Write specs for new Ruby 2.5 features and changes

[eregon]

ruby/spec already contains many specs for 2.5, but we should aim to cover all new features and important changes.
This will improve the test coverage of these features (and maybe discover a few bugs along the way), allow other Ruby implementations to implement the changes faster with more confidence and document clearly the new behavior.

The new specs should be within a version guard block:

ruby_version_is "2.5" do
  # New specs
end

From https://github.com/ruby/ruby/blob/trunk/NEWS.
Only relevant items are kept for brevity.

NEWS for Ruby 2.5.0

Changes since the 2.4.0 release

Language changes

• rescue/else/ensure are allowed inside do/end blocks. Feature #12906

• Top-level constant look-up is removed. Feature #11547

• refinements take place in string interpolations. Feature #13812

Core classes updates (outstanding ones only)

Array

• Array#append Feature #12746
• Array#prepend Feature #12746

Data

• Is deprecated. It was a base class for C extensions, and it's not
  necessary to expose in Ruby level. Feature #3072

Exception

• Exception#full_message to retrieve a String expression of an exception,
  formatted in the same way in which Ruby prints out an uncaught exception.
  Feature #14141 [experimental]

Dir

• Dir.glob provides new optional keyword argument, :base. Feature #13056
• Dir.children Feature #11302
• Dir.each_child Feature #11302

Enumerable

• Enumerable#{any?,all?,none?,one?} accept a pattern argument Feature #11286

File

• File.open accepts :newline option to imply text mode. Bug #13350
• File#path raises an IOError for files opened with
  File::Constants::TMPFILE option. Feature #13568
• File::Stat#{atime,mtime,ctime} support fractional second timestamps on
  Windows 8 and later Feature #13726
• File::Stat#ino and File.indentical? support ReFS 128bit ino on Windows 8.1
  and later Feature #13731
• File.lutime Feature #4052

Hash

• Hash#transform_keys Feature #13583
• Hash#transform_keys! Feature #13583
• Hash#slice Feature #8499

IO

• IO#pread Feature #4532
• IO#pwrite Feature #4532
• IO#copy_stream tries copy offload with copy_file_range(2) Feature #13867
• IO#write accepts multiple arguments Feature #9323

IOError

• IO#close might raise an error with message "stream closed",
  but it is refined to "stream closed in another thread". The new message
  is more clear for user.
  Bug #13405

Integer

• Integer#step no longer hides errors from coerce method when
  given a step value which cannot be compared with #> to 0.
  Feature #7688
• Integer#{round,floor,ceil,truncate} always return an Integer.
  Bug #13420
• [Ruby 2.5] Integer#pow accepts modulo argument for calculating modular exponentiation #573 Integer#pow accepts modulo argument for calculating modular
  exponentiation. Feature #12508 Feature #11003
• Integer#allbits?, Integer#anybits?, Integer#nobits? Feature #12753
• Integer.sqrt Feature #13219

Kernel

• Kernel#yield_self Feature #6721
• Kernel#pp Feature #14123
• Kernel#warn(..., uplevel:n) Feature #12882

Method

• Method#=== that invokes Method#call, as same as Proc#=== Feature #14142

Module

• Module#{attr,attr_accessor,attr_reader,attr_writer} become public Feature #14132
• Module#{define_method,alias_method,undef_method,remove_method} become public Feature #14133

Numeric

• Numerical comparison operators (<,<=,>=,>) no longer hide exceptions
  from #coerce method internally. Return nil in #coerce if the coercion is
  impossible. Feature #7688

Process

• Precision of Process.times is improved if getrusage(2) exists. Feature #11952
• Process.last_status as an alias of $? Feature #14043

Range

• Range#initialize no longer hides exceptions when comparing begin and
  end with #<=> and raise a "bad value for range" ArgumentError
  but instead lets the exception from the #<=> call go through.
  Feature #7688

Regexp

• Support absent operator Implement Absent Operator k-takata/Onigmo#82
• Support new 5 emoji-related Unicode character properties

String

• [Ruby 2.5] String#-@ #586 String#-@ deduplicates unfrozen strings. Already-frozen
  strings remain unchanged for compatibility. Feature #13077
• -"literal" (String#-@) optimized to return the same object
  (same as "literal".freeze in Ruby 2.1+) Feature #13295
• String#{casecmp,casecmp?} return nil for non-string arguments
  instead of raising a TypeError. Bug #13312
• [Ruby 2.5] Add spec for String#start_with? supporting regexp #551 String#start_with? accepts a regexp Feature #13712
• String#delete_prefix, String#delete_prefix! Feature #12694
• String#delete_suffix, String#delete_suffix! Feature #13665
• String#each_grapheme_cluster and String#grapheme_clusters to
  enumerate grapheme clusters Feature #13780
• String#undump to unescape String#dump'ed string Feature #12275

Struct

• Struct.new takes keyword_init: true option to initialize members
  with keyword arguments. Feature #11925

• Regexp/String: Update Unicode version from 9.0.0 to 10.0.0 Feature #13685

Thread

• Thread#fetch Feature #13009
• The default of Thread.report_on_exception is now true,
  showing unhandled exceptions terminating threads on $stderr.
  Feature #14143

Time

• Time#at receives 3rd argument which specifies the unit of 2nd argument.
  Feature #13919

KeyError

• Adds coverage for KeyError#receiver / KeyError#key #574 KeyError#receiver Feature #12063
• Adds coverage for KeyError#receiver / KeyError#key #574 KeyError#key Feature #12063

FrozenError

• #557 [Ruby 2.5] Update specs to use frozen_error_class for FrozenError #568 New exception class. Feature #13224

Stdlib updates

Only stdlib which do not have tests are listed.
Many stdlib live in their own projects with their own tests.

RbConfig

• RbConfig::LIMITS is added to provide the limits of C types.
  This is available when rbconfig/sizeof is loaded.

Compatibility issues (excluding feature bug fixes)

Socket

• BasicSocket#read_nonblock and BasicSocket#write_nonblock no
  longer set the O_NONBLOCK file description flag as side effect
  (on Linux only) Feature #13362

Random

• Random.raw_seed renamed to become Random.urandom. It is now
  applicable to non-seeding purposes due to Bug #9569.

Stdlib compatibility issues (excluding feature bug fixes)

mathn.rb

• 49cdead 82145c5 Removed from stdlib. Feature #10169

[eregon]

@marcandre @andrykonchin Could you tick the checkboxes of features you added specs for?
Unfortunately clicking on the checkbox is not enough,
instead you need to edit the issue and add x inside the [ ].
Please also do the same when adding new specs, first tick it to avoid conflicts :)

For contributors without commit access, please write here when you start working on a spec, to avoid having multiple people working on the same specs.

[cadwallion]

Just a heads-up, but it appears you fixed Integer#floor/truncate/ceil/round in October, but it hasn't been checked off yet. :)

[eregon]

@cadwallion Thanks, I checked it now :)

[cadwallion]

Working on FrozenError, PR should be up shortly.

[eregon]

For ruby/spec committers: I made the checkboxes all top-level, which allows to just tick them.

[eregon]

@cadwallion I think FrozenError is already done in #568.

[cadwallion]

Why yes, yes it is. And much more thoroughly than mine. Thanks for the heads up!

[eregon]

Process.last_status as an alias of $?, Thread#fetch and KeyError should all be mostly straightforward and afaik not yet done.

[cadwallion]

I'll start in on both KeyError features.

[znz]

Pathname.glob with base: and Pathname#glob are missing?