1 files changed, 63 insertions, 0 deletions
diff --git a/test/files/neg/t8431.scala b/test/files/neg/t8431.scala
new file mode 100644
index 0000000000..29b852d0c0
--- /dev/null
+++ b/test/files/neg/t8431.scala
@@ -0,0 +1,63 @@
+trait Covariant[+A]
+trait Invariant[A] extends Covariant[A @annotation.unchecked.uncheckedVariance] 
+ 
+trait Combinable[G] {
+  def combined = 0
+}
+
+trait CanBuildFrom[+C]
+ 
+object C {
+  implicit def convert1[G, TRAVONCE[+e] <: Covariant[e]]
+    (xs: TRAVONCE[G]): Combinable[G] = ???
+ 
+  implicit def convert2[G, SET[e] <: Invariant[e]]
+    (xs: SET[_ <: G])
+    (implicit cbf: CanBuildFrom[SET[G]]): Combinable[G] = ???
+
+  implicit def cbf[A]: CanBuildFrom[Invariant[A]] = ???
+}
+
+class Test1 {
+  import C.{cbf, convert1, convert2}
+  val s: Invariant[Nothing] = ???
+  s.combined // fail
+}
+
+class Test2 {
+  import C.{cbf, convert2, convert1}
+
+  val s: Invariant[Nothing] = ???
+
+  // Non-uniformity with Test1 was due to order of typechecking implicit candidates:
+  // the last candidate typechecked was the only one that could contribute undetermined type parameters
+  // to the enclosing context, due to mutation of `Context#undetparam` in `doTypedApply`.
+  s.combined // was okay!
+}
+
+
+class TestExplicit {
+  import C.{cbf, convert2}
+
+  val s: Invariant[Nothing] = ???
+
+  // Now the implicit Test fail uniformly as per this explicit conversion
+  convert2(s).combined
+
+  // Breaking this expression down makes it work.
+  {val c1 = convert2(s); c1.combined}
+}
+
+// These ones work before and after; infering G=Null doesn't need to contribute an undetermined type param.
+class Test3 {
+   import C.{cbf, convert1, convert2}
+   val s: Invariant[Null] = ???
+   s.combined // okay
+}
+
+class Test4 {
+   import C.{cbf, convert2, convert1}
+
+   val s: Invariant[Null] = ???
+   s.combined // okay
+}