Fix invalid sig generated for anonymous block param (&)

.ruby-version

@@ -1 +1 @@
-4.0.0
+3.4.4

lib/spoom/sorbet/translate/rbs_comments_to_sorbet_sigs.rb

@@ -184,6 +184,16 @@ def rewrite_def(def_node, comments)
 
             sig = translator.result
 
+            # When the method uses Ruby 3.1+ anonymous block forwarding (`&` with no name),
+            # the RBI translator names the block param `&block`, producing `&block:` inside
+            # `params()` which is a syntax error. Detect that case and rename it to `block`
+            # (without the `&`) so the generated sig is valid Ruby.
+            if anonymous_block_param?(def_node)
+              sig.params.each do |param|
+                param.name = param.name.delete_prefix("&") if param.name.start_with?("&")
+              end
+            end
+
             apply_member_annotations(comments.method_annotations, sig)
 
             # Sorbet runtime doesn't support `sig` on `method_added` or
@@ -348,6 +358,14 @@ def apply_member_annotations(annotations, sig)
           end
         end
 
+        # Returns true if the def node uses an anonymous block parameter (`&` with no name),
+        # i.e. Ruby 3.1+ anonymous block forwarding like `def foo(&); end`.
+        #: (Prism::DefNode) -> bool
+        def anonymous_block_param?(def_node)
+          block_param = def_node.parameters&.block
+          block_param.is_a?(Prism::BlockParameterNode) && block_param.name.nil?
+        end
+
         #: (Prism::ClassNode | Prism::ModuleNode | Prism::SingletonClassNode, Regexp) -> bool
         def already_extends?(node, constant_regex)
           node.child_nodes.any? do |c|
@@ -438,4 +456,4 @@ def apply_type_aliases(comments)
       end
     end
   end
-end
+end

test/spoom/sorbet/translate/rbs_comments_to_sorbet_sigs_test.rb

@@ -890,6 +890,45 @@ def rbs_comments_to_sorbet_sigs(ruby_contents, max_line_length: nil, overloads_s
             overloads_strategy: overloads_strategy,
           ).rewrite
         end
+
+        def test_rbs_comments_to_sorbet_sigs_anonymous_block_param
+          res = Spoom::Sorbet::Translate.rbs_comments_to_sorbet_sigs(<<~RUBY, file: "test.rb")
+            # typed: true
+            class Foo
+              #: (String) ?{ (String) -> void } -> String
+              def bar(request, &); end
+            end
+          RUBY
+
+          assert_equal(<<~RUBY, res)
+            # typed: true
+            class Foo
+              sig { params(request: String, block: ::T.nilable(::T.proc.params(arg0: String).void)).returns(String) }
+              def bar(request, &); end
+            end
+          RUBY
+
+          # Must also be valid Ruby
+          assert RubyVM::InstructionSequence.compile(res)
+        end
+
+        def test_rbs_comments_to_sorbet_sigs_named_block_param_unchanged
+          res = Spoom::Sorbet::Translate.rbs_comments_to_sorbet_sigs(<<~RUBY, file: "test.rb")
+            # typed: true
+            class Foo
+              #: (String) ?{ (String) -> void } -> String
+              def bar(request, &block); end
+            end
+          RUBY
+
+          assert_equal(<<~RUBY, res)
+            # typed: true
+            class Foo
+              sig { params(request: String, block: ::T.nilable(::T.proc.params(arg0: String).void)).returns(String) }
+              def bar(request, &block); end
+            end
+          RUBY
+        end
       end
     end
   end