[nomaster] SI-8764 fix return type of case class productElement under Xexperimental

Under Xexperimental, productElement now returns the lub instead
of the weak lub of case class parameter types (numeric widening
shouldn't magically happen *inside* productElement).

This was removed from 2.12.x in 6317ae2.

Author: gourlaysama

src/compiler/scala/tools/nsc/typechecker/SyntheticMethods.scala

@@ -95,7 +95,7 @@ trait SyntheticMethods extends ast.TreeDSL {
     // which they shouldn't.
     val accessorLub  = (
       if (settings.Xexperimental) {
-        global.weakLub(accessors map (_.tpe.finalResultType)) match {
+        global.lub(accessors map (_.tpe.finalResultType)) match {
           case RefinedType(parents, decls) if !decls.isEmpty => intersectionType(parents)
           case tp                                            => tp
         }

test/files/neg/t8764.check

@@ -0,0 +1,6 @@
+t8764.scala:8: error: type mismatch;
+ found   : AnyVal
+ required: Double
+  val d: Double = a.productElement(0)
+                                  ^
+one error found

test/files/neg/t8764.flags

@@ -0,0 +1 @@
+-Xexperimental

test/files/neg/t8764.scala

@@ -0,0 +1,9 @@
+object Main {
+
+  case class IntAndDouble(i: Int, d: Double)
+
+  // a.productElement used to be Int => Double
+  // now: Int => AnyVal
+  val a = IntAndDouble(1, 5.0)
+  val d: Double = a.productElement(0)
+}

test/files/run/t8764.check

@@ -0,0 +1,5 @@
+IntOnly: should return an unboxed int
+Int: int
+IntAndDouble: should just box and return Anyval
+Double: class java.lang.Double
+Int: class java.lang.Integer

test/files/run/t8764.flags

@@ -0,0 +1 @@
+-Xexperimental

test/files/run/t8764.scala

@@ -0,0 +1,16 @@
+object Test extends App {
+case class IntOnly(i: Int, j: Int)
+
+println("IntOnly: should return an unboxed int")
+val a = IntOnly(1, 2)
+val i: Int = a.productElement(0)
+println(s"Int: ${a.productElement(0).getClass}")
+
+case class IntAndDouble(i: Int, d: Double)
+
+println("IntAndDouble: should just box and return Anyval")
+val b = IntAndDouble(1, 2.0)
+val j: AnyVal = b.productElement(0)
+println(s"Double: ${b.productElement(1).getClass}")
+println(s"Int: ${b.productElement(0).getClass}")
+}