

        const char in[] = "Jan";
        err = std::ios_base::goodbit;
        t = std::tm();
#ifdef TEST_GET_MONTHNAME_WEEKDAY
        I i = f.get_monthname(I(in), I(in+sizeof(in)-1), str, err, &t);
#else
        I i = f.get(I(in), I(in+sizeof(in)-1), str, err, &t, 'b');
#endif
        assert(i == in+3);
        assert(t.tm_mon == 0);
        if (err == std::ios::eofbit) {
            std::cout << "eofbit is set\n";
        } else {
            assert(err == std::ios::goodbit);
        }
    }

    {
        std::cout << "Test 2: Get weekday\n";
        const char in[] = "Sun";
        err = std::ios_base::goodbit;
        t = std::tm();
#ifdef TEST_GET_MONTHNAME_WEEKDAY
        I i = f.get_weekday(I(in), I(in+sizeof(in)-1), str, err, &t);
#else
        I i = f.get(I(in), I(in+sizeof(in)-1), str, err, &t, 'a');
#endif
        assert(i == in+3);
        assert(t.tm_mday == 0);
        if (err == std::ios::eofbit) {
            std::cout << "eofbit is set\n";
        } else {
            assert(err == std::ios::goodbit);
        }
    }
}

D:\test>cl /EHsc /DTEST_GET_MONTHNAME_WEEKDAY test-time-get.cpp

Microsoft (R) C/C++ Optimizing Compiler Version 19.50.35725 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

test-time-get.cpp
Microsoft (R) Incremental Linker Version 14.50.35725.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:test-time-get.exe
test-time-get.obj

D:\test>cl /EHsc test-time-get.cpp /Fe: test-time-get-with-get.exe

Microsoft (R) C/C++ Optimizing Compiler Version 19.50.35725 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

test-time-get.cpp
Microsoft (R) Incremental Linker Version 14.50.35725.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:test-time-get-with-get.exe
test-time-get.obj

D:\test>test-time-get.exe

Test 1: Get month name
Test 2: Get weekday

D:\test>test-time-get-with-get.exe

Test 1: Get month name
eofbit is set
Test 2: Get weekday
eofbit is set

Expected behavior

test-time-get.exe has the same output as test-time-get-with-get.exe.

STL version

117ca96

Additional context

[muellerj2]

I think this is N5032 [locale.time.get.general]/2 (https://eel.is/c++draft/locale.time.get.general#2)

If the end iterator is reached during parsing by any of the get() member functions, the member sets ios_base::eofbit in err.

[cpplearner]

In LWG-2036, it is mentioned that:

Yes, this matches the stdcxx test suite for num_get and time_get but not money_get when the currency symbol is last. I don't see where in the locale.money.get.virtuals section we specify whether eofbit is or isn't set and when.

IMO, if we try to fix the char extractor to be consistent we should also fix all the others extractors and manipulators that aren't consistent (including std::get_money and std::get_time).