Fix GenericRepr dropping args of a secondary inspected class with no defaults - #13478

Summary

In GenericRepr (util/langhelpers.py), the loop over to_inspect guards the argument slice with if default_len for the first inspected class (i == 0), falling back to spec.args[1:] when a class has no default arguments. The branch for subsequent inspected classes does not:

if i == 0:
    ...
    if default_len:
        pos_args.extend(spec.args[1:-default_len])
    else:
        pos_args.extend(spec.args[1:])
else:
    kw_args.update(
        [(arg, missing) for arg in spec.args[1:-default_len]]   # no guard
    )

When a secondary inspected class has no default arguments, default_len == 0, so spec.args[1:-default_len] becomes spec.args[1:-0] == spec.args[1:0] == [], and all of that class's arguments are silently dropped from the repr.

Reproduction

class Foo:            # secondary, no defaults
    def __init__(self, a): ...

class Bar(Foo):
    def __init__(self, b=2, **kw): ...

generic_repr(Bar(a="a", b="b"), to_inspect=[Bar, Foo])
# before: "Bar(b='b')"          <- 'a' dropped
# after:  "Bar(b='b', a='a')"

Fix

Mirror the i == 0 guard:

-                    [(arg, missing) for arg in spec.args[1:-default_len]]
+                    [
+                        (arg, missing)
+                        for arg in (
+                            spec.args[1:-default_len]
+                            if default_len
+                            else spec.args[1:]
+                        )
+                    ]

Tests

Added GenericReprTest.test_multi_kw_secondary_no_defaults, which fails on the previous behavior (drops the secondary class's arg) and passes with the fix.