Fix #8067: Don't constrain with types of wrong kinds

The example got into a pickle since it tried to constrain a type variable
A[_] with a type (G[_] => G[String]). It should have rejected this outright.
Instead the constraint addition succeeded and the compiler got into a dead-end
with it afterwards.

Author: odersky

compiler/src/dotty/tools/dotc/core/ConstraintHandling.scala

@@ -524,13 +524,18 @@ trait ConstraintHandling[AbstractContext] {
           pruneLambdaParams(bound)
       }
 
+      def sameKind(tp1: Type, tp2: Type): Boolean =
+        tp1.typeParams.corresponds(tp2.typeParams)((p1, p2) =>
+          sameKind(p1.paramInfo, p2.paramInfo))
+
       try bound match {
         case bound: TypeParamRef if constraint contains bound =>
           addParamBound(bound)
         case _ =>
           val pbound = prune(bound)
-          pbound.exists && (
-            if (fromBelow) addLowerBound(param, pbound) else addUpperBound(param, pbound))
+          pbound.exists
+          && sameKind(param, pbound)
+          && (if fromBelow then addLowerBound(param, pbound) else addUpperBound(param, pbound))
       }
       finally addConstraintInvocations -= 1
     }

tests/pos/i8067.scala

@@ -0,0 +1,9 @@
+object Test {
+
+trait Tc[F[_]]
+type OfString[G[_]] = G[String]
+
+def foo[A[_]](fs: A[String])(implicit f: Tc[A]): A[String] = ???
+def bar[B[_]](fs: OfString[B]): B[String] = foo(fs)(new Tc[B] {})
+
+}