work around libstdc++ `<chrono>` duration conversion bug

[akyrtzi]

Bugzilla Link	14340
Version	trunk
OS	All
Attachments	preprocessed file
CC	@DougGregor,@efriedma-quic,@zygoloid

Extended Description

The attached preprocessed file would parse without compiler errors, but after r167514 it gives this error:

error: no matching constructor for initialization of 'duration'

Run on clang like this:

$ clang -fsyntax-only main.cpp -std=c++11

[efriedma-quic]

As far as I can tell, the code is pretty clearly wrong; the only reason we previously didn't print the error is that the template in question is never instantiated.

Richard's giant FIXME is relevant here:

+    // FIXME: This instantiates constexpr functions too frequently. If this is
+    // really an unevaluated context (and we're not just in the definition of a
+    // function template or overload resolution or other cases which we
+    // incorrectly consider to be unevaluated contexts), and we're not in a
+    // subexpression which we actually need to evaluate (for instance, a
+    // template argument, array bound or an expression in a braced-init-list),
+    // we are not permitted to instantiate this constexpr function definition.

[zygoloid]

Agreed. This diagnostic is correct, and the code is broken. The expression on line 74417 has type duration<int64_t, nanoseconds> and it's being used to initialize a variable of type duration<int64_t, microseconds>. Lossy conversions between std::duration types are not allowed.

This seems to be the only error we encounter when parsing libstdc++-4.6 or -4.7 in C++11 mode, so perhaps we should try to work around it.

An easy fix is to replace

        const __clock_t::time_point __s_atime = __s_entry + __delta;

with

        const auto __s_atime = __s_entry + __delta;

[AaronBallman]

I cannot reproduce this issue, but I also don't have access to ancient libstdc++ versions. Closing this issue as worksforme, but if the issue does still reproduce, please reopen!

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Argyrios Kyrtzidis (akyrtzi)

Details

| | | | --- | --- | | Bugzilla Link | [14340](https://llvm.org/bz14340) | | Version | trunk | | OS | All | | Attachments | [preprocessed file](https://user-images.githubusercontent.com/46970/143746353-3c562775-ed20-4a97-a7fd-d40aae7fa310.gz) | | CC | @DougGregor,@efriedma-quic,@zygoloid |

Extended Description

The attached preprocessed file would parse without compiler errors, but after r167514 it gives this error:

error: no matching constructor for initialization of 'duration'

Run on clang like this:

$ clang -fsyntax-only main.cpp -std=c++11