Fix stale Hash::_Key expectations from upstream RBS core drift (#1224)

* Fix stale Hash::_Key expectations from upstream RBS core drift

RBS core's `Hash#[]` now types its key parameter as the `_Key` duck-type
interface rather than the generic `K`, so instantiating `Hash{Symbol =>
untyped}` no longer substitutes `Symbol` for the key param type. The two
specs added for #1042 asserted the old `Symbol` value; update them to the
now-correct `::Hash::_Key`.

* Make Hash::_Key expectation conditional on RBS version

CI's rspec matrix pins RBS versions 3.10.0 through 4.0.2, all of which
predate the _Key duck-type interface change (introduced in RBS 4.1.0) - only
my local environment's latest RBS picked up the new behavior. Make the
expectation depend on the installed RBS::VERSION so both old and new RBS
are covered correctly, following the existing pattern used elsewhere in this
file for RBS-version-gated behavior.

Author: apiology

spec/convention/activesupport_concern_spec.rb

@@ -187,8 +187,17 @@ class Sub < Hash[Symbol, untyped]
       end
 
       it 'finds superclass method pin parameter type' do
+        # RBS core's Hash#[] started taking its key as the _Key duck-type
+        # interface instead of the generic K as of RBS 4.1.0, so instantiating
+        # Hash{Symbol => untyped} no longer substitutes the param type on
+        # newer RBS - see ruby/rbs core/hash.rbs.
+        expected = if Gem::Version.new(RBS::VERSION) >= Gem::Version.new('4.1.0')
+                     ['::Hash::_Key']
+                   else
+                     ['Symbol']
+                   end
         expect(sup_method_stack.flat_map(&:signatures).flat_map(&:parameters).map(&:return_type).map(&:rooted_tags)
-                 .uniq).to eq(['Symbol'])
+                 .uniq).to eq(expected)
       end
     end
   end

spec/rbs_map/conversions_spec.rb

@@ -142,8 +142,17 @@ class Sub < Hash[Symbol, untyped]
       end
 
       it 'finds superclass method pin parameter type' do
+        # RBS core's Hash#[] started taking its key as the _Key duck-type
+        # interface instead of the generic K as of RBS 4.1.0, so instantiating
+        # Hash{Symbol => untyped} no longer substitutes the param type on
+        # newer RBS - see ruby/rbs core/hash.rbs.
+        expected = if Gem::Version.new(RBS::VERSION) >= Gem::Version.new('4.1.0')
+                     ['::Hash::_Key']
+                   else
+                     ['Symbol']
+                   end
         expect(sup_method_stack.flat_map(&:signatures).flat_map(&:parameters).map(&:return_type).map(&:rooted_tags)
-                 .uniq).to eq(['Symbol'])
+                 .uniq).to eq(expected)
       end
     end
   end