[Bug 2089259] [NEW] GCC14 C++ CTAD failure due to ambiguous overload

[Paul Fee]

Public bug reported:

While using a C++ compile time fixed_string library, I encountered an
issue with Ubuntu's version of GCC 14.

I tried a number of combinations:
* GCC-13, C++20: Pass
* GCC-13, C++23: Pass
* GCC-14, C++20: Pass
* GCC-14, C++23: Fail

Same results on both Ubuntu 24.04 (g++-14 14.2.0-4ubuntu2~24.04) and
Ubuntu 24.10 (g++-14 14.2.0-4ubuntu2).

Testing on another distro (OpenSUSE Tumbleweed,
gcc-c++-14-2.1.x86_64.rpm) all combinations passed.

Distro compiler versions reported by "g++-14 -v"
Ubuntu 24.04: gcc version 14.2.0 (Ubuntu 14.2.0-4ubuntu2~24.04)
Ubuntu 24.10: gcc version 14.2.0 (Ubuntu 14.2.0-4ubuntu2)
Tumbleweed: gcc version 14.2.1 20241007 [revision 4af44f2cf7d281f3e4f3957efce10e8b2ccb2ad3] (SUSE Linux)

I've stripped down the code in question to produce a minimal
reproducible example (see attachment).

Since the code works with GCC-14 on another distro, is Ubuntu's package
lacking upstream updates?

** Affects: gcc-14 (Ubuntu)
     Importance: Undecided
         Status: New

** Attachment added: "Illustration of compilation failure on Ubuntu"
   https://bugs.launchpad.net/bugs/2089259/+attachment/5839139/+files/ambiguous_ctad.cpp

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https://bugs.launchpad.net/bugs/2089259

Title:
  GCC14 C++ CTAD failure due to ambiguous overload

Status in gcc-14 package in Ubuntu:
  New

Bug description:
  While using a C++ compile time fixed_string library, I encountered an
  issue with Ubuntu's version of GCC 14.

  I tried a number of combinations:
  * GCC-13, C++20: Pass
  * GCC-13, C++23: Pass
  * GCC-14, C++20: Pass
  * GCC-14, C++23: Fail

  Same results on both Ubuntu 24.04 (g++-14 14.2.0-4ubuntu2~24.04) and
  Ubuntu 24.10 (g++-14 14.2.0-4ubuntu2).

  Testing on another distro (OpenSUSE Tumbleweed,
  gcc-c++-14-2.1.x86_64.rpm) all combinations passed.

  Distro compiler versions reported by "g++-14 -v"
  Ubuntu 24.04: gcc version 14.2.0 (Ubuntu 14.2.0-4ubuntu2~24.04)
  Ubuntu 24.10: gcc version 14.2.0 (Ubuntu 14.2.0-4ubuntu2)
  Tumbleweed: gcc version 14.2.1 20241007 [revision 4af44f2cf7d281f3e4f3957efce10e8b2ccb2ad3] (SUSE Linux)

  I've stripped down the code in question to produce a minimal
  reproducible example (see attachment).

  Since the code works with GCC-14 on another distro, is Ubuntu's
  package lacking upstream updates?

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