[clang] Poor diagnostic with wrong template argument type

[antoniofrighetto]

Clang diagnostic for invalid bool-to-enum template parameter conversion lacks failure detail when compared to GCC, as the following C++ snippet:

enum endianness { little = 0, big };

template <endianness e>
inline void foo() {}

void bar() {
    foo<false>();
}

Fails as follows on latest clang:

<source>:7:5: error: no matching function for call to 'foo'
    7 |     foo<false>();
      |     ^~~~~~~~~~
<source>:4:13: note: candidate template ignored: invalid explicitly-specified argument for template parameter 'big_endian'
    4 | inline void foo() {}

GCC explicitly indicates the conversion failure instead:

<source>:4:13: note: template argument deduction/substitution failed:
<source>:7:15: error: could not convert 'false' from 'bool' to 'endianness'
    7 |     foo<false>();
      |     ~~~~~~~~~~^~
      |               |
      |               bool

Godbolt: https://godbolt.org/z/hnazTn3M7 (cc/ @shafik).

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Antonio Frighetto (antoniofrighetto)

Clang diagnostic for invalid bool-to-enum template parameter conversion lacks failure detail when compared to GCC, as the following C++ snippet:

enum endianness { little = 0, big };

template <endianness e>
inline void foo() {}

void bar() {
    foo<false>();
}

Fails as follows on latest clang:

<source>:7:5: error: no matching function for call to 'foo'
    7 |     foo<false>();
      |     ^~~~~~~~~~
<source>:4:13: note: candidate template ignored: invalid explicitly-specified argument for template parameter 'big_endian'
    4 | inline void foo() {}

GCC explicitly indicates the conversion failure instead:

<source>:4:13: note: template argument deduction/substitution failed:
<source>:7:15: error: could not convert 'false' from 'bool' to 'endianness'
    7 |     foo<false>();
      |     ~~~~~~~~~~^~
      |               |
      |               bool

Godbolt: https://godbolt.org/z/hnazTn3M7 (cc/ @shafik).

[shafik]

CC @erichkeane

[erichkeane]

I agree, we could definitely do a much better job here. I suspect our template diagnostics would need an additional 'type' for the note. It isn't quite a good first issue, but this shouldn't be too awful. I'll mark as 'good first issue', though note this'll be one of the harder ones

[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: Antonio Frighetto (antoniofrighetto)

Clang diagnostic for invalid bool-to-enum template parameter conversion lacks failure detail when compared to GCC, as the following C++ snippet:

enum endianness { little = 0, big };

template <endianness e>
inline void foo() {}

void bar() {
    foo<false>();
}

Fails as follows on latest clang:

<source>:7:5: error: no matching function for call to 'foo'
    7 |     foo<false>();
      |     ^~~~~~~~~~
<source>:4:13: note: candidate template ignored: invalid explicitly-specified argument for template parameter 'big_endian'
    4 | inline void foo() {}

GCC explicitly indicates the conversion failure instead:

<source>:4:13: note: template argument deduction/substitution failed:
<source>:7:15: error: could not convert 'false' from 'bool' to 'endianness'
    7 |     foo<false>();
      |     ~~~~~~~~~~^~
      |               |
      |               bool

Godbolt: https://godbolt.org/z/hnazTn3M7 (cc/ @shafik).

[AidanGoldfarb]

I would be happy to pick this up and give it a shot