Adding a type signature changes heap allocation into stack allocation without changing the actual type

According to Milan Straka, changing

insert :: Ord k => k -> a -> Map k a -> Map k a
insert = go
  where
    STRICT_1_OF_3(go)
    go kx x Tip = singleton kx x
    go kx x (Bin sz ky y l r) = ...

to

insert :: Ord k => k -> a -> Map k a -> Map k a
insert = go
  where
    go :: Ord k => k -> a -> Map k a -> Map k a
    STRICT_1_OF_3(go)
    go kx x Tip = singleton kx x
    go kx x (Bin sz ky y l r) = ...

changes how GHC allocates the argument, from heap to stack. Here's the relevant commit: https://github.com/haskell/containers/commit/32d84ba5eb82f34dbb8a8fabf07077d848cdb408

It includes this comment:

-- [Note: Type of local 'go' function]
-- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-- If the local 'go' function uses an Ord class, it must be given a type
-- which mentions this Ord class. Otherwise it is not passed as an argument and
-- it is instead heap-allocated at the entry of the outer method.

I find this quite alarming. The type of k above is already Ord k, so the extra type signature shouldn't make a difference in my opinion.

Trac metadata

Trac field	Value
Version	7.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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