SI-8023 Fix symbol-completion-order type var pattern bug

Removing the `isComplete` check altogether leads to cycles in,
for instatnce, F-bound type parameters:

    trait LSO[+A, +Repr <: LSO[A, Repr]] // error: illegal cyclic reference involving type Repr

But, I believe that we can (and must) eagerly initialize the type
parameter symbols if we are typechecking a pattern.

While this appeared to regress in 2.11.x, but the problem was in fact
dormant and was merely uncovered in the fix for SI-7756, 3df1d77fc.

2 files changed, 37 insertions, 7 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index 6d799b0098..a8d332f96a 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -4879,14 +4879,22 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
 
           if (sameLength(tparams, args)) {
             // @M: kind-arity checking is done here and in adapt, full kind-checking is in checkKindBounds (in Infer)
-            val args1 =
-              if (!isComplete)
-                args mapConserve (typedHigherKindedType(_, mode))
-                // if symbol hasn't been fully loaded, can't check kind-arity
-              else map2Conserve(args, tparams) { (arg, tparam) =>
-                //@M! the polytype denotes the expected kind
-                typedHigherKindedType(arg, mode, GenPolyType(tparam.typeParams, AnyTpe))
+            val args1 = map2Conserve(args, tparams) { (arg, tparam) =>
+              //@M! the polytype denotes the expected kind
+              def pt = GenPolyType(tparam.typeParams, AnyTpe)
+
+              // if symbol hasn't been fully loaded, can't check kind-arity
+              // ... except, if we're in a pattern, where we can and must (SI-8023)
+              if (mode.typingPatternOrTypePat) {
+                tparam.initialize
+                typedHigherKindedType(arg, mode, pt)
               }
+              else if (isComplete)
+                typedHigherKindedType(arg, mode, pt)
+              else
+                // This overload (without pt) allows type constructors, as we don't don't know the allowed kind.
+                typedHigherKindedType(arg, mode)
+            }
             val argtypes = args1 map (_.tpe)
 
             foreach2(args, tparams) { (arg, tparam) =>
diff --git a/test/files/pos/t8023.scala b/test/files/pos/t8023.scala
new file mode 100644
index 0000000000..86824084ed
--- /dev/null
+++ b/test/files/pos/t8023.scala
@@ -0,0 +1,22 @@
+import language._
+
+
+object Test {
+  def foo = (null: Any) match {
+    case a: A[k] =>
+      // error: kinds of the type arguments (k) do not conform to the 
+      // expected kinds of the type parameters (type K) in class B.
+      new B[k]()
+  }
+}
+
+class A[K[L[_]]]
+
+class B[K[M[_]]]
+
+
+object Test2 {
+  def foo = (null: Any) match {
+    case a: A[k] => new B[k]() // this one worked before as the info of `A` was complete
+  }
+}