Improve error message for invalid lambda captures

[ilya-biryukov]

For the following code (https://gcc.godbolt.org/z/GGoWq45hs) Clang produces an error message that some people find confusing:

struct X {
    int x;
    void foo() {
        [x](){};
    }
};
// <source>:4:10: error: 'x' in capture list does not name a variable

It seems that the error message could indeed be improved, a few ideas that come to mind:

• mentioning what kind of symbol we are actually referring to: 'class member 'x' cannot appear in capture list as it is not a variable'
• we could suggest fixes in some situations did you mean to capture 'this'? or did you mean to use a capture with initializer?
• variable is technically correct and what Clang is using, but maybe it would be better to saylocal variable or parameter? I might be missing some cases, but it sounds more user-friendly than the similar error about "automatic storage duration` that we have today:

<source>:4:10: error: 'x' cannot be captured because it does not have automatic storage duration

[Sirraide]

mentioning what kind of symbol we are actually referring to: 'class member 'x' cannot appear in capture list as it is not a variable'

I like this option the most personally. It makes it pretty clear what the problem is.

A fix-it might require some more thinking (e.g. don’t suggest adding this if it’s already there etc.), and I’m not sure how helpful it’d be. My suggestion would be to just add a new diagnostic for this case because that would already make this situation a lot less confusing.

At a glance, it looks like adding an else if branch for FieldDecl here might do the trick:

llvm-project/clang/lib/Sema/SemaLambda.cpp

Lines 1247 to 1250 in bd6e324

	if (auto *BD = R.getAsSingle<BindingDecl>())
	Var = BD;
	else
	Var = R.getAsSingle<VarDecl>();

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

1. Check that no other contributor has already been assigned to this issue. If you believe that no one is actually working on it despite an assignment, ping the person. After one week without a response, the assignee may be changed.
2. In the comments of this issue, request for it to be assigned to you, or just create a pull request after following the steps below. Mention this issue in the description of the pull request.
3. Fix the issue locally.
4. Run the test suite locally. Remember that the subdirectories under test/ create fine-grained testing targets, so you can e.g. use make check-clang-ast to only run Clang's AST tests.
5. Create a Git commit.
6. Run git clang-format HEAD~1 to format your changes.
7. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation. Mention this issue in the description of the pull request.

If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Ilya Biryukov (ilya-biryukov)

For the following code (https://gcc.godbolt.org/z/GGoWq45hs) Clang produces an error message that some people find confusing: ```cpp struct X { int x; void foo() { [x](){}; } }; // <source>:4:10: error: 'x' in capture list does not name a variable ```

It seems that the error message could indeed be improved, a few ideas that come to mind:

• mentioning what kind of symbol we are actually referring to: 'class member 'x' cannot appear in capture list as it is not a variable'
• we could suggest fixes in some situations did you mean to capture 'this'? or did you mean to use a capture with initializer?
• variable is technically correct and what Clang is using, but maybe it would be better to saylocal variable or parameter? I might be missing some cases, but it sounds more user-friendly than the similar error about "automatic storage duration` that we have today:

<source>:4:10: error: 'x' cannot be captured because it does not have automatic storage duration

[CedricSwa]

I would like to work on this for the weekend.

[Sirraide]

Feel free to @ me as a reviewer when you open a pr