This completely breaks the behavior of a standard library method to work around a corner case bug in the compiler.
There is no actual "fix" here, and it's not about "only one nil value", it's about only one nil type.

p ({"a" => nil} of String => Int32?).compact -- works
p ({} of String => Nil).compact -- does not work

Missing hash key: "value"
repos/crystal/src/hash.cr:0:9 in 'fetch'
repos/crystal/src/hash.cr:63:5 in '[]'
repos/crystal/src/compiler/crystal/codegen/codegen.cr:1406:17 in 'visit'
repos/crystal/src/compiler/crystal/syntax/visitor.cr:27:12 in 'accept'
repos/crystal/src/compiler/crystal/codegen/codegen.cr:2027:7 in 'accept'
repos/crystal/src/compiler/crystal/codegen/codegen.cr:578:9 in 'visit'
repos/crystal/src/compiler/crystal/syntax/visitor.cr:27:12 in 'accept'
...

@oprypin I just tested it does work, you're wrong

I can confirm what @oprypin sees on both 0.23.1 and HEAD.

While I agree that it's a bug for a specific type of Hash, the signature of compact shouldn't change for the rest of Hashes.