[jiro-by-nori]

Authored by Jiro, an autonomous coding agent, on behalf of @sam-xif — the code and this description are machine-written.

Fixes #93.

What changed

Pathname#initialize in the prelude assigned path.to_s. CRuby's constructor is @path = File.path(path).dup, so:

• a String (subclasses included) is the path, unchanged;
• anything else converts through #to_path, then #to_str — never #to_s;
• whether a conversion exists is rb_check_funcall's question, not respond_to?'s, so the constructor asks the prelude's __conv_callable? (the same predicate String.try_convert uses): a #to_path that only method_missing serves is called, a custom respond_to_missing? that denies the name is believed, and a #to_path that answers nil does not fall back to #to_str;
• if the result is not a String: TypeError: Pathname.new requires a String, #to_path or #to_str;
• a path with a null byte, given directly or produced by #to_path: ArgumentError: path name contains null byte;
• the path is duped, so mutating the argument afterwards leaves the Pathname alone.

Three wrong answers follow from the to_s coercion, all fixed here: Pathname.new(123)/nil/:sym/[1]/Object.new answered a Pathname where CRuby raises TypeError (a type-error-family exception the validator claims to exclude, so a wrong answer rather than a gate); an object with both #to_path and a different #to_s got the wrong path; and a mutated argument changed the path.

ruby-lean/RubyCore/Generated/Prelude.lean and PreludeJson.lean are the make gen output of that prelude edit.

Test plan

New regression case difftest/corpus/regressions/pathname-new-requires-a-path.rb (plus a fixed sidecar) covering, against CRuby 4.0.5: the six rejection shapes (Integer, nil, true, Symbol, Array, Object.new), Kernel#Pathname on a non-String, #to_path preferred over #to_str, a bad and a nil #to_path not falling back, the three method_missing/respond_to_missing? shapes, both null-byte paths, a Pathname and a String subclass as controls, and the dup.

Command	Result
bin/ruby-lean --compare difftest/corpus/regressions/pathname-new-requires-a-path.rb	AGREE — identical stdout, no exception
the same case against the pre-change prelude	red on every coercion line (integer => #<Pathname:123> vs CRuby's TypeError, to_path wins => "/from-to-s" vs "/from-to-path", argument mutated after => "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/before/after" vs "/before"), so the case pins the defect instead of passing trivially
make conformance + scripts/check-conformance.sh	1287 ran, 1087 agree, 194 declined, 0 disagree — CONFORMANCE OK, the recorded baseline held exactly
difftest run --tier regressions --sut lean	223 agree, 1 gated (pre-existing sorbet-hash-gate), 0 failures; the new case reconciles as held
difftest replay corpus/tier3 --sut lean	57 agree, 13 declined, 0 disagreements
make books	1405 jobs, Build completed successfully (the booted heap is regenerated from the new prelude)
make metatheory	OK — theorems rest on propext/Classical.choice/Quot.sound only, and the booted heap satisfies the assumptions
make gen-check	all four generated sources fresh
scripts/check-not-weakened.py	CHECKS NOT WEAKENED

Not run locally, with the reason: make lint and the Bundler-backed suites (desugar-unit, emitter-unit, difftest-test, soundness, book-checks). The machine this was written on has no C compiler, so bundle install cannot build the pinned json gem and make deps fails before those targets start. The change adds no linted file — ruby-lean/prelude/ and difftest/corpus/ are excluded from RuboCop — and lint passes in CI. The soundness and comparator halves are covered by CI's proofs job.

Related, deliberately not in this PR

• Pathname#empty? answers String#empty?; a separate wrong answer, tracked in Pathname#empty? answers String#empty? instead of asking the filesystem, so it is a wrong answer #91.
• The prelude's File path helpers still coerce with to_s (File path helpers coerce non-String paths with to_s instead of refusing, so the model answers where CRuby raises TypeError #57, PRs File path helpers convert arguments the way CRuby does, and raise TypeError where CRuby does (fixes #57) #148/File path helpers run rb_get_path, not to_s, so a non-String path is refused (fixes #57) #153).