Fix #4493: Fail level checks for macro type parameters at level 0

They require a type tag.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/transform/ReifyQuotes.scala

@@ -144,6 +144,9 @@ class ReifyQuotes extends MacroTransformWithImplicits with InfoTransformer {
     /** We are in a `~(...)` context that is not shadowed by a nested `'(...)` */
     def inSplice = outer != null && !inQuote
 
+    /** We are not in a `~(...)` or a `'(...)` */
+    def isRoot = outer != null
+
     /** A map from type ref T to expressions of type `quoted.Type[T]`".
      *  These will be turned into splices using `addTags` and represent type variables
      *  that can be possibly healed.
@@ -242,8 +245,8 @@ class ReifyQuotes extends MacroTransformWithImplicits with InfoTransformer {
      */
     def levelOK(sym: Symbol)(implicit ctx: Context): Boolean = levelOf.get(sym) match {
       case Some(l) =>
-        l == level ||
-        sym.is(Inline) && sym.owner.is(Macro) && sym.info.isValueType && l - 1 == level
+        if (level == 1 && l == level && sym.isType && sym.owner.is(Macro) && outer.isRoot) false
+        else l == level || (l - 1 == level && sym.is(Inline) && sym.owner.is(Macro) && sym.info.isValueType)
       case None =>
         !sym.is(Param) || levelOK(sym.owner)
     }

tests/neg/i4493.scala

@@ -0,0 +1,6 @@
+class Index[K]
+object Index {
+  inline def succ[K]: Unit = ~{
+    '(new Index[K]) // error: missing typetag Type[K]. May not be an error if we synthesize them #4515.
+  }
+}

tests/pos/i4493-b.scala

@@ -0,0 +1,7 @@
+class Index[K]
+object Index {
+  inline def succ[K](x: K): Unit = ~{
+    implicit val t: quoted.Type[K] = '[K]
+    '(new Index[K])
+  }
+}

tests/pos/i4493.scala

@@ -0,0 +1,7 @@
+class Index[K]
+object Index {
+  inline def succ[K]: Unit = ~{
+    implicit val t: quoted.Type[K] = '[K]
+    '(new Index[K])
+  }
+}