Fix #7969: Don't let typevars be bounded by ExprTypes

Author: odersky

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

@@ -508,6 +508,8 @@ trait ConstraintHandling[AbstractContext] {
             case inst =>
               prune(inst)
           }
+        case bound: ExprType =>
+          NoType
         case _ =>
           pruneLambdaParams(bound)
       }

tests/neg/i7969.scala

@@ -0,0 +1,5 @@
+object O{
+  def f(i: => Int): Int = 1
+  def m[A](a: A => Int) = 2
+  def n = m(f)  // error: cannot eta expand
+}