Require discriminator instance to be an object #427

Job	Run time
test (ubuntu-latest, 2.7)	55s
test (ubuntu-latest, 3)	20s
test (ubuntu-latest, 3.1)	36s
test (ubuntu-latest, 3.2)	38s
test (ubuntu-latest, 3.3)	37s
test (ubuntu-latest, jruby)	2m 44s
test (ubuntu-latest, jruby-head)	2m 38s
test (ubuntu-latest, truffleruby)	1m 52s
test (ubuntu-latest, truffleruby-head)	2m 24s
test (windows-latest, 2.7)	3m 33s
test (windows-latest, 3)	2m 12s
test (windows-latest, 3.1)	3m 3s
test (windows-latest, 3.2)	2m 19s
test (windows-latest, 3.3)	2m 56s
test (windows-latest, head)	1m 33s
test (windows-latest, jruby)	1m 59s
test (windows-latest, jruby-head)	2m 20s
test (macos-latest, 2.7)	43s
test (macos-latest, 3)	19s
test (macos-latest, 3.1)	34s
test (macos-latest, 3.2)	32s
test (macos-latest, 3.3)	31s
test (macos-latest, jruby)	2m 31s
test (macos-latest, jruby-head)	1m 54s
test (macos-latest, truffleruby)	1m 18s
test (macos-latest, truffleruby-head)	1m 26s
	42m 27s

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