8000 [clang] Deduction guides in anonymous namespaces or nested classes trigger -Wunused-function · Issue #141281 · llvm/llvm-project · GitHub
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[clang] Deduction guides in anonymous namespaces or nested classes trigger -Wunused-function #141281

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armandomontanez opened this issue May 23, 2025 · 3 c 8000 omments
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[clang] Deduction guides in anonymous namespaces or nested classes trigger -Wunused-function #141281

armandomontanez opened this issue May 23, 2025 · 3 comments
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clang:frontend Language frontend issues, e.g. anything involving "Sema"

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@armandomontanez
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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 llvmbot added the clang Clang issues not falling into any other category label May 23, 2025
@EugeneZelenko EugeneZelenko added clang:frontend Language frontend issues, e.g. anything involving "Sema" and removed clang Clang issues not falling into any other category labels May 24, 2025
@llvmbot
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llvmbot commented May 24, 2025

@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:

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

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
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@hokein

Sorry, something went wrong.

@zygoloid
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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.

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