Refactor constant folding of applications (#20099)

Move them in typedApply/typedTypeApply instead of leaving them until
adapt. This aligns these folds with folds of unary operations, which are
done already in typedSelect and avoids potentially several calls to
ConstFold when arguments are passed to overloaded methods.

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

@@ -1124,12 +1124,13 @@ trait Applications extends Compatibility {
         }
         app
       }
-    app1 match {
+    val app2 = app1 match {
       case Apply(Block(stats, fn), args) =>
         tpd.cpy.Block(app1)(stats, tpd.cpy.Apply(app1)(fn, args))
       case _ =>
         app1
     }
+    ConstFold(app2)
   }
 
   /** Typecheck an Apply node with a typed function and possibly-typed arguments coming from `proto` */

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

@@ -4196,7 +4196,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
 
     def adaptToSubType(wtp: Type): Tree =
       // try converting a constant to the target type
-      ConstFold(tree).tpe.widenTermRefExpr.normalized match
+      tree.tpe.widenTermRefExpr.normalized match
         case ConstantType(x) =>
           val converted = x.convertTo(pt)
           if converted != null && (converted ne x) then

tests/pos/constfold.scala

@@ -15,4 +15,12 @@ object Test extends App {
     Console.println(A.y);
     Console.println(A.z);
     Console.println(A.s);
+
+    def f(x: 12): Int = 1
+    def f(x: Int): Double = 2
+    val x = f(12)
+    val _: Int = x
+    val y = f(2 * 6)
+    val _: Int = x
+
 }