[tannevaled]

Closes #655.

The defect, and how big it was

ShouldProxy inherited Object. Every predicate Object already answers was
therefore dispatched on the proxy and its result discarded, so the
assertion was never made. x.should.equal?(y) compared the proxy with y,
got false, threw it away, and the example passed whatever the two values were.

Eleven spellings were affected — nine predicates and two operators. Counted in
language/ + core/ of the pinned corpus (87b16319):

spelling	should.	should_not.	files	what it did instead
equal?	765	147	376	Object#equal? on the proxy → always false → discarded
instance_of?	738	2	341	Object#instance_of? on the proxy
eql?	542	26	68	Object#eql? on the proxy
is_a?	418	2	181	Object#is_a? on the proxy
frozen?	68	35	35	Object#frozen? on the proxy
respond_to?	21	30	32	Object#respond_to? → always true (respond_to_missing? returned true)
nil?	2	6	3	Object#nil? on the proxy
kind_of?	2	0	2	Object#kind_of? on the proxy
instance_variable_defined?	0	2	1	Object#instance_variable_defined? on the proxy
should === y	4	1	3	Object#=== → the proxy's ==, comparing the wrong pair — a false FAILURE
should !~ re	11	0	1	Object#!~ = !(x =~ y) → ran the proxy's positive =~ → the exact INVERSE

2 806 assertion sites across 811 files asserted nothing (or, for the last
two rows, asserted something else).

The fix: inherit less

Upstream mspec is immune by construction, and this reproduces it. Its operator
matchers subclass BasicObject
(mspec/lib/mspec/matchers/base.rb — class SpecPositiveOperatorMatcher < BasicObject), which answers only ==, !=, !, equal?, __send__,
__id__, instance_eval, instance_exec and method_missing. mspec spells
out the three it must and lets every other predicate fall through
#method_missing onto the real receiver, where its truthiness is asserted
(SpecExpectation.fail_predicate,
mspec/lib/mspec/expectations/expectations.rb).

So: class ShouldProxy < BasicObject, with equal? spelled out beside the
existing ==/!=, !~ spelled out, the deprecated matcher spellings
equal/eql kept (mspec/lib/mspec/matchers/equal.rb — the receiver has no
such method to forward to), and respond_to_missing? dropped. This also covers
spellings the corpus has not used yet: a predicate is broken exactly when the
proxy answers it, so the proxy must answer as little as possible.

Every constant inside the class is now ::-qualified. From inside a
BasicObject subclass MRI does not reach the top-level constants, so a bare
Exception raised NameError: uninitialized constant ShouldProxy::Exception
under MRI 4.0.5 — while rbgo resolved it happily. Since MRI through this shim
is the judge that separates a real rbgo defect from an inexpressible assertion,
that difference would have broken the judge and not us.

The measurement

One host, serially (an 8-way sweep of this corpus is not reproducible:
core/kernel/require_spec.rb reads 69 alone and 68 under load), three runs
each side, same corpus, same rbgo binary built from this branch.

	run 1	run 2	run 3	median
before	22340	22338	22339	22339
after	22136	22137	22136	22136

Decomposed per example: 205 examples newly FAIL, 2 newly PASS, and
core/random/bytes_spec.rb drifts ±1 between any two runs (it and
core/module/autoload_spec.rb are the only files that move between two
identical runs). 2 − 205 = −203; the −202 median-to-median difference is that
flake.

Counting note, because it bit me: matching failures by (kind, description)
first read 207 and 4. Two examples changed kind from error to fail —
a NoMethodError raised downstream of a vacuous assertion became the
assertion's own failure — and a kind change looks like one loss plus one
gain. Keyed on description alone the pair is 205 and 2.

106 files moved: 103 down, 3 up (one of the three up is the flake).

