Merge pull request #975 from adriaanm/ticket-4881b

SI-4881 infer variance from formals, then result

1 files changed, 13 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Infer.scala b/src/compiler/scala/tools/nsc/typechecker/Infer.scala
index 960c210649..47cf80807a 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Infer.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Infer.scala
@@ -445,7 +445,7 @@ trait Infer {
       val tvars = tparams map freshVar
       if (isConservativelyCompatible(restpe.instantiateTypeParams(tparams, tvars), pt))
         map2(tparams, tvars)((tparam, tvar) =>
-          instantiateToBound(tvar, varianceInTypes(formals)(tparam)))
+          instantiateToBound(tvar, inferVariance(formals, restpe)(tparam)))
       else
         tvars map (tvar => WildcardType)
     }
@@ -575,13 +575,24 @@ trait Infer {
             "argument expression's type is not compatible with formal parameter type" + foundReqMsg(tp1, pt1))
         }
       }
+
       val targs = solvedTypes(
-        tvars, tparams, tparams map varianceInTypes(formals),
+        tvars, tparams, tparams map inferVariance(formals, restpe),
         false, lubDepth(formals) max lubDepth(argtpes)
       )
       adjustTypeArgs(tparams, tvars, targs, restpe)
     }
 
+    /** Determine which variance to assume for the type paraneter. We first chose the variance
+     *  that minimizes any formal parameters. If that is still undetermined, because the type parameter
+     *  does not appear as a formal parameter type, then we pick the variance so that it minimizes the
+     *  method's result type instead.
+     */
+    private def inferVariance(formals: List[Type], restpe: Type)(tparam: Symbol): Int = {
+      val v = varianceInTypes(formals)(tparam)
+      if (v != VarianceFlags) v else varianceInType(restpe)(tparam)
+    }
+
     private[typechecker] def followApply(tp: Type): Type = tp match {
       case NullaryMethodType(restp) =>
         val restp1 = followApply(restp)