[PATCH] D62845: [clang-tidy] Fix make-unique tests on C++2a.

[Dmitri Gribenko via Phabricator via cfe-commits]

gribozavr added a comment.

In D62845#1528887 <https://reviews.llvm.org/D62845#1528887>, @hokein wrote:

> In D62845#1528791 <https://reviews.llvm.org/D62845#1528791>, @gribozavr wrote:
>
> > I'd suggest to add a separate file that covers the exact language modes needed.
> >
> > The C++14 test that we have right now is about C++14-or-later, testing the availability of std::make_unique.
>
>
> The test file name ("modernize-make-unique-cxx14") indicates this test is for C++14, and since we change the existing `modernize-make-unique` test (which covers more cases) to C++14 or later, I think it is reasonable to restrict the `cxx14` test to run only on C++14. or am I missing anything?


I think we should be looking at the intent of the test rather than its name.

The intent looks like testing how the check works when `std::make_unique` is available from the standard library (as opposed to some kind of replacement like `absl::make_unique`).  See the patch that introduced it: https://reviews.llvm.org/D43766

So modernize-make-unique-cxx14 is actually "C++14 or later".  (Probably it should be renamed.)

>> I'm also not sure what the intent behind these tests is. Maybe the right fix is to add a constructor that can be called?
> 
> sorry, the check is too complicated to catch up, the test cases are testing the code path https://github.com/llvm/llvm-project/blob/master/clang-tools-extra/clang-tidy/modernize/MakeSmartPtrCheck.cpp#L405.
> 
> Unfortunately, adding a default constructor doesn't fix the problem: the AST tree is different in C++14/17 and C++2a, which causes different behavior in the check.
> 
> e.g. `new NoCopyMoveCtor{}`
> 
> In C++14/17, it looks like below, the check thinks it is an aggregate initialization (with deleted copy/move constructor) and doesn't generate the fix.
> 
>   |-CXXNewExpr <line:26:3, col:22> 'NoCopyMoveCtor *' Function 0x5614cfece8f0 'operator new' 'void *(std::size_t)'
>       | `-InitListExpr <col:21, col:22> 'NoCopyMoveCtor'
> 
> 
> However, in C++2a, the AST is like below, the check thinks it is a direct initialization with default constructor, and generate the fix.
> 
>   `-CXXNewExpr <line:26:3, col:22> 'NoCopyMoveCtor *' Function 0x55c9b1b24ba0 'operator new' 'void *(std::size_t)'
>         `-CXXConstructExpr <col:7, col:22> 'NoCopyMoveCtor' 'void () noexcept' list zeroing

I see.  Assuming it is desired behavior, I'd say for these cases we should create separate files that are specifically run in C++14 and 17, and another one for C++2a onward.

But is it desired behavior?  That is, can we generate a call to `std::make_unique` in C++14 in practice -- would it compile?


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