More type stability for GSets #16912

Job	Run time
doctest (1.10, ubuntu-latest)	26m 33s
doctest (1.11-nightly, ubuntu-latest)	27m 12s
doctest (nightly, ubuntu-latest)	29m 19s
doctest (1.10, ubuntu-latest, depwarn=error)	27m 23s
doctest (1.10, macOS, RPTU)	12m 47s
test (1.6, short, ubuntu-latest)	1h 29m 52s
test (1.6, long, ubuntu-latest)	1h 2m 13s
test (1.10, short, ubuntu-latest)	1h 11m 31s
test (1.10, long, ubuntu-latest)	45m 28s
test (1.11-nightly, short, ubuntu-latest)	1h 25m 55s
test (1.11-nightly, long, ubuntu-latest)	53m 1s
test (nightly, short, ubuntu-latest)	1h 24m 31s
test (nightly, long, ubuntu-latest)	53m 29s
test (1.10, short, ubuntu-latest, depwarn=error)	1h 12m 29s
test (1.10, long, ubuntu-latest, depwarn=error)	46m 59s
test (1.10, short, macOS, RPTU)	13m 48s
test (1.10, long, macOS, RPTU)	14m 33s
test (1.10, book, ubuntu-latest)	50m 59s
test (nightly, book, ubuntu-latest)	1h 4m 11s
	15h 32m 13s

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