Merge pull request #3088 from retronym/ticket/7944

SI-7944 FOUND: stray undetermined type params in vicinity of implicits

2 files changed, 29 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Implicits.scala b/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
index 987a3d2202..1e89e79cdd 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
@@ -78,7 +78,11 @@ trait Implicits {
       })
       debuglog("update buffer: " + implicitSearchContext.reportBuffer.errors)
     }
-    context.undetparams = context.undetparams filterNot result.subst.from.contains
+    // SI-7944 undetermined type parameters that result from inference within typedImplicit land in
+    //         `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
 
     if (Statistics.canEnable) Statistics.stopTimer(implicitNanos, start)
     if (Statistics.canEnable) Statistics.stopCounter(rawTypeImpl, rawTypeStart)
diff --git a/test/files/pos/t7944.scala b/test/files/pos/t7944.scala
new file mode 100644
index 0000000000..2fe2c5866d
--- /dev/null
+++ b/test/files/pos/t7944.scala
@@ -0,0 +1,24 @@
+class M[+A, +B]
+
+object Test {
+  implicit class EitherOps[A, B](self: Either[A, B]) {
+    def disjunction: M[A, B] = null
+  }
+
+  def foo = {
+    val l: Either[Int, Nothing] = Left[Int, Nothing](1)
+
+    var ok = EitherOps(l).disjunction
+
+    val runawayTypeVar = l.disjunction
+
+    // reported bug:
+    // found   : M[Int,B]; required: M[Int,Nothing]
+    val assign: M[Int, Nothing] = runawayTypeVar
+
+    // variations on the theme, all failed before similarly.
+    val assign1: M[Int, Nothing] = {val temp = runawayTypeVar; temp}
+    val assign2: M[Int, String] = runawayTypeVar
+    val assign3: M[Int, Nothing] = {val temp = Left(1).disjunction; temp}
+  }
+}