Compiler: merge procs with the same arguments type and Nil | T return type to Nil return type #7527
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Fixes #3655
The idea here is that a proc that returns any type
T
is compatible with a proc with the same argument types that returnsNil
. The union type of those things is just the type of the proc that returnsNil
, as in "in the general case we don't know anything about the value being returned".This is a rule that's currently being applied in a few places, this fixes the language to apply it in more (all?) places.