Restore blockless overloads for optional-block methods

[mybys]

Summary

Array#sum, Array#count and Hash#transform_keys! were converted from explicit overloads to optional blocks (?{ ... }) in 4.1. This makes block-pass arguments fail to type check under Steep, so code that worked with 4.0.3 now reports Ruby::BlockTypeMismatch. This PR restores the blockless overload for those three methods and adds a test/typecheck case covering them.

Motivation

Steep cannot check a block-pass argument against an optional block. The expected type becomes a union with nil, and the special case that lets a plain ::Proc (what Symbol#to_proc returns) satisfy a block type does not apply to the union — see soutaro/steep#1207, open since 2024.

With master as of 7534c7e:

[1, 2, 3].sum(&:to_r)
[1, 2, 3].count(&:even?)

hash = { 1 => 2 } #: Hash[Integer, Integer]
hash.transform_keys!({ 1 => 3 }, &:itself)

test.rb:1:14: [error] Cannot pass a value of type `::Proc` as a block-pass-argument of type `(^(::Integer) -> untyped | nil)`
│   ::Proc <: (^(::Integer) -> untyped | nil)
│     ::Proc <: ^(::Integer) -> untyped
│
│ Diagnostic ID: Ruby::BlockTypeMismatch
│
└ [1, 2, 3].sum(&:to_r)
                ~~~~~~

The same call with a literal block is accepted, and [1, 2, 3].map(&:to_r) is accepted too because Enumerable#map declares a required block. Writing the blockless case as its own overload restores the pre-4.1 behaviour: the block-pass argument then matches the overload with a required block.

This was found while upgrading an application from 4.0.3 to 4.1.1, where it produced 37 Ruby::BlockTypeMismatch errors, all from sum(&:some_method).

Changes

• core/array.rbs: split Array#sum and Array#count back into blockless and block-taking overloads.
• core/hash.rbs: same for the (hash[K, K] replacements) branch of Hash#transform_keys!.
• test/typecheck/block_pass/: new steep check case that fails on master and passes here.

The overloads accept exactly the same calls as the optional-block form, so this is not a change in what type checks apart from block-pass arguments.

Not included

Module#initialize was converted the same way in 4.1, but restoring its overloads does not help: the block gained a [self: self] binding, and Steep rejects a ::Proc against a self-bound block whether it is required or optional. Class#initialize already used ?{ ... } in 4.0.3, so it is not a regression. Both are left alone.

Testing

• rake typecheck_test — passes, including the new test/typecheck/block_pass.
• rake test — 967 tests, 0 failures.
• rake test/stdlib/Array_test.rb, rake test/stdlib/Hash_test.rb — no new failures. (Array_test has a pre-existing error for Array#rfind when run on Ruby 3.4, present on master too.)
• rake validate and bin/steep check — pass.

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