- builder.rb: catch encoding errors when parsing invalid encoding reg…

…exp (#1033)

This errors when trying to set the regexp source string to binary.

prism parser logic: https://github.com/ruby/prism/blob/89c22f0e6cbe7aac81c789ca35786a9a97afcb57/src/prism.c#L7169-L7198

Closes #1032

lib/parser/builders/default.rb

@@ -426,7 +426,7 @@ def regexp_options(regopt_t)
     def regexp_compose(begin_t, parts, end_t, options)
       begin
         static_regexp(parts, options)
-      rescue RegexpError => e
+      rescue RegexpError, Encoding::UndefinedConversionError => e
         diagnostic :error, :invalid_regexp, { :message => e.message },
                    loc(begin_t).join(loc(end_t))
       end

test/test_parser.rb

@@ -585,6 +585,18 @@ def test_regex_error
       SINCE_1_9)
   end
 
+  def test_regexp_error_invalid_encoding_conversion
+    message = if defined?(JRUBY_VERSION)
+      '"\\xE3\\x81\\x82" from UTF-8 to ASCII-8BIT'
+    else
+      'U+3042 from UTF-8 to ASCII-8BIT'
+    end
+    assert_diagnoses(
+      [:error, :invalid_regexp, { message: message }],
+      %q[/あ/n],
+      %q(~~~ location))
+  end
+
   # Arrays
 
   def test_array_plain