SI-7944 FOUND: stray undetermined type params in vicinity of implicits #3088
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Review by @adriaanm |
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And I didn't even have to go around and put up MISSING posters! |
| // `implicitSearchContext.undetparams`, *not* in `context.undetparams` | ||
| // Here, we copy them up to parent context (analogously to the way the errors are copied above), | ||
| // and then filter out any which *were* inferred and are part of the substitutor in the implicit search result. | ||
| context.undetparams = ((context.undetparams ++ implicitSearchContext.undetparams) filterNot result.subst.from.contains).distinct |
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Thanks for the pointer. I traced this through to better understand the process.
By way of example, in:
scala> val ls = List[Option[Any]]()
ls: List[Option[Any]] = List()
scala> ls.flattenThe undetermined type params from the outer context are B(in method flatten), during the implicit search for the implicit parameter of type Option[Any] => scala.collection.GenTraversableOnce[B].
That is first used to approximate the sought type as Option[Any] => scala.collection.GenTraversableOnce[?] (wildPt) which is used to prune the candidate implicits.
Later, when typechecking the plausible candidates, occurrences of the undet params are replaced by type variables in the expected type, and matchesPt is called to see if the candidate is suitable:
itree2 = {
((xo: Option[Any]) => scala.this.Option.option2Iterable[Any](xo))
}: Option[Any] => Iterable[Any]
matchesPt(
tp = Option[Any] => Iterable[Any],
pt = Option[Any] => scala.collection.GenTraversableOnce[?B]
undet = [type B]
)
What does matchesPt do with the undet? Ultimately, they are used in isApplicableToMethod:
def typesCompatible(args: List[Type]) = undetparams match {
case Nil => isCompatibleArgs(args, formals) && isWeaklyCompatible(mt resultType args, pt)
case _ => tryInstantiating(args)
}
def tryInstantiating(args: List[Type]) = falseIfNoInstance {
val restpe = mt resultType args
val AdjustedTypeArgs.Undets(okparams, okargs, leftUndet) = methTypeArgs(undetparams, formals, restpe, args, pt)
val restpeInst = restpe.instantiateTypeParams(okparams, okargs)
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Does this failure look familiar? The method reported as missing was recently added to SBT: |
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Zinc master has been updated (typesafehub/zinc@54f5665) to use new apis introduced in sbt/sbt@30cdb1a but in PR validation zinc master is compiled against stable release of sbt. I believe we should either compile stable versions of everything or masters of everything. I'd prefer to compile everything from stable versions (tags). |
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Actually, it uses a bit old branch: https://github.com/typesafehub/sbt-builds-for-ide/blob/master/sbt-on-2.11.x#L56 |
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However, I can do it only tomorrow's evening. Feel free to beat on this. |
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I'm working on the sbt build for #3087, so I'll take this. (Does the 0.13 branch build as-is on 2.11 these days?) |
Implicit search created a nested Context into which the results of
its typechecking, namely, errors and undetermined type parameters
(roughly: those inferred as Nothing) are stashed.
The code the drives the process was checking for errors, but
discarded those undetermined type parameters.
This commit copies them from the child context to the parent,
which lets `Typer#adapt` to get to:
else if (hasUndetsInMonoMode) { // (9)
assert(!context.inTypeConstructorAllowed, context) //@m
instantiatePossiblyExpectingUnit(tree, mode, pt)
}
Our lost TypeVar has found its way home! The reward for which
is being instantiated, based on another type inference session
adapting the expression's type to the expected type.
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The latest force push just expands the test case a little. |
ghost
assigned
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LGTM |
SI-7944 FOUND: stray undetermined type params in vicinity of implicits
Implicit search created a nested Context into which the results of
its typechecking, namely, errors and undetermined type parameters
(roughly: those inferred as Nothing) are stashed.
The code the drives the process was checking for errors, but
discarded those undetermined type parameters.
This commit copies them from the child context to the parent,
which lets
Typer#adaptto get to:Our lost TypeVar has found its way home! The reward for which
is being instantiated, based on another type inference session
adapting the expression's type to the expected type.