GHC fails to inline methods of single-method classes

Consider the following module:

module A where

class C a where
  f :: a

g :: C a => (a,a)
g = (f,f)

GHC has a nice optimization where the dictionaries for classes with just a single method (and no super-classes) are represented with a newtype. This means that, essentially, the "dictionary" is the method itself, and the code that "extracts" a method from a dictionary is just the identity function. For this reason, we can completely eliminate the extraction code by always inlining the method.

This works as expected in GHC 6.12.2 but something seems to have changed in the current HEAD, so that the "extraction code" (i.e., the method "f") does not seem to get eliminated:

[GHC 6.12.2]

f :: forall a. (C a) => a
GblId[ClassOp]

f = \ (@ a) (tpl [ALWAYS Once Nothing] :: C a) -> tpl `cast` ...

g :: forall a. (C a) => (a, a)
GblId

g = __inline_me (\ (@ a) (d :: C a) -> (d `cast` ..., d `cast` ...)

[HEAD as of Nov 14 2010]

f [InlPrag=[NEVER]] :: forall a. C a => a
[GblId[ClassOp],
 Arity=1,
 Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=1, Value=True,
         ConLike=True, Cheap=True, Expandable=True,
         Guidance=ALWAYS_IF(unsat_ok=True,boring_ok=True)},
 RULES: Built in rule for f: "Class op f"]
f = \ (@ a) (tpl [Occ=Once] :: C a) -> tpl `cast` ...


g :: forall a. C a => (a, a)
[GblId,
 Arity=1,
 Unf=Unf{Src=InlineStable, TopLvl=True, Arity=1, Value=True,
         ConLike=True, Cheap=True, Expandable=True,
         Guidance=ALWAYS_IF(unsat_ok=True,boring_ok=False)
         Tmpl= \ (@ a) (d :: C a) -> (f @ a d, f @ a d)}]

g = \ (@ a) (d :: C a) -> (f @ a d, f @ a d)

Trac metadata

Trac field	Value
Version	7.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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