FIX Accept layers_to_transform=0 together with layers_pattern - #3425

No linked issue — found by reading the code.

What this fixes

Every tuner config validates the layers_pattern / layers_to_transform pairing in __post_init__ with:

if self.layers_pattern and not self.layers_to_transform:
    raise ValueError("When `layers_pattern` is specified, `layers_to_transform` must also be specified.")

Because 0 is falsy, passing the valid first-layer index layers_to_transform=0 together with a layers_pattern wrongly raises — even though layers_to_transform was specified:

from peft import LoraConfig

LoraConfig(target_modules=["q_proj"], layers_to_transform=0, layers_pattern="blocks")
# ValueError: When `layers_pattern` is specified, `layers_to_transform` must also be specified.

LoraConfig(target_modules=["q_proj"], layers_to_transform=1,  layers_pattern="blocks")  # OK
LoraConfig(target_modules=["q_proj"], layers_to_transform=[0], layers_pattern="blocks")  # OK

Only index 0 is affected — 1 and [0] (the same logical request) are accepted.

Why is None is the correct check

• The consuming matcher already treats 0 as a valid index — check_target_module_exists in src/peft/tuners/tuners_utils.py gates on layer_indexes is not None, and the sibling check a few lines above in the same __post_init__ also uses self.layers_to_transform is not None.
• The docstring states the pattern is "used only if layers_to_transform is different from None."

So the guard should compare against None, not truthiness. The check was added in #2159 and its message improved in #2169, but the falsy-0 case remained.

Scope

The identical guard is duplicated across all 19 tuner configs that support layer indexing (lora, adalora, boft, deft, delora, fourierft, frod, hra, lily, loha, lokr, oft, peanut, psoft, pvera, tinylora, vblora, vera, waveft), so the one-line fix is applied to every one.

Testing

Added test_config_layers_to_transform_index_zero, parametrized over every config in ALL_CONFIG_CLASSES that supports layers_to_transform (25 configs), asserting layers_to_transform=0 + layers_pattern is accepted. It fails before this change (19 of the configs raise) and passes after. ruff check / ruff format --check are clean and tests/test_config.py passes (1218 passed).