Compute isCompleted together with typeParams for kind-arity check

Being complete is a dynamic property. If we take `typeParams`
before being complete they are unusable. But inbetween we might
get completed and suddenly start passing the check. This is why
the problem manifests in the second parameter of `S`.

src/compiler/scala/tools/nsc/typechecker/Typers.scala

@@ -5535,18 +5535,17 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
       }
 
       def typedAppliedTypeTree(tree: AppliedTypeTree) = {
-        val tpt        = tree.tpt
-        val args       = tree.args
-        val tpt1       = typed1(tpt, mode | FUNmode | TAPPmode, WildcardType)
-        def isPoly     = tpt1.tpe.isInstanceOf[PolyType]
-        def isComplete = tpt1.symbol.rawInfo.isComplete
+        val tpt = tree.tpt
+        val args = tree.args
+        val tpt1 = typed1(tpt, mode | FUNmode | TAPPmode, WildcardType)
 
         if (tpt1.isErrorTyped) {
           tpt1
         } else if (!tpt1.hasSymbolField) {
           AppliedTypeNoParametersError(tree, tpt1.tpe)
         } else {
           val tparams = tpt1.symbol.typeParams
+          val isComplete = tpt1.symbol.rawInfo.isComplete
 
           if (sameLength(tparams, args)) {
             // @M: kind-arity checking is done here and in adapt, full kind-checking is in checkKindBounds (in Infer)
@@ -5594,6 +5593,7 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
             }
             val original = treeCopy.AppliedTypeTree(tree, tpt1, args1)
             val result = TypeTree(appliedType(tpt1.tpe, argtypes)) setOriginal original
+            val isPoly = tpt1.tpe.isInstanceOf[PolyType]
             if (isPoly) // did the type application (performed by appliedType) involve an unchecked beta-reduction?
               TypeTreeWithDeferredRefCheck() { () =>
                 // wrap the tree and include the bounds check -- refchecks will perform this check (that the beta reduction was indeed allowed) and unwrap

test/files/pos/t12187.scala

@@ -0,0 +1,8 @@
+object Test {
+  trait Foo[S[_[_], _[_]]] extends Bar[S] {
+    def m[F[_]](x: S[({ type G[A] = Bar[S] })#G, F]): Unit
+  }
+  trait Bar[S[_[_], _[_]]] {
+    def m[F[_]](x: S[({ type G[A] = Bar[S] })#G, F]): Unit
+  }
+}