[PATCH] D66186: [Sema] Don't warn on printf('%hd', [char]) (PR41467)

[Roman Lebedev via Phabricator via cfe-commits]

lebedev.ri added a comment.

In D66186#1631921 <https://reviews.llvm.org/D66186#1631921>, @Nathan-Huckleberry wrote:

> In D66186#1630427 <https://reviews.llvm.org/D66186#1630427>, @aaron.ballman wrote:
>
> > There was a request in the linked bug for some code archaeology to see why this behavior exists in the first place. What were the results of that? I'm not opposed to the patch, but I would like to understand why it behaves the way it does.
>
>
> Since printf is a variadic function, integral argument types are promoted to int. The warning code runs the matchesType check twice, once to check if the promoted type (int) is able to be printed with the format and once to check if the original type (char) is able to be printed with the format.
>
> `printf("%d", [char])` is caught by the first case
>  `printf("%hhd", [char])` is caught by the second case.
>
> `printf("%hd", [char])` is a warning because an exception has not been made for that case.


That explains what the implementation does, but does not attempt to answer the question *why* things are the way they are.

I read https://bugs.llvm.org/show_bug.cgi?id=41467#c4 as

- any narrowing is always diagnosed
- promotion to wider than int is diagnosed
- passthrough is not diagnosed
- promotion to something smaller than int is diagnosed (the current case)

I can interpret it as: we already know that

In D66186#1631921 <https://reviews.llvm.org/D66186#1631921>, @Nathan-Huckleberry wrote:

> Since printf is a variadic function, integral argument types are promoted to int.


therefore why are you first implicitly promoting to int and then implicitly truncating?
Did you mean to print the original value? Did you mean to print int?

That doesn't sound too outlandish to me.


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