[clang] Deduction guides in anonymous namespaces or nested classes trigger -Wunused-function

[armandomontanez]

Summary: Unused deduction guides in anonymous namespaces or for nested classes trigger -Wunused-function.

Example error message:

<source>:12:3: error: unused function '<deduction guide for Bar>' [-Werror,-Wunused-function]
   12 |   Bar() -> Bar<0>;
      |   ^~~

Reproducer: https://godbolt.org/z/zW1KWdK84

This is basically the same failure mode as https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106604.

Appears to occur on all existing releases of clang.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (armandomontanez)

Summary: Unused deduction guides in anonymous namespaces or for nested classes trigger `-Wunused-function`.

Example error message:

<source>:12:3: error: unused function '<deduction guide for Bar>' [-Werror,-Wunused-function]
   12 |   Bar() -> Bar<0>;
      |   ^~~

Reproducer: https://godbolt.org/z/zW1KWdK84

This is basically the same failure mode as https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106604.

Appears to occur on all existing releases of clang.

[cor3ntin]

@hokein

[zygoloid]

It seems to me that there are two problems here:

• We shouldn't be warning for Foo::Bar at all -- the type has external linkage and is accessible in other files, so we shouldn't be warning just because the deduction guide is unused in the current file.
• The warning for Qux has surprising text and a surprising warning group. A separate warning that talks about deduction guides explicitly, with its own warning group under -Wunused, would make more sense.