Classification of the new failures — MRI 4.0.5 as the per-file ceiling

Every one of the 205 newly-failing examples was matched by description against
the same file run under MRI 4.0.5 through the same fixed shim:

• (a) real rbgo defect the vacuous assertion was hiding: 205 / 205
• (b) assertion the shim still cannot express: 0 / 205

A sample, with the ceiling beside it:

file	rbgo before	rbgo after	MRI 4.0.5	what the hidden defect is
core/io/internal_encoding_spec.rb	79	67	79	IO#internal_encoding returns nil where MRI returns IBM866
core/io/external_encoding_spec.rb	59	48	59	IO#external_encoding returns the wrong Encoding object
core/rational/exponent_spec.rb	24	17	24	Rational#eql? is false for (1/1).eql?((1/1))
language/predefined_spec.rb	124	117	151	$& carries the wrong encoding; $& assignment raises NameError, not SyntaxError
language/singleton_class_spec.rb	42	35	53	singleton classes of true/false/nil
core/builtin_constants/builtin_constants_spec.rb	17	11	17	RUBY_VERSION, RUBY_COPYRIGHT … are not frozen
core/exception/receiver_spec.rb	7	1	8	NameError#receiver returns nil
core/numeric/step_spec.rb	208	202	208	float step yields 12.700000000000001 for 12.7
core/time/subsec_spec.rb	6	1	6	Time#subsec Rational identity (eql?)
core/dir/children_spec.rb	13	9	13	Dir.children ignores the requested encoding
core/hash/constructor_spec.rb	20	16	20	Hash.[] returns Hash where a subclass is required
core/integer/to_s_spec.rb	10	6	10	Integer#to_s returns UTF-8, not US-ASCII

Both honest gains are examples the shadowing made fail, not pass:
x.should === y reached Object#=== → the proxy's == and compared a class
with an instance (core/exception/case_compare_spec.rb,
core/range/case_compare_spec.rb).

One divergence this could not fix, stated rather than papered over

x.should !~ /re/ is now spelled out on the proxy and is correct under MRI.
rbgo never dispatches !~ as a method — it compiles a !~ b into
!(a =~ b) (and inlines ! the same way), where MRI sends :!~. So under
rbgo the definition is dead code and those 11 sites in
core/exception/full_message_spec.rb still run the positive =~ assertion,
i.e. the inverse of what the spec wrote. That is an rbgo/MRI divergence of its
own and worth its own issue; the shim cannot reach it from here. It changes no
number in this PR.

FLOOR: 22345 → 22170

The number fell because the measurement got stricter, not because the VM
regressed. No Go code changed on this branch; CGO_ENABLED=0 GOWORK=off go test ./... is green.

The floor is taken from the lane that enforces it, which is also where the
22345 came from:

	before	after
this host, serially, ×3	22340 / 22338 / 22339	22136 / 22137 / 22136
ruby/spec ratchet CI	22345 (frozen)	22170 (measured on this branch)

The 34-example gap between the two environments is the environment, not the
change: on this host the unchanged shim read 22339, six below the frozen
22345, so part of that number was never reproducible here either. A floor 34
below what the enforcing lane reads would be slack rather than safety, so the
gate moves 22345 → 22170, a drop of 175, all of it measurement.

A floor that no longer reflects a truthful measurement is worse than a lower
one. The 205 examples that now fail were never passing on their merits, and
each one is a real rbgo defect with a named ceiling.

Per-file movement, all 106 files (before → after, median of three runs each)

