[PATCH] D125919: Drop qualifiers from return types in C (DR423)

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman added a comment.

In D125919#3554599 <https://reviews.llvm.org/D125919#3554599>, @rjmccall wrote:

> Ah, true, the standard doesn't describe it as a normal qualifier.  From the DR, it sounds like the committee certainly expected that this reasoning would also apply to `_Atomic`, even if that's not quite what they've written, but that the DR submitter seems to not want that behavior.  Have you been able to track down the solicited paper mentioned in that DR?  In the absence of further indication, I think dropping `_Atomic` like anything else is the right thing to do; like the other qualifiers, it isn't meaningful for r-values.

I think it's less clear to me that the committee expected to drop `_Atomic` because the standard basically makes `_Atomic` introduce a new type rather than a qualified use like `const`; it's more akin to `_Complex` in that regard. e.g., it's not clear to me that these are redeclarations:

  int func(void);
  _Atomic(int) func(void);

When I survey non-Clang C compilers which support `_Atomic`, it's evenly split between thinking these are valid redeclarations or not (GCC and ICC say they're not valid redeclarations, ICC and TCC say they are) but when I use `const int` and `int` as the return types, only ICC claims they're incompatible types.

Further, this seems like a gratuitous incompatibility with C++ where the qualifiers are *not* dropped, given that function call expressions explicitly drop the qualifiers anyway. (So you're right, they are effectively useless on the return type, but why should C and C++ differ in what's a valid redeclaration?)


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