Fix method type

Author: soutaro

core/range.rbs

@@ -1017,8 +1017,10 @@ class Range[out Elem] < Object
   #     ('a'..'e').step { p _1 }
   #     # Default step 1; prints: a, b, c, d, e
   #
-  def step: (?Numeric | int n) -> Enumerator[Elem, self]
-          | (?Numeric | int n) { (Elem element) -> void } -> self
+  def step: (?Numeric | int) -> Enumerator[Elem, self]
+          | (?Numeric | int) { (Elem element) -> void } -> self
+          | (untyped) -> Enumerator[Elem, self]
+          | (untyped) { (Elem element) -> void } -> self
 
   # <!--
   #   rdoc-file=range.c

test/typecheck/range_step/Steepfile

@@ -0,0 +1,7 @@
+D = Steep::Diagnostic
+
+target :test do
+  signature "."
+  check "."
+  configure_code_diagnostics(D::Ruby.all_error)
+end

test/typecheck/range_step/test.rb

@@ -0,0 +1,10 @@
+# These are valid because step is a number.
+
+(1...3).step(1) { }
+("A".."C").step(1) { }
+
+# This works because "A" + "" is valid.
+("A".."C").step("") { }
+
+# This doesn't work but the type checker cannot detect it.
+("A".."C").step(Exception.new) { }