delta	file	before	after	note
-12	core/io/internal_encoding_spec.rb	79	67
-11	core/io/external_encoding_spec.rb	59	48
-7	core/rational/exponent_spec.rb	24	17
-7	language/predefined_spec.rb	124	117
-7	language/singleton_class_spec.rb	42	35
-6	core/builtin_constants/builtin_constants_spec.rb	17	11
-6	core/exception/receiver_spec.rb	7	1
-6	core/numeric/step_spec.rb	208	202
-5	core/time/subsec_spec.rb	6	1
-4	core/dir/children_spec.rb	13	9
-4	core/hash/constructor_spec.rb	20	16
-4	core/integer/to_s_spec.rb	10	6
-3	core/marshal/load_spec.rb	229	226
-3	core/range/step_spec.rb	76	73
-3	core/string/uminus_spec.rb	6	3
-3	core/time/new_spec.rb	115	112
-2	core/dir/entries_spec.rb	7	5
-2	core/env/shift_spec.rb	4	2
-2	core/exception/name_spec.rb	4	2
-2	core/false/to_s_spec.rb	3	1
-2	core/float/ceil_spec.rb	8	6
-2	core/float/floor_spec.rb	7	5
-2	core/float/to_s_spec.rb	27	25
-2	core/kernel/Float_spec.rb	99	97
-2	core/matchdata/post_match_spec.rb	4	2
-2	core/matchdata/pre_match_spec.rb	4	2
-2	core/module/name_spec.rb	18	16
-2	core/nil/to_s_spec.rb	3	1
-2	core/proc/dup_spec.rb	2	0
-2	core/range/frozen_spec.rb	4	2
-2	core/rational/ceil_spec.rb	9	7
-2	core/rational/divide_spec.rb	9	7
-2	core/rational/divmod_spec.rb	7	5
-2	core/rational/floor_spec.rb	8	6
-2	core/rational/minus_spec.rb	6	4
-2	core/rational/multiply_spec.rb	6	4
-2	core/rational/plus_spec.rb	6	4
-2	core/string/each_grapheme_cluster_spec.rb	15	13
-2	core/string/grapheme_clusters_spec.rb	13	11
-2	core/time/at_spec.rb	39	37
-2	core/time/dup_spec.rb	6	4
-2	core/time/to_r_spec.rb	2	0
-2	core/true/to_s_spec.rb	3	1
-1	core/argf/argf_spec.rb	2	1
-1	core/array/max_spec.rb	13	12
-1	core/array/min_spec.rb	13	12
-1	core/array/to_a_spec.rb	3	2
-1	core/array/to_ary_spec.rb	2	1
-1	core/basicobject/basicobject_spec.rb	7	6
-1	core/complex/divide_spec.rb	9	8
-1	core/dir/exist_spec.rb	12	11
-1	core/dir/to_path_spec.rb	3	2
-1	core/enumerable/to_set_spec.rb	3	2
-1	core/enumerator/arithmetic_sequence/each_spec.rb	2	1
-1	core/env/each_pair_spec.rb	5	4
-1	core/env/element_reference_spec.rb	5	4
-1	core/exception/backtrace_spec.rb	13	12
-1	core/exception/frozen_error_spec.rb	1	0
-1	core/fiber/new_spec.rb	5	4
-1	core/file/expand_path_spec.rb	26	25
-1	core/float/arg_spec.rb	6	5
-1	core/float/to_i_spec.rb	2	1
-1	core/float/truncate_spec.rb	3	2
-1	core/hash/element_set_spec.rb	11	10
-1	core/hash/merge_spec.rb	25	24
-1	core/hash/to_hash_spec.rb	2	1
-1	core/integer/ceil_spec.rb	10	9
-1	core/integer/floor_spec.rb	9	8
-1	core/io/read_spec.rb	105	104
-1	core/kernel/Rational_spec.rb	31	30
-1	core/kernel/backtick_spec.rb	3	2
-1	core/kernel/singleton_class_spec.rb	5	4
-1	core/kernel/sprintf_spec.rb	167	166
-1	core/matchdata/string_spec.rb	4	3
-1	core/module/define_method_spec.rb	85	84
-1	core/module/module_eval_spec.rb	16	15
-1	core/numeric/clone_spec.rb	4	3
-1	core/numeric/conjugate_spec.rb	1	0
-1	core/numeric/dup_spec.rb	2	1
-1	core/numeric/quo_spec.rb	7	6
-1	core/objectspace/_id2ref_spec.rb	11	10
-1	core/proc/clone_spec.rb	2	1
-1	core/proc/new_spec.rb	11	10
-1	core/proc/source_location_spec.rb	5	4
-1	core/range/clone_spec.rb	2	1
-1	core/range/dup_spec.rb	2	1
-1	core/range/new_spec.rb	10	9
-1	core/range/to_set_spec.rb	4	3
-1	core/string/chars_spec.rb	11	10
-1	core/string/chomp_spec.rb	65	64
-1	core/string/each_char_spec.rb	11	10
-1	core/string/freeze_spec.rb	3	2
-1	core/string/modulo_spec.rb	265	264
-1	core/thread/start_spec.rb	4	3
-1	core/time/ceil_spec.rb	7	6
-1	core/time/floor_spec.rb	5	4
-1	core/time/inspect_spec.rb	8	7
-1	core/time/local_spec.rb	47	46
-1	core/time/now_spec.rb	17	16
-1	core/time/round_spec.rb	5	4
-1	core/time/to_s_spec.rb	3	2
-1	core/tracepoint/raised_exception_spec.rb	2	1
-1	language/range_spec.rb	5	4
+1	core/exception/case_compare_spec.rb	5	6
+1	core/random/bytes_spec.rb	8	9	±1 run-to-run flake
+1	core/range/case_compare_spec.rb	41	42

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