Better return type inference for proc and lambda values

[jez]

Input

→ View on sorbet.run

# typed: true
extend T::Sig

f = proc {|x| 2}
T.reveal_type(f)

g = -> (x) {2}
T.reveal_type(g)

h = Proc.new {|x| 2}
T.reveal_type(h)

i = lambda {|x| 2}
T.reveal_type(i)

Observed output

editor.rb:5: Revealed type: T.proc.params(arg0: T.untyped).returns(T.untyped) https://srb.help/7014
     5 |T.reveal_type(f)
        ^^^^^^^^^^^^^^^^
  Got T.proc.params(arg0: T.untyped).returns(T.untyped) originating from:
    editor.rb:4:
     4 |f = proc {|x| 2}
            ^^^^^^^^^^^^

editor.rb:8: Revealed type: T.proc.params(arg0: T.untyped).returns(T.untyped) https://srb.help/7014
     8 |T.reveal_type(g)
        ^^^^^^^^^^^^^^^^
  Got T.proc.params(arg0: T.untyped).returns(T.untyped) originating from:
    editor.rb:7:
     7 |g = -> (x) {2}
            ^^^^^^^^^^

editor.rb:11: Revealed type: T.untyped https://srb.help/7014
    11 |T.reveal_type(h)
        ^^^^^^^^^^^^^^^^
  Got T.untyped originating from:
    editor.rb:10:
    10 |h = Proc.new {|x| 2}
            ^^^^^^^^^^^^^^^^

editor.rb:14: Revealed type: T.proc.params(arg0: T.untyped).returns(T.untyped) https://srb.help/7014
    14 |T.reveal_type(i)
        ^^^^^^^^^^^^^^^^
  Got T.proc.params(arg0: T.untyped).returns(T.untyped) originating from:
    editor.rb:13:
    13 |i = lambda {|x| 2}
            ^^^^^^^^^^^^^^
Errors: 4

Expected behavior

• It's very unexpected that Proc.new returns T.untyped

• It's somewhat unexpected that the inferred return types are T.untyped

• It's actually expected that the arguments don't have an inferred type. Sorbet makes no effort to infer arguments (this is true for methods too). We might want to expose a way to annotate argument types for proc/lambda values, but that is out of scope for this issue.

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