Fix #6828: Support by-name synthesized implicits

Author: nicolasstucki

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

@@ -1067,7 +1067,7 @@ trait Implicits { self: Typer =>
             }
             val base = baseWithRefinements(formal)
             val result =
-              if (base <:< formal) {
+              if (base <:< formal.widenExpr) {
                 // With the subtype test we enforce that the searched type `formal` is of the right form
                 handler(base, span)(ctx)
               }

tests/pos/i6828.scala

@@ -0,0 +1,5 @@
+class Foo {
+  inline def foo[T](implicit ct: =>scala.reflect.ClassTag[T]): Unit = Unit
+  foo[Int]
+  foo[String]
+}