Fix #6826: Drop ExprType before getting the arg infos

Author: nicolasstucki

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

@@ -1167,7 +1167,7 @@ trait Implicits { self: Typer =>
           err.userDefinedErrorString(
             raw,
             pt.typeSymbol.typeParams.map(_.name.unexpandedName.toString),
-            pt.argInfos))
+            pt.widenExpr.argInfos))
 
         def hiddenImplicitsAddendum: String = arg.tpe match {
           case fail: SearchFailureType =>

tests/neg/i6826.scala

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