Silence one build-time warning

[jkeenan]

Observed in clang 9 and 10.

Partial solution for #17015

[jkeenan]

Given the discussion in #17015, I'm surprised that a solution to one of the build-time warnings discussed in that ticket was as simple as putting parentheses around one term.

However, smoke-test results suggest that this did no harm.

[hvds]

Oh! I had completely misunderstood what was being warned about here, how embarrassing - and my apologies for repeatedly denigrating clang's warnings based on that misunderstanding.

I now realise that what it is really complaining about is this bit of C, and I have no idea how it would be interpreted:

#define UVuf "lu"  /* typically */
#define UTF8f "d%" UVuf "%4p"
(UTF8f + 2)

I've never understood C string concatenation; but if the last line is not interpreted the same as ((UTF8f) + 2), then perhaps a more correct and complete fix would instead be to move the parens into the macro definition:

#define UTF8f ("d%" UVuf "%4p")

However I don't know if that risks causing problems for the more common case where we're concatenating it further.

I'll defer to those who know the C (or preprocessor?) spec better.

[tonycoz]

I've never understood C string concatenation; but if the last line is not interpreted the same as ((UTF8f) + 2), then perhaps a more correct and complete fix would instead be to move the parens into the macro definition:

#define UTF8f ("d%" UVuf "%4p")

However I don't know if that risks causing problems for the more common case where we're concatenating it further.

I'll defer to those who know the C (or preprocessor?) spec better.

I'm pretty sure that would break concatenating it into larger string literals.

The relevant part of the standard (6.4.5 paragraph 5)

In translation phase 6, the multibyte character sequences specified by any sequence of adjacent character and identically-prefixed string literal tokens are concatenated into a single multibyte character sequence. If any of the tokens has an encoding prefix, the resulting multibyte character sequence is treated as having the same prefix; otherwise, it is treated as a character string literal. Whether differently-prefixed wide string literal tokens can be concatenated and, if so, the treatment of the resulting multibyte character sequence are implementation-defined.

and the ( between the string literal tokens would make them non-adjacent.