Fix return type restrictions crystal-lang#10583

src/enumerable.cr

@@ -238,7 +238,7 @@ module Enumerable(T)
   # ```
   # [1, 2, 3, 4].count { |i| i % 2 == 0 } # => 2
   # ```
-  def count
+  def count(& : T -> _) : Int32
     count = 0
     each { |e| count += 1 if yield e }
     count
@@ -624,7 +624,7 @@ module Enumerable(T)
   # ```
   #
   # Returns `nil` if the block didn't return `true` for any element.
-  def index
+  def index(& : T -> _) : Int32?
     each_with_index do |e, i|
       return i if yield e
     end
@@ -638,7 +638,7 @@ module Enumerable(T)
   # ```
   #
   # Returns `nil` if *obj* is not in the collection.
-  def index(obj)
+  def index(obj) : Int32?
     index { |e| e == obj }
   end
 

src/string.cr

@@ -817,7 +817,7 @@ class String
   # s[-2..-4] # => ""
   # s[-2..1]  # => ""
   # s[3..-4]  # => ""
-  # ``` : String
+  # ```
   def [](range : Range) : String
     self[*Indexable.range_to_index_and_count(range, size) || raise IndexError.new]
   end
@@ -5010,7 +5010,7 @@ class String
   # Raises an `ArgumentError` if `self` has null bytes. Returns `self` otherwise.
   #
   # This method should sometimes be called before passing a `String` to a C function.
-  def check_no_null_byte(name = nil) : String
+  def check_no_null_byte(name = nil) : self
     if byte_index(0)
       name = "`#{name}` " if name
       raise ArgumentError.new("String #{name}contains null byte")