Handle case where expected type of a SeqLiteral has an undetermined element type.

Test case is isApplicableSafe -Ycheck:first.

Author: odersky

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

@@ -895,7 +895,11 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
   }
 
   def typedSeqLiteral(tree: untpd.SeqLiteral, pt: Type)(implicit ctx: Context): SeqLiteral = track("typedSeqLiteral") {
-    val proto1 = pt.elemType orElse WildcardType
+    val proto1 = 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

tests/pending/pos/isApplicableSafe.scala

@@ -0,0 +1,54 @@
+import reflect.ClassTag
+
+// The same problems arise in real arrays.
+class A {
+
+  class Array[T]
+  object Array {
+    def apply[T: ClassTag](xs: T*): Array[T] = ???
+    def apply(x: Int, xs: Int*): Array[Int] = ???
+  }
+
+  // Any of Array[List[Symbol]], List[Array[Symbol]], or List[List[Symbol]] compile.
+  var xs: Array[Array[Symbol]] = _
+  var ys: Array[Map[Symbol, Set[Symbol]]] = _
+
+  //xs = Array(Array())
+    // gives:
+    //
+    // isApplicableSafe.scala:15: error: type mismatch:
+    // found   : A.this.Array[Nothing]
+    // required: A.this.Array[Symbol]
+    // xs = Array(Array())
+    //
+    // Here's the sequence of events that leads to this problem:
+    //
+    // 1. the outer Array.