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Subject: Are ill-formed UTF literals well-formed?
From: Corentin (corentin.jabot_at_[hidden])
Date: 2021-04-24 06:25:58


Hello,

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,

Corentin



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