[PATCH] D134453: Introduce the `AlwaysIncludeTypeForNonTypeTemplateArgument` into printing policy

[Nenad Mikša via Phabricator via cfe-commits]

DoDoENT added inline comments.


================
Comment at: clang/include/clang/AST/PrettyPrinter.h:307
+  /// decltype(s) will be printed as "S<Point{1,2}>" if enabled and as "S<{1,2}>" if disabled,
+  /// regardless if PrintCanonicalTypes is enabled.
+  unsigned AlwaysIncludeTypeForNonTypeTemplateArgument : 1;
----------------
aaron.ballman wrote:
> dblaikie wrote:
> > DoDoENT wrote:
> > > dblaikie wrote:
> > > > What does `PrintCanonicalTypes` have to do with this? Does it overlap with this functionality in some way, but doesn't provide the functionality you want in particular?
> > > Thank you for the question. If you set the `PrintCanonicalTypes` to `false`, the `S<Point{1, 2}>` would be printed as `S<Point{1, 2}>` even without this patch. However, if you set it to `true`, it will be printed as `S<{1, 2}>`.
> > > 
> > > I don't fully understand why it does that, but it's quite annoying.
> > > 
> > > For a better example, please take a look at the `TemplateIdWithComplexFullTypeNTTP` unit tests that I've added: if `PrintCanonicalTypes` is set to `true`, the original print output of type is `NDArray<float, {{{0}}}, {{{0}}}, {{{0}}}>`, and if set to `false` (which is default), the output is `NDArray<float, Height{{{0}}}, Width{{{0}}}, Channels{{{0}}}>` - so the NTTP type is neither fully written nor fully omitted, which is weird.
> > > 
> > > As I said, I don't really understand the idea behind `PrintCanonicalTypes`, but when my new `AlwaysIncludeTypeForNonTypeTemplateArgument` is enabled, you will get the full type printed, regardless of value of `PrintCanonicalTypes` setting.
> > > 
> > Perhaps this might be more of a bug in PrintCanonicalTypes than something to add a separate flag for.
> > 
> > @aaron.ballman D55552 for context here... 
> > 
> > Hmm, actually, just adding the top level `Height{{0}}, Width{{0}}, Channels{{0}}` is sufficient to make this code compile (whereas with the `{{{0}}}` it doesn't form a valid identifier.
> > 
> > So what's your use case for needing more explicitness than that top level? 
> > Perhaps this might be more of a bug in PrintCanonicalTypes than something to add a separate flag for.
> >
> > @aaron.ballman D55552 for context here...
> 
> I looked over D55552 again and haven't spotted anything with it that seems amiss; the change there is to grab the canonical type before trying to print it which is all the more I'd expect `PrintCanonicalTypes` to impact.
> 
> This looks like the behavior you'd get when you desugar the type. Check out the AST dump for `s`: https://godbolt.org/z/vxh5j6qWr
> ```
> `-VarDecl <line:3:1, col:20> col:20 s 'S<Point{1, 2}>':'S<{1, 2}>' callinit
> ```
> We generate that type information at https://github.com/llvm/llvm-project/blob/main/clang/lib/AST/TextNodeDumper.cpp#L663 for doing the AST dump, note how the second type printed is the desugared type and that matches what we're seeing from the pretty printer.
> So what's your use case for needing more explicitness than that top level?

As I described in the [github issue](https://github.com/llvm/llvm-project/issues/57562), I'm trying to write a clang-based tool that will have different behavior if the printed `{{{0}}}` is actually `Width` than if its `Height` or anything else.

You can see the the issue in the AST dump for `bla`: https://godbolt.org/z/fMr4f13o3

The type is
```
`-VarDecl <line:20:1, col:21> col:21 bla 'NDArray<float, W>':'NDArray<float, {{{0}}}>' callinit
  `-CXXConstructExpr <col:21> 'NDArray<float, W>':'NDArray<float, {{{0}}}>' 'void () noexcept'
```

so it's unknown whether `{{{0}}}` represent the `Width` or `Height`. My patch makes it work exactly like GCC (see the comparison of error message between [clang 15 and GCC 12.1](https://godbolt.org/z/WenWe8caf).

> Perhaps this might be more of a bug in PrintCanonicalTypes than something to add a separate flag for.

This was also my first thought and the first version of my patch (before even submitting it here) was to actually change the behavior of `PrintCanonicalTypes`. However, that change made some tests fail, as I described in the patch description:

- CodeGenCXX/debug-info-template.cpp
- SemaCXX/constexpr-printing.cpp
- SemaCXX/cxx2a-nttp-printing.cpp
- SemaTemplate/temp_arg_string_printing.cpp

Of course, it's possible to simply update the tests, but I actually don't fully understand what is the goal of `PrintCanonicalTypes` and whether its current behavior is actually desired or not, so I played it safe and introduced a new policy that is disabled by default until I get more feedback from more experienced LLVM developers.

The patch does solve my problem and since I'm building LLVM from source anyway, I can have it enabled in my fork.

I just want to see if it would also be beneficial to be introduced into the upstream LLVM.


Repository:
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  https://reviews.llvm.org/D134453/new/

https://reviews.llvm.org/D134453