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It regressed in fada1ef6b#L4L614. Partially reverting just
this change restores the correct behaviour:
```
- if (sym.isStable && pre.isStable && !isByNameParamType(tree.tpe) &&
+ if (treeInfo.admitsTypeSelection(tree) &&
```
This patch embeds the check for by-name parameter types into
`TreeInfo.isStableIdentifier`. That code already checks for
`Symbol#isStable`, which exludes direct references to by-name
parameters. But the additional check is required to deal with
by-name parameters in function types, e.g `(=> Int) => Any`.
Open question: should we go further and embed this check in `isStable`?
Currently:
final def isStable = isTerm && !isMutable && !(hasFlag(BYNAMEPARAM)) && (!isMethod || hasStableFlag)
Such function types are an underspecified corner of the language,
albeit one that is pretty useful writing, for example, in the
signature of a lazy foldRight that can operate over infinite
structures:
def foldRight[A, B](fa: F[A], z: => B)(f: (A, => B) => B): B
The next commit subjects them to a little testing.
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