It only attempts conversion
<https://github.com/llvm-mirror/llvm/blob/master/lib/Support/raw_ostream.cpp#L655-L657>
if the check for a Windows console succeeds
<https://github.com/llvm-mirror/llvm/blob/master/lib/Support/raw_ostream.cpp#L691-L697>. 
If the conversion fails, the code falls back to where the check for a
Windows console was performed and then the raw bytes are written.

Tom.

On 7/28/21 8:51 PM, Charlie Barto via SG16 wrote:
>
> does it do two passes to decide if it’s going to write straight
> through or not? I.e. if I have some valid utf-8 followed by some bogus
> UTF-8 will it transcode the valid bits then pass the invalid bits
> right through?
>
> *From:* SG16 <sg16-bounces_at_[hidden]> *On Behalf Of *Tom
> Honermann via SG16
> *Sent:* Tuesday, July 13, 2021 10:41 PM
> *To:* sg16_at_[hidden]
> *Cc:* Tom Honermann <tom_at_[hidden]>; Corentin
> <corentin.jabot_at_[hidden]>
> *Subject:* Re: [SG16] std::print usage experience
>
> On 7/12/21 5:33 PM, Victor Zverovich via SG16 wrote:
>
> Thanks, Corentin, that's a great find. This is indeed very close
> to what is proposed and a nice addition to the implementation
> experience in {fmt} and Rust.
>
> There are some notable differences relative to what we've been discussing.
>
> 1. The LLVM implementation has a wide contract; there is no UB if the
> input is not valid UTF-8.
> 2. If the provided text is not valid UTF-8, then the raw input is
> written through the file descriptor rather than being transcoded
> to UTF-16 with substitution characters and then written directly
> to the console.
>
> Tangent: the method LLVM is using to determine if a file descriptor
> corresponds to a console
> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fllvm-mirror%2Fllvm%2Fblob%2Fmaster%2Flib%2FSupport%2Fraw_ostream.cpp%23L589-L593&data=04%7C01%7CCharles.Barto%40microsoft.com%7C8abe3f543e014c1fec7608d9468a0362%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637618380859365692%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=LFmRFUfIDdH99IPt3M6o8caVqnrFOTY5DjLK8sARk9Q%3D&reserved=0>
> looks wrong to me and, I think, will result in attempts to write text
> to the console when it was actually directed elsewhere.  Such writes
> likely fail with the result that the original input ends up getting
> written to the file descriptor anyway.  We previously discussed how to
> detect output directed to a console here
> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.isocpp.org%2Fsg16%2F2021%2F01%2F2008.php&data=04%7C01%7CCharles.Barto%40microsoft.com%7C8abe3f543e014c1fec7608d9468a0362%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637618380859375632%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=QeO6vYYjWRz92A2%2FsPDgAAUEJqorlNeN7NA3bsWBM4Y%3D&reserved=0>
> (and some day we'll have email archives that aren't embarrassing.  I
> hope).
>
> Tom.
>
> Cheers,
>
> Victor
>
> On Mon, Jul 12, 2021 at 1:59 PM Corentin via SG16
> <sg16_at_[hidden] <mailto:sg16_at_[hidden]>> wrote:
>
> Hello,
>
> LLVM implemented exactly what is proposed by std::print 3
> years ago
>
> https://github.com/llvm-mirror/llvm/blob/master/lib/Support/raw_ostream.cpp#L638-L685
> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fllvm-mirror%2Fllvm%2Fblob%2Fmaster%2Flib%2FSupport%2Fraw_ostream.cpp%23L638-L685&data=04%7C01%7CCharles.Barto%40microsoft.com%7C8abe3f543e014c1fec7608d9468a0362%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637618380859375632%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=crqvJh3W0odkygOj5R7RMXMHubQT%2F6PgNoBCheyPRaM%3D&reserved=0>
>
> I thought this might be of interest.
>
> Regards,
>
> Corentin
>
> --
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