[PATCH] D112491: Add `LambdaCapture`-related matchers.

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman added a comment.

In D112491#3088363 <https://reviews.llvm.org/D112491#3088363>, @jcking1034 wrote:

> @aaron.ballman for the purpose of these matchers, what @ymandel said is correct - the goal is to allow `LambdaCapture`s themselves to be bindable.

Should we be discussing deprecating the non-bindable matchers for lambda captures? I guess the part that worries me is there are now two ways to match the same sorts of constructs, and I'm a bit worried it won't be clear which one to reach for and when. Do you think this will be a usability concern for users?



================
Comment at: clang/include/clang/ASTMatchers/ASTMatchers.h:4629-4630
+///   matches `[x](){}`.
+AST_MATCHER_P(LambdaCapture, refersToVarDecl, internal::Matcher<VarDecl>,
+              InnerMatcher) {
+  auto *capturedVar = Node.getCapturedVar();
----------------
jcking1034 wrote:
> ymandel wrote:
> > aaron.ballman wrote:
> > > The name here is a bit unclear -- whether it is the matcher matching `int x;` or the `x` from the capture is not clear from the name. The comment suggests it's matching `x` from the capture, but I think it's actually matching the `int x;` variable declaration.
> > > 
> > > Being clear on what's matched here is important when we think about initializers:
> > > ```
> > > void foo() {
> > >   int x = 12;
> > >   auto f = [x = 100](){};
> > > }
> > > ```
> > > and
> > > ```
> > > lambdaExpr(hasAnyCapture(lambdaCapture(refersToVarDecl(hasName("x"), hasInitializer(integerLiteral(equals(100))))))
> > > ```
> > > Would you expect this to match? (This might be a good test case to add.)
> > In a similar vein, do we want a separate matcher on the name of the capture itself? e.g. an overload of `hasName`? And what about matchers for the initializers?  Those don't have to land in this patch, but do you think those would be doable?
> I would expect @aaron.ballman's initializer example to match, and I added a similar test case to the one  described. I think that if a capture does not have an initializer, then `refersToVarDecl` will match on the variable declaration before the lambda. However, if a capture does have an initializer, that initializer itself seems to be represented as a `VarDecl` in the AST, which is the `VarDecl` that gets matched.
> 
> For that reason, I think we may not need to have a separate matcher on the name of the capture itself. Additionally, since captures with/without initializers are both represented the same way, there may not be a good way to distinguish between them, so matchers for initializers may not be possible.
> I think that if a capture does not have an initializer, then refersToVarDecl will match on the variable declaration before the lambda. However, if a capture does have an initializer, that initializer itself seems to be represented as a VarDecl in the AST, which is the VarDecl that gets matched.

Oof, that'd be confusing! :-(

> For that reason, I think we may not need to have a separate matcher on the name of the capture itself.

Er, but there are init captures where you can introduce a whole new declaration. I think we do want to be able to match on that, right? e.g.,
```
[x = 12](){ return x; }();
```

> Additionally, since captures with/without initializers are both represented the same way, there may not be a good way to distinguish between them, so matchers for initializers may not be possible.

That's a bummer! :-( If this turns out to be a limitation, we should probably document it as such.


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  https://reviews.llvm.org/D112491/new/

https://reviews.llvm.org/D112491