Backport "Refactor constant folding of applications" to LTS (#21043)

Backports #20099 to the LTS branch.

PR submitted by the release tooling.
[skip ci]

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

@@ -1106,12 +1106,13 @@ 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

@@ -4129,7 +4129,7 @@ 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
+
 }