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

[Markus Blatt <markus@dr-blatt.de>]

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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