[cppyy] fix: aliasing of TemplateProxy

[Vipul-Cariappa]

Fixes: #21405

[guitargeek]

Nice, makes sense to me! Thanks for fixing this issue.

[vepadulano]

Nice thank you! I just had a minor suggestion, feel free to disregard.

[github-actions]

Test Results

    22 files      22 suites   3d 2h 24m 0s ⏱️
 3 831 tests  3 829 ✅ 1 💤 1 ❌
75 737 runs  75 727 ✅ 9 💤 1 ❌

For more details on these failures, see this check.

Results for commit 8f21994.

[hahnjo]

FWIW this fixes the "original" minimal reproducer #21405, but using it once "breaks" other overloads:

>>> import ROOT
>>> ROOT.gInterpreter.Declare("struct A { template <typename... Ts> void m(std::vector<int> v) {} template <typename T = void> void m(int i) {} };")
True
>>> a = ROOT.A()
>>> type(a)._m = type(a).m
>>> type(a).m = lambda self, x: self._m(x)
>>> a.m(42)
>>> a.m([1, 2, 3])
>>> a.m(42)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: void A::m(vector<int> v) =>
    TypeError: could not convert argument 1

I'm not sure if this should be addressed here, or if it's a different issue

[dpiparo]

@Vipul-Cariappa ?