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Sometimes (e.g. here) the user wants to write

f (IfLFCon n)       = unitNameSet n
f (IfLFReEntrant _) = emptyNameSet
f (IfLFThunk _ _)   = emptyNameSet
f (IfLFUnknown _)   = emptyNameSet
f IfLFUnlifted      = emptyNameSet

enumerating all the constructors for the sake of explicitness. But we don't want to do all those tests at runtime! We'd prefer to execute

f (IfLFCon n)       = unitNameSet n
f _                 = emptyNameSet

Some bits of GHC already combine identical alternatives:

• The Simplifier uses combineIdenticalAlts
• CSE uses combineAlts, which tries a bit harder

But neither does the full job:

• CSE doesn't use occurrence analysis to look for dead binders, so it only looks for nullary constructors
• The Simplifier uses only cheapEqExpr, and only combines the first alternative.

What we want is to

• Look at alternatives whose binders are all dead
• Among them pick the most popular RHS
• Combine them

Not really hard. Probably appropriate to do this in CSE.

Then we could guarantee that we wouldn't make redundant runtime tests.

See also #14684 for an earlier version of the same issue.