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The builtin Warning exception hierarchy is missing

Repro

print(Warning, UserWarning, DeprecationWarning)



pxx:     pascal26: error, near: print  Warning >>>  UserWarning
CPython: <class 'Warning'> <class 'UserWarning'> <class 'DeprecationWarning'>

Why a shim cannot paper over it

These are builtins, not members of warnings — calling code names them bare, and subclasses them:

# html5lib/constants.py:2940
class DataLossWarning(UserWarning):
    pass

So mimic_warnings cannot supply them however it is written, and a mimic_warnings shipped without them would resolve import warnings and then fail one line later at the first category argument. That is worse than the honest missing module and violates the T1 rule in devdocs/dev/python-compat-tiers.md (a shim states its subset and fails loudly rather than approximating).

Blast radius, measured not estimated

Every warnings.warn call in non-test html5lib code passes a category:

category sites
DataLossWarning 14
DeprecationWarning 2

The 14 are indirect — they need UserWarning only so that DataLossWarning can be declared — so all 16 sit behind this one gap.

Note on scope

The full CPython warning tree is large (SyntaxWarning, RuntimeWarning, BytesWarning, …). What is measured as load-bearing here is three names: Warning, UserWarning, DeprecationWarning. Whether to add the three or the whole tree is a judgement for whoever owns the builtin exception table — the rest are cheap once the base exists, and a partial tree that silently misses a name is the failure mode worth avoiding.

Gate

class D(UserWarning): pass compiles, warnings.warn(msg, D) accepts it, and the three names print as classes like CPython.

2026-08-17 (frank3) — working. Premises re-measured on the current tree.

Assigned to Track B rather than N because the fix is runtime-library ground: the exception table lives in compiler/builtin/pylib.pas, not in the NilPy frontend files, and is disjoint from the parser.inc / ir*.inc work Track A holds tonight. No shared-core file is touched.

Re-measured, unchanged

print(Warning, UserWarning, DeprecationWarning)
  -> pascal26:1: error: undefined variable (Warning)

while Exception, ValueError, TypeError, RuntimeError and KeyError all resolve. So the gap is exactly the Warning subtree and nothing adjacent.

Where it goes, and why it is not a shim

compiler/builtin/pylib.pas already carries the whole builtin exception tree as a flat table of X = class(Y) end; — OSError, LookupError, RecursionError, the ConnectionError family, the IOError = OSError alias. The Warning names are peers of those, and they are builtins named bare by calling code (class DataLossWarning(UserWarning)), which is what makes a mimic_warnings structurally unable to supply them.

Scope: the whole tree, not the three that are load-bearing

The earlier note left this open ("whether to add the three or the whole tree is a judgement"). Resolved: all twelve, read off CPython 3.12's builtins rather than from memory —

Warning(Exception), then UserWarning, DeprecationWarning, PendingDeprecationWarning, SyntaxWarning, RuntimeWarning, FutureWarning, ImportWarning, UnicodeWarning, BytesWarning, ResourceWarning, EncodingWarning, all deriving directly from Warning; none nest.

A partial table is the worse failure mode here and the ticket said so: the missing name does not announce itself, it surfaces later as undefined variable (ResourceWarning) inside somebody's library — and each addition costs another round trip through a pin, which is a repo-wide lock.

Landed

compiler/builtin/pylib.pas gains the twelve names beside the existing builtin exception table. Verified against the freshly built compiler:

print(Warning, UserWarning, DeprecationWarning)
  -> <class '__main__.Warning'> <class '__main__.UserWarning'> <class '__main__.DeprecationWarning'>

test/test_nilpy_warning_hierarchy.npy + .expected, wired into test-nilpy. It runs unmodified under CPython and the two outputs are byte-identical, so the expectations come from the oracle rather than from memory. It asserts the tree shape (Warning under Exception, all eleven others directly under Warning), that they are SIBLINGS rather than a chain (issubclass(DeprecationWarning, UserWarning) is False — the easy thing to get wrong in a hand-written table), subclassing (class DataLossWarning(UserWarning), the shape html5lib actually uses), raise/catch at each level, and passing the class itself as an argument, which is what warnings.warn(msg, Category) needs.

All twelve are named individually rather than looped over a list. Two reasons: issubclass() here takes class NAMES and refuses a variable holding a class (clear diagnostic, not a crash — my first draft hit it), and naming them makes a missing one a compile error in this file rather than a silent gap.

Note for whoever picks up the warnings shim

This unblocks the categories, not the module. import warnings still needs mimic_warnings — warn(msg, category) writing to stderr, with simplefilter/catch_warnings/resetwarnings as no-ops. That is the remaining half of [[feature-nilpy-six-and-warnings-shims]] and is now genuinely writable, which it was not this morning.

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