Require discriminator instance to be an object #439

Job	Run time
test (ubuntu-latest, 2.7)	52s
test (ubuntu-latest, 3)	38s
test (ubuntu-latest, 3.1)	36s
test (ubuntu-latest, 3.2)	37s
test (ubuntu-latest, 3.3)	40s
test (ubuntu-latest, 3.4)	22s
test (ubuntu-latest, jruby)	2m 42s
test (ubuntu-latest, jruby-head)	2m 45s
test (ubuntu-latest, truffleruby)	1m 59s
test (ubuntu-latest, truffleruby-head)	2m 19s
test (windows-latest, 2.7)	5m 10s
test (windows-latest, 3)	4m 39s
test (windows-latest, 3.1)	2m 6s
test (windows-latest, 3.2)	2m 9s
test (windows-latest, 3.3)	3m 30s
test (windows-latest, 3.4)	2m 41s
test (windows-latest, head)	1m 36s
test (windows-latest, jruby)	1m 23s
test (windows-latest, jruby-head)	2m 1s
test (macos-latest, 2.7)	49s
test (macos-latest, 3)	26s
test (macos-latest, 3.1)	31s
test (macos-latest, 3.2)	30s
test (macos-latest, 3.3)	36s
test (macos-latest, 3.4)	22s
test (macos-latest, jruby)	1m 37s
test (macos-latest, jruby-head)	1m 47s
test (macos-latest, truffleruby)	1m 18s
test (macos-latest, truffleruby-head)	1m 44s
	48m 25s

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