Adopt params and sig from other when merging a Method with no sig

lib/rbi/rewriters/merge_trees.rb

@@ -479,10 +479,12 @@ def merge_with(other)
       super
 
       # If self is the all-anonymous side (since compatible_with? ensures
-      # at most one side is), adopt other's non-anonymous param names and sigs.
-      if params.all?(&:anonymous?)
+      # at most one side is), or if self has no sigs but other does, adopt
+      # other's non-anonymous param names and sigs.
+      if params.all?(&:anonymous?) || (sigs.empty? && !other.sigs.empty?)
         @params = other.params.dup
         @sigs = other.sigs.dup unless other.sigs.empty?
+        return
       end
 
       other.sigs.each do |sig|

test/rbi/rewriters/merge_trees_test.rb

@@ -1333,5 +1333,30 @@ def m2(_a); end
       RBI
       refute_empty(res.conflicts)
     end
+
+    def test_merge_methods_without_sig_adopts_params_and_sig_from_other
+      tree1 = parse_rbi(<<~RBI)
+        class Foo
+          def expand=(value); end
+        end
+      RBI
+
+      tree2 = parse_rbi(<<~RBI)
+        class Foo
+          sig { params(_a_different_name: T.nilable(T::Array[String])).returns(T.nilable(T::Array[String])) }
+          def expand=(_a_different_name); end
+        end
+      RBI
+
+      res = tree1.merge(tree2)
+
+      assert_equal(<<~RBI, res.string)
+        class Foo
+          sig { params(_a_different_name: T.nilable(T::Array[String])).returns(T.nilable(T::Array[String])) }
+          def expand=(_a_different_name); end
+        end
+      RBI
+      assert_empty(res.conflicts)
+    end
   end
 end