Bug#1120951: marked as done (g++-15: Compilation error about class not being a non-union class type for valid C++ code)

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Your message dated Mon, 13 Jul 2026 11:57:03 +0200
with message-id <[email protected]>
and subject line fixed in 15.3.0
has caused the Debian Bug report #1120951,
regarding g++-15: Compilation error about class not being a non-union class=
 type for valid C++ code
to be marked as done.

This means that you claim that the problem has been dealt with.
If this is not the case it is now your responsibility to reopen the
Bug report if necessary, and/or fix the problem forthwith.

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1120951: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=3D1120951
Debian Bug Tracking System
Contact [email protected] with problems

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From: Markus Blatt <[email protected]>
To: Debian Bug Tracking System <[email protected]>
Subject: g++-15: Compilation error about class not being a non-union class type for valid C++ code
Message-ID: <176349029740.935681.13301472264724354397.reportbug@frau-mahlzahn.dr-blatt.de>
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Date: Tue, 18 Nov 2025 19:24:57 +0100
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Package: g++-15
Version: 15.2.0-8
Severity: important
Tags: upstream

Dear Maintainer,

the recently uploaded version of g++ caused compilation errors in some packages
during rebuils, e.g. opm-simulators [1].

This simple C++ program fails to compile:

#include <set>

template <class S>
class A
{
  typedef S VertexSet;

public:
  typename VertexSet::size_type size();

private:
  VertexSet vertices_;
};

template <class S>
inline typename A<S>::VertexSet::size_type A<S>::size()
{
  return vertices_.size();
}

int main()
{
  A<std::set<int>> a;
  return a.size();
}

with the error:

<source>:16:55: error: 'class A<S>::VertexSet' resolves to 'A<S>::VertexSet',
which is not a non-union class type [-Wtemplate-body]
   16 | inline typename A<S>::VertexSet::size_type A<S>::size()
      |                                                       ^
Compiler returned: 1

See also [2].

The expected outcome would be that the program compiles without any errors.

One can work around this problem by defining a typename size_type for
VertexSet::size_type and using that instead. A similar fix has been applied to
dune-istl in [3]

Best,

Markus

[1] https://buildd.debian.org/status/fetch.php?pkg=opm-
simulators&arch=loong64&ver=2025.10%2Bds-1%2Bb1&stamp=1763168796&raw=0
[2] https://godbolt.org/z/xodooafea
[3] https://salsa.debian.org/science-team/dune-
istl/-/commit/51fedd3623c511f79ef5c2c6321fb62b9c08b5cf


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