Fix #4375: Properly erase SeqLiteral

Before this commit, the `SeqLiteral#elems` were typed based on the
SeqLiteral prototype, which means that nothing ensured that the elems
types conformed to the type of `SeqLiteral#elemtpt`. In i4375.scala this
means that erasing `SeqLiteral([1, 2], Object)` did not box each element
because the expected element type was `WildcardType`.

To prevent this sort of issue, we now use the type of SeqLiteral#elemtpt
if it exists as the expected type of each element in the sequence.

Author: smarter

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

@@ -1126,20 +1126,31 @@ class Typer extends Namer
   }
 
   def typedSeqLiteral(tree: untpd.SeqLiteral, pt: Type)(implicit ctx: Context): SeqLiteral = track("typedSeqLiteral") {
-    val proto1 = pt.elemType match {
+    val elemProto = pt.elemType match {
       case NoType => WildcardType
       case bounds: TypeBounds => WildcardType(bounds)
       case elemtp => elemtp
     }
-    val elems1 = tree.elems mapconserve (typed(_, proto1))
-    val proto2 = // the computed type of the `elemtpt` field
-      if (!tree.elemtpt.isEmpty) WildcardType
-      else if (isFullyDefined(proto1, ForceDegree.none)) proto1
-      else if (tree.elems.isEmpty && tree.isInstanceOf[Trees.JavaSeqLiteral[_]])
-        defn.ObjectType // generic empty Java varargs are of type Object[]
-      else ctx.typeComparer.lub(elems1.tpes)
-    val elemtpt1 = typed(tree.elemtpt, proto2)
-    assignType(cpy.SeqLiteral(tree)(elems1, elemtpt1), elems1, elemtpt1)
+
+    def assign(elems1: List[Tree], elemtpt1: Tree) =
+      assignType(cpy.SeqLiteral(tree)(elems1, elemtpt1), elems1, elemtpt1)
+
+    if (!tree.elemtpt.isEmpty) {
+      val elemtpt1 = typed(tree.elemtpt, elemProto)
+      val elems1 = tree.elems.mapconserve(typed(_, elemtpt1.tpe))
+      assign(elems1, elemtpt1)
+    } else {
+      val elems1 = tree.elems.mapconserve(typed(_, elemProto))
+      val elemtptType =
+        if (isFullyDefined(elemProto, ForceDegree.none))
+          elemProto
+        else if (tree.elems.isEmpty && tree.isInstanceOf[Trees.JavaSeqLiteral[_]])
+          defn.ObjectType // generic empty Java varargs are of type Object[]
+        else
+          ctx.typeComparer.lub(elems1.tpes)
+      val elemtpt1 = typed(tree.elemtpt, elemtptType)
+      assign(elems1, elemtpt1)
+    }
   }
 
   def typedInlined(tree: untpd.Inlined, pt: Type)(implicit ctx: Context): Inlined = {

tests/pos/i4375.scala

@@ -0,0 +1,13 @@
+object Test {
+  type Id[A] >: A
+  def test: Unit = {
+    val a: Array[_ >: Id[Int]] = Array(1, 2)
+    val b = a(0)
+  }
+
+  class VC(i: String) extends AnyVal
+  def test2: Unit = {
+    val c: Array[_ >: Id[VC]] = Array(new VC(""))
+    val d = c(0)
+  }
+}