Fix Issue 22760 - Segmentation fault in CppMangleVisitor.template_arg

[RazvanN7]

For aggregate declarations at the template level the parent is not the template instance but the aggregate. For example:

struct T(U)
{
      struct S {}
}

For a given instantiation, say T!int, the parent of S is not T!int but T!int.T. That is because the compiler rewrites the struct to an eponymous template. That is why, in this PR, I am checking the parent of parent for writing the cpp mangled name. I am keeping the old case because there are situations such as:

template T(U)
{
     int a;
     struct T {}
}

In this case, the parent of a is going to be T!whatever, not T!whatever.T.

@Geod24 can you please take a look at this, since you are the original author? I don't particularly like this fix, it looks like a special case handling, however, I have no idea if a better fix is possible.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	22760	critical	Segmentation fault in CppMangleVisitor.template_arg

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "stable + dmd#15099"

[dkorpel]

@Geod24

[dkorpel]

@RazvanN7 Can you rebase to rerun tests, or force merge at some point?

[RazvanN7]

@dkorpel done