[PATCH] D133289: [C2X] N3007 Type inference for object definitions

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman added a comment.

In D133289#4489883 <https://reviews.llvm.org/D133289#4489883>, @aaron.ballman wrote:

> - `_Atomic auto x = 12;` is now something we should be accepting and deduce as an `_Atomic int`

The committee is discussing this again on the reflectors. Thus far, almost everyone reads the standard the same way as GCC did with their implementation, which matches what I suggest above. However, there are folks who are claiming we should not be able to deduce the derived type because `_Atomic` forms an entirely new type and thus isn't actually a qualifier (and there are some words in the standard that could maybe be read as supporting that). The most conservative approach is to reject using `_Atomic auto` for right now so users don't build a reliance on it. Eventually WG14 will make a decision and we can relax that diagnostic then if we need to. Sorry for the confusion on this topic!



================
Comment at: clang/test/Sema/c2x-auto.c:119
+  return x;
+}
----------------
aaron.ballman wrote:
> Some additional test cases to consider:
> ```
> _Complex auto i = 12.0; // Should be rejected because _Complex is a type specifier; however, 
>                         // when auto becomes a type specifier, this should be accepted. GCC
>                         // rejects but if we end up accepting, I won't be sad -- we'll need an
>                         // extension warning in that case though.
> 
> void foo(void) {
>   extern void foo(int a, int array[({ auto x = 12; x;})]); // This is a use of `auto` within function prototype
>                                                            // scope, but should still be accepted.
> }
> ```
The suggested comment I have isn't fully correct. It should be rejected because _Complex is a type specifier, but when auto becomes a type specifier, I think _Complex perhaps should still not deduce. Consider this analogous case (which could be a fun test case as well):
```
signed auto a = 1L;
```
`signed` is a type specifier as well, and this is not accepted in C++ (so we don't want to accept it in C either).


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D133289/new/

https://reviews.llvm.org/D133289