Fix valueOf

ValueOf would happily return a value for non-value types, e.g. valueOf[String].
This would crash the backend later.

This commit replaces the crash with a type error. Ideally, though, this should
work as expected in inlined code.

Author: odersky

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -773,7 +773,7 @@ trait Implicits { self: Typer =>
               success(Literal(c))
             case TypeRef(_, sym) if sym == defn.UnitClass =>
               success(Literal(Constant(())))
-            case n: NamedType =>
+            case n: TermRef =>
               success(ref(n))
             case tp =>
               EmptyTree

tests/neg/i6241.scala

@@ -0,0 +1,5 @@
+object Test extends App {
+  inline def v[T] = valueOf[T]  // error
+
+  println(v[String])
+}