Fix #8749: Warn on $ at the start of a name in quoted patterns

These are just part of the name.

Author: nicolasstucki

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

@@ -229,6 +229,10 @@ trait QuotesAndSplices {
               }
             }
             cpy.AppliedTypeTree(tree)(transform(tpt), args1)
+        case tree: NamedDefTree =>
+          if tree.isTerm && !tree.nameSpan.isSynthetic && tree.symbol.name.startsWith("$") then
+            ctx.error("Names cannot start with $ quote pattern", ctx.source.atSpan(tree.nameSpan))
+          super.transform(tree)
         case _ =>
           super.transform(tree)
       }

tests/neg/i8749.scala

@@ -0,0 +1,9 @@
+import scala.quoted._
+
+object FunObject {
+  def fun(t: String => String) = t
+}
+
+def test(using QuoteContext)(x: Expr[String => String]) =
+  x match
+    case '{ FunObject.fun(($arg: String) => $out) } => // error