No inlining in the presence of non-instantiated phantom type

Consider the following code:

data U a = U

g :: [U a]
g = [U]

f :: [Int]
f = map (\U -> 2) g

Compiled with -O1, g does not get inlined, and f stays unchanged. Inline pragmas don't seem to help. However, if we change the type signature of g to instantiate the phantom type parameter, such as with g :: [U ()], then f is nicely optimised to become [2]. I believe this is a bug, and we'd like f to be simplified even when g has its most general type signature.

Trac metadata

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