Add regression test

Closes scala#9531

Author: nicolasstucki

tests/pos/i9531.scala

@@ -0,0 +1,10 @@
+trait Scope:
+  type Expr[+T]
+
+def exprQuote[T](x: T)(using s: Scope, dummy: Null = null): s.Expr[T] = ???
+def exprQuote[T <: Singleton](x: T)(using s: Scope): s.Expr[T] = ???
+
+def test(using s: Scope): Unit =
+  val t1: s.Expr[1] = exprQuote(1)
+  val t2 = exprQuote(1)
+  val t3: s.Expr[1] = t2