Fix #9321: Make not-null detection more robust

We can't rely on a test whether a tree transform yields the same tree,
since typed tree copiers do not always map equal sub-trees to equal results.

Author: odersky

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

@@ -480,10 +480,12 @@ object Nullables:
       if mt.paramInfos.exists(_.isInstanceOf[ExprType]) && !fn.symbol.is(Inline) =>
         app match
           case Apply(fn, args) =>
-            val dropNotNull = new TreeMap:
+            object dropNotNull extends TreeMap:
+              var dropped: Boolean = false
               override def transform(t: Tree)(using Context) = t match
                 case AssertNotNull(t0) if t0.symbol.is(Mutable) =>
                   nullables.println(i"dropping $t")
+                  dropped = true
                   transform(t0)
                 case t: ValDef if !t.symbol.is(Lazy) => super.transform(t)
                 case t: MemberDef =>
@@ -502,7 +504,7 @@ object Nullables:
             def postProcess(formal: Type, arg: Tree): Tree =
               val nestedCtx = ctx.fresh.setNewTyperState()
               val arg1 = dropNotNull.transform(arg)(using nestedCtx)
-              if arg1 eq arg then arg
+              if !dropNotNull.dropped then arg
               else
                 val arg2 = retyper.typed(arg1, formal)(using nestedCtx)
                 if nestedCtx.reporter.hasErrors || !(arg2.tpe <:< formal) then

tests/pos-macros/i9321/macros.scala

@@ -0,0 +1,16 @@
+import scala.quoted._
+
+type Foo
+type F[X]
+def varargsFunc(funcs0: Foo*) = ???
+
+inline def mcr1: F[String] = ${ mcr1Impl }
+def mcr1Impl(using QuoteContext): Expr[F[String]] = '{???}
+
+inline def mcr2: Unit = ${mcr2Impl}
+def mcr2Impl(using ctx: QuoteContext): Expr[Unit] =
+  val func: Expr[Seq[Foo] => Unit] =
+    '{ (esx: Seq[Foo]) => varargsFunc(esx: _*) }
+  val trees: Expr[Seq[Foo]] =
+    '{Nil}
+  Expr.betaReduce(func)(trees)

tests/pos-macros/i9321/test.scala

@@ -0,0 +1,5 @@
+def f(x: => Any) = ???
+def g = f {
+  mcr1
+  mcr2
+}