Specs for type validation of writes to $@ #1987

Job	Run time
RuboCop	53s
RuboCop	53s
specs (ubuntu, 3.2.7)	1m 31s
specs (ubuntu, 3.2.7)	1m 31s
specs (ubuntu, 3.3.7)	1m 28s
specs (ubuntu, 3.3.7)	1m 28s
specs (ubuntu, 3.4.2)	1m 34s
specs (ubuntu, 3.4.2)	1m 34s
specs (macos, 3.2.7)	1m 25s
specs (macos, 3.2.7)	1m 25s
specs (macos, 3.3.7)	54s
specs (macos, 3.3.7)	1m 9s
specs (macos, 3.4.2)	1m 13s
specs (macos, 3.4.2)	1m 13s
specs (windows, 3.2.7)	3m 4s
specs (windows, 3.2.7)	3m 4s
specs (windows, 3.3.7)	3m 7s
specs (windows, 3.3.7)	3m 7s
specs (windows, 3.4.2)	3m 5s
specs (windows, 3.4.2)	3m 5s
specs (ubuntu, 3.4.2, --enable-frozen-string-literal)	1m 34s
specs (ubuntu, 3.4.2, --enable-frozen-string-literal)	1m 34s
	39m 51s

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