I believe the intent, and the wording, is to say that the associated encoding is UTF-n. 
The real issue, in my opinion, is that string literal is not a type, and any well-formedness guarantees about literals does not and can not apply to char8_t[N] or char8_t*. 
char8_t s[] = {0xff, 0x0} 
should not be undefined behavior.
More than that, a standard utf8->scalar_value decoder should not have undefined behavior when handed ill-formed encodings. 

On Sat, Apr 24, 2021 at 7:26 AM Corentin via SG16 <sg16@lists.isocpp.org> wrote:


Consider the following:

auto x = u8"\xC0";

In [decl.init.string], we say

> An array of ordinary character type ([basic.fundamental]), char8_­t array, char16_­t array, char32_­t array, or wchar_­t array can be initialized by an ordinary string literal, UTF-8 string literal, UTF-16 string literal, UTF-32 string literal, or wide string literal, respectively [..]

In ISO 10646, 9.1 UTF-8

> Table 3 lists all the ranges (inclusive) of the octet sequences that are well-formed in UTF-8. Any UTF-8 sequence that does not match the patterns listed in table 3 is ill-formed [..]
As a consequence of the well-formedness conditions specified in table 9.2, the following octet values are disallowed in UTF-8: C0-C1, F5-FE

A reading of both standards would lead me to believe that the code is ill-formed.
Either the standard represents the intent, this is ill-formed and all implementations need fixing (and we might add a note to the standard), or the standard does not describe the intent.

I would argue that it should be ill-formed, exactly because there is no such thing as invalid UTF-8 and allowing that defeats the purpose of UTF-8 literals and char8_t.
This doesn't really contradict P2029 : no value in preventing numeric escape sequences, but there should be a well-formedness check after all other transformations. 

But regardless of whether we agree on that design question, my reading is that the standard contradicts ISO 10646.

Have a great week-end,


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