clang: Missing line and file in message error when ‘auto’ is used in a not permitted context (and bad diagnostic)

[jonathanpoelen]

With this code:

int main()
{ 
  struct A
  { 
    void foo(auto x) {} // auto not permitted
  };
} 

And clang-18,19,20 or trunk and -std=c++20 or -std=c++23:

error: templates cannot be declared inside of a local class

No line number, no filename and refer to template.

With -std=c++17:

test.cpp:5:14: error: 'auto' not allowed in function prototype
    5 |     void foo(auto x) {}
      |              ^~~~

With gcc (same message for C++17 or C++20):

test.cpp: In function ‘int main()’:
test.cpp:5:14: error: ‘auto’ parameter not permitted in this context
    5 |     void foo(auto x) {}
      |              ^~~~

https://gcc.godbolt.org/z/5YGdh6Pnb

[AaronBallman]

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

Line 8242 in 2485c51

return Diag(TemplateParams->getTemplateLoc(),

The problem is that it's presuming there's a template keyword to specify as the location, but it's not there for auto. (The same issue happens elsewhere in that function.)

CC @erichkeane @mizvekov

Possibly a good first issue? I'm thinking: add a select to the diagnostic so that there's a distinction between auto and template usage, and use the correct source location for the problematic keyword?

[erichkeane]

Ooof, yeah. Probably a good first issue, but yeah, a better diagnostic wording + some fall-back on the location would be the right idea here.

[mizvekov]

Yeah, this looks like a good first issue. Another possibility here is that for the 'auto' parameter case, we already know the templated entity at that point (foo), and could point to that instead, also pointing to the source range of the auto parameter which introduces the function template.

[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:

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[llvmbot]

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

Author: Jonathan Poelen (jonathanpoelen)

With this code:

int main()
{ 
  struct A
  { 
    void foo(auto x) {} // auto not permitted
  };
} 

And clang-18,19,20 or trunk and -std=c++20 or -std=c++23:

> error: templates cannot be declared inside of a local class

No line number, no filename and refer to template.

With -std=c++17:

test.cpp:5:14: error: 'auto' not allowed in function prototype
    5 |     void foo(auto x) {}
      |              ^~~~

With gcc (same message for C++17 or C++20):

test.cpp: In function ‘int main()’:
test.cpp:5:14: error: ‘auto’ parameter not permitted in this context
    5 |     void foo(auto x) {}
      |              ^~~~

https://gcc.godbolt.org/z/5YGdh6Pnb

[emily-dror]

Hi, I'd like to work on this issue. Would someone assign it to me please?

[emily-dror]

July 17th update:

Hi team, I'm updating that I was able to find a solution to the problem above:

✗ bin/clang++ -std=c++20 ../error.cpp
../error.cpp:3:19: error: templates cannot be declared inside of a local class
    3 |     void foo(auto x) {} // auto not permitted
      |                   ^
1 error generated.

I'm posting this update to avoid reassigning the case since it's been a week.
Thanks

[mizvekov]

@emily-dror Thanks! Can you make sure to mention this issue from your PR, by appending a Fixes #147324 to the PR description?

This will also make sure this issue is automatically closed when that PR is merged.