Always check for __dictrefoffset__ and __weakoffset__ in dataclasses with slots

Bug report

We now correctly check for __dictoffset__ and __weakoffset__ on C types without __slots__ defined:

Lines 1201 to 1212 in c68acb1

    
           match cls.__dict__.get('__slots__'): 
        
               # `__dictoffset__` and `__weakrefoffset__` can tell us whether 
        
               # the base type has dict/weakref slots, in a way that works correctly 
        
               # for both Python classes and C extension types. Extension types 
        
               # don't use `__slots__` for slot creation 
        
               case None: 
        
                   slots = [] 
        
                   if getattr(cls, '__weakrefoffset__', -1) != 0: 
        
                       slots.append('__weakref__') 
        
                   if getattr(cls, '__dictrefoffset__', -1) != 0: 
        
                       slots.append('__dict__') 
        
                   yield from slots

But, not in this case:

cpython/Lib/dataclasses.py

Lines 1213 to 1218 in c68acb1

    
           case str(slot): 
        
               yield slot 
        
           # Slots may be any iterable, but we cannot handle an iterator 
        
           # because it will already be (partially) consumed. 
        
           case iterable if not hasattr(iterable, '__next__'): 
        
               yield from iterable

So, in theory there might be C types with __slots__ and __dictoffset__ and __weakoffset__.

I will investigate!

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Always check for `dictrefoffset` and `weakoffset` in `dataclasses` with slots #118829

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	match cls.__dict__.get('__slots__'):
	# `__dictoffset__` and `__weakrefoffset__` can tell us whether
	# the base type has dict/weakref slots, in a way that works correctly
	# for both Python classes and C extension types. Extension types
	# don't use `__slots__` for slot creation
	case None:
	slots = []
	if getattr(cls, '__weakrefoffset__', -1) != 0:
	slots.append('__weakref__')
	if getattr(cls, '__dictrefoffset__', -1) != 0:
	slots.append('__dict__')
	yield from slots

	case str(slot):
	yield slot
	# Slots may be any iterable, but we cannot handle an iterator
	# because it will already be (partially) consumed.
	case iterable if not hasattr(iterable, '__next__'):
	yield from iterable

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Always check for __dictrefoffset__ and __weakoffset__ in dataclasses with slots #118829

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Always check for `dictrefoffset` and `weakoffset` in `dataclasses` with slots #118829