[tannevaled]

Summary

Advances MRI 4.0.6 conformance of Kernel#sprintf / Kernel#format / String#%. core/string/modulo_spec rises 252 → 263 examples passing (fail 11 → 6, error 8 → 2); the remaining modulo failures are all encoding-subsystem (Encoding::CompatibilityError, US-ASCII/BINARY tracking) or the $DEBUG IOStub harness, outside this cluster.

Binding changes (internal/vm/format_bind.go)

• %c operand coercion — formatValue.CoerceChar resolves a user object with MRI's to_str-then-to_int protocol: a String feeds first-character rendering, an Integer a code point, a non-String/#to_int result raises the matching can't convert X into String/Integer TypeError, and an object answering neither raises no implicit conversion of X into Integer.
• %<name> / %{name} Hash#default — formatNamedArgs registers a NamedArgs default resolver, so a reference to an absent key consults the hash's own #[] (Hash#default / default_proc): a non-nil default is used, a nil one raises MRI's KeyError (with #key/#receiver set).
• formatRespondsTo — probes a coercion method via the method table and only dispatches #respond_to? (hence #respond_to_missing?) to an object that actually provides it, matching MRI's rb_check_funcall. Fixes "%c" % basic_object_with_to_str, which previously raised NoMethodError: undefined method 'respond_to?' for BasicObject while unpacking the % right-hand side.

The hex-float alternate-form (%#a → 0x1.p+4) and zero-fill (%020a → 0x000…1.9p+3), the argument-numbered star width/precision (%*1$.*2$3$d), and the width/precision too big ArgumentError are fixed in the format engine (go-ruby-format/format#7) and land here through the dependency bump.

Dependency

Important

go.mod bumps github.com/go-ruby-format/format to a commit of the branch behind go-ruby-format/format#7, which is not yet merged. Merge that engine PR first, then re-pin this module to the resulting main pseudoversion before merging here.

Testing

• GOWORK=off CGO_ENABLED=0 go test ./... — green.
• GOOS=js GOARCH=wasm go build ./... — green.
• gofmt -l on changed files — empty.
• Merged coverage (-coverpkg over internal/...) — 100.0%; the new CoerceChar, formatRespondsTo, and formatNamedArgs are each 100%.
• New internal/vm/format_c_named_test.go covers the coercion, default-resolver, and KeyError branches; every case verified byte-for-byte against ruby 4.0.

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