Add `TCDiag` type class.

12 jobs for master in 29 minutes and 48 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed build-coq.8.6.0 #8873
fp

00:08:39

passed build-coq.8.6.1 #8872
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passed build-coq.8.7.0 #8871
fp

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passed build-coq.8.7.1 #8870
fp

00:09:02

passed build-coq.8.7.2 #8869
fp-timing

00:07:33

passed build-coq.dev #8868
fp

00:21:11

failed build-coq.8.6.0 #8824
fp

01:02:46

failed build-coq.8.6.1 #8823
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01:02:58

failed build-coq.8.7.0 #8822
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failed build-coq.8.7.1 #8821
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failed build-coq.8.7.2 #8820
fp-timing

01:03:23

failed build-coq.dev #8819
fp

01:03:18