Add type check (conversion) in `String#[]=`

Before this patch:

  'a'[0] = 1        #=> 1
  'a'[:a] = '1'     #=> ArgumentError
  'a'[:a, 0] = '1'  #=> ArgumentError
  'a'[0, :a] = '1'  #=> ArgumentError
  'a'[0, 1] = 1     #=> 1

After this patch / Ruby:

  'a'[0] = 1        #=> TypeError
  'a'[:a] = '1'     #=> TypeError
  'a'[:a, 0] = '1'  #=> TypeError
  'a'[0, :a] = '1'  #=> TypeError
  'a'[0, 1] = 1     #=> TypeError

1 files changed, 7 insertions, 0 deletions

diff --git a/test/t/string.rb b/test/t/string.rb
index cf3702cbe..404cf03e1 100644
--- a/test/t/string.rb
+++ b/test/t/string.rb
@@ -161,6 +161,9 @@ assert('String#[]=') do
    assert_equal 'aXc', e
   end
 
+  assert_raise(TypeError) { 'a'[0] = 1 }
+  assert_raise(TypeError) { 'a'[:a] = '1' }
+
   # length of args is 2
   a1 = 'abc'
   assert_raise(IndexError) do
@@ -197,6 +200,10 @@ assert('String#[]=') do
   assert_raise(IndexError) do
     b3['XX'] = 'Y'
   end
+
+  assert_raise(TypeError) { 'a'[:a, 0] = '1' }
+  assert_raise(TypeError) { 'a'[0, :a] = '1' }
+  assert_raise(TypeError) { 'a'[0, 1] = 1 }
 end
 
 assert('String#capitalize', '15.2.10.5.7') do