Fix inconsistent typeSize for TupleN vs nested pairs

Previously, `typeSize` reported different values for standard `TupleN`
types (e.g., `(A, B)`) compared to their equivalent recursive pair
encodings (e.g., `A *: B *: EmptyTuple`). This discrepancy occurred
because `TupleN` is a flat `AppliedType`, while the nested encoding
forms a deeper tree structure.

This patch modifies `TypeSizeAccumulator` to canonicalize `TupleN`
types into their recursive `*:` representation before calculating
the size. This ensures that the size metric is consistent regardless
of whether the tuple is represented syntactically or structurally.

This change is verified by a new unit test in `TypesTest`, which
confirms that both `Tuple3[Int, Boolean, Double]` and its recursive
equivalent `Int *: Boolean *: Double *: EmptyTuple` now yield
identical `typeSize` values.

Fixes #24730

Author: Lluc24

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -7124,7 +7124,15 @@ object Types extends TypeUtils {
   class TypeSizeAccumulator(using Context) extends TypeAccumulator[Int] {
     var seen = util.HashSet[Type](initialCapacity = 8)
     def apply(n: Int, tp: Type): Int =
-      tp match {
+
+      val canonicTp = tp match
+        case AppliedType(_, args) if defn.isTupleNType(tp) =>
+          args.foldRight(defn.EmptyTupleClass.typeRef: Type) { (elemType, acc) =>
+            defn.PairClass.typeRef.appliedTo(elemType, acc)
+          }
+        case _ => tp
+
+      canonicTp match {
         case tp: AppliedType =>
           val tpNorm = tp.tryNormalize
           if tpNorm.exists then apply(n, tpNorm)

compiler/test/dotty/tools/dotc/core/TypesTest.scala

@@ -0,0 +1,18 @@
+package dotty.tools.dotc.core
+
+import dotty.tools.DottyTest
+import dotty.tools.dotc.core.Symbols.defn
+import dotty.tools.dotc.core.TypeOps
+
+import org.junit.Test
+import org.junit.Assert.assertEquals
+
+class TypesTest extends DottyTest:
+
+  @Test def tuple3TypeSize =
+    val tpe = defn.TupleType(3).nn.appliedTo(List(defn.IntType, defn.BooleanType, defn.DoubleType))
+    assertEquals(3, tpe.typeSize)
+
+  @Test def tuple3ConsTypeSize =
+    val tpe = TypeOps.nestedPairs(List(defn.IntType, defn.BooleanType, defn.DoubleType))
+    assertEquals(3, tpe.typeSize)

compiler/test/dotty/tools/dotc/typer/DivergenceChecker.scala

@@ -50,8 +50,8 @@ class DivergenceCheckerTests extends DottyTest {
           1,
           1,
           1,
-          3,
-          5
+          4,
+          6
         )
 
         tpes.lazyZip(expectedSizes).lazyZip(expectedCoveringSets).foreach {