Fix #4515: Synthesize implicit type tags in transparent methods

Author: nicolasstucki

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

@@ -620,6 +620,8 @@ trait Implicits { self: Typer =>
         val tag = bindFreeVars(arg)
         if (bindFreeVars.ok) ref(defn.QuotedType_apply).appliedToType(tag)
         else EmptyTree
+      case arg :: Nil if ctx.inTransparentMethod =>
+        ref(defn.QuotedType_apply).appliedToType(arg)
       case _ =>
         EmptyTree
     }

tests/run/i4515/Macro_1.scala

@@ -0,0 +1,5 @@
+
+object Macro {
+  transparent def foo[X](x: X): Unit = ~fooImpl('(x))
+  def fooImpl[X: quoted.Type](x: quoted.Expr[X]): quoted.Expr[Unit] = '()
+}

tests/run/i4515/Test_2.scala

@@ -0,0 +1,6 @@
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    Macro.foo(4)
+  }
+}