Fix bug in MatchMapping class #671

Job	Run time
lint	1m 37s
coverage	3m 15s
docs	2m 19s
Rust unit tests (ubuntu-latest)	2m 48s
Rustfmt	12s
Rust unit tests (macos-latest)	3m 48s
test (macos-latest, 3.8)	6m 25s
Rust unit tests (windows-latest)	3m 41s
typecheck	1m 28s
test (macos-latest, 3.9)	7m 24s
test (macos-latest, 3.10)	7m 42s
test (macos-latest, 3.11)	10m 17s
test (macos-latest, 3.12-dev)	11m 19s
test (ubuntu-latest, 3.8)	3m 40s
test (ubuntu-latest, 3.9)	3m 22s
test (ubuntu-latest, 3.10)	3m 53s
test (ubuntu-latest, 3.11)	3m 40s
test (ubuntu-latest, 3.12-dev)	4m 46s
test (windows-latest, 3.8)	5m 57s
test (windows-latest, 3.9)	5m 10s
test (windows-latest, 3.10)	6m 20s
test (windows-latest, 3.11)	7m 7s
test (windows-latest, 3.12-dev)	7m 6s
	1h 53m 16s

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