Merge castwide#1228: allow arguments to satisfy RBS interface-typed parameters

castwide#1228 fixes the same underlying bug as the already-merged
castwide#1266 (issue castwide#1227: RBS 4.1's Hash#fetch takes its
key as the Hash::_Key duck-type interface instead of a generic,
causing Solargraph to fall back to the unresolved generic<X> from the
block-form overload) but via a different, earlier mechanism: a blanket
:allow_unmatched_interface bypass in Pin::Parameter#compatible_arg?,
rather than castwide#1266's later structural Conformance check.

Verified castwide#1228's own regression test already passes unmodified on
this branch without its compatible_arg? change (isolated it into a
standalone spec file and ran it against HEAD before resolving the
conflict) - castwide#1266's structural interface verification already covers
this case, making castwide#1228's code change redundant here. Kept HEAD's
compatible_arg? as-is (including literal_arg_matches?, from an
earlier-merged PR that castwide#1228's branch, based directly on
castwide/master, never saw) and dropped castwide#1228's interface-bypass hunk
entirely.

Conflict in spec/type_checker/levels/strong_spec.rb: kept castwide#1228's new
regression test (issue castwide#1227) as a sibling of HEAD's intersection-type
test block (from castwide#1231), which castwide#1228's branch also never saw.

.github/workflows/rspec.yml auto-merged cleanly, taking castwide#1228's RBS
matrix bump (4.0.0/4.0.1/4.0.2 -> 3.10.0/4.0.3/4.1.1) - core to what
this PR is actually testing (RBS 4.1's Hash#fetch signature change).

Verified: spec/type_checker/levels/strong_spec.rb, spec/pin/parameter_spec.rb
(104 examples, 0 failures, 5 pending), and a broader safety net -
spec/type_checker, spec/source, spec/source_map/clip_spec.rb,
spec/complex_type, spec/complex_type_spec.rb (807 examples, 0
failures, 35 pending) - all passing locally.

Author: apiology

.github/workflows/rspec.yml

@@ -25,14 +25,12 @@ jobs:
         #   It currently 404s ("Unavailable version head for ruby"), failing CI:
         #   https://github.com/castwide/solargraph/actions/runs/25863741955/job/76000137015?pr=1187
         ruby-version: ['3.1', '3.2', '3.3', '3.4', '4.0']
-        rbs-version: ['3.10.0', '4.0.0', '4.0.1', '4.0.2']
+        rbs-version: ['3.10.0', '4.0.3', '4.1.1']
         exclude:
           - ruby-version: '3.1'
-            rbs-version: '4.0.0'
+            rbs-version: '4.0.3'
           - ruby-version: '3.1'
-            rbs-version: '4.0.1'
-          - ruby-version: '3.1'
-            rbs-version: '4.0.2'
+            rbs-version: '4.1.1'
     steps:
     - uses: actions/checkout@v3
     - name: Set up Ruby

spec/type_checker/levels/strong_spec.rb

@@ -1291,5 +1291,35 @@ def make
         expect(checker.problems.map(&:message)).to be_empty
       end
     end
+
+    it 'resolves Hash#fetch return type on Hash{Symbol => Class<X>} without leaking a generic placeholder' do
+      # https://github.com/castwide/solargraph/issues/1227
+      #
+      # As of RBS 4.1.0, Hash#fetch's single-argument overload takes
+      # its key as the ::Hash::_Key duck-type interface instead of the
+      # generic K (see ruby/rbs core/hash.rbs). Solargraph couldn't
+      # prove a Symbol argument satisfies that interface, so it fell
+      # back to merging the return types of all of Hash#fetch's
+      # overloads (including the unresolved block-form's `generic<X>`)
+      # instead of picking the single-argument overload.
+      checker = type_checker(%(
+        class Foo; end
+
+        class Holder
+          # @return [Hash{Symbol => Class<Foo>}]
+          def registry
+            { x: Foo }
+          end
+
+          # @return [Foo]
+          def use_it
+            # @type [Class<Foo>]
+            clazz = registry.fetch(:x)
+            clazz.new
+          end
+        end
+      ))
+      expect(checker.problems.map(&:message)).to eq([])
+    end
   end
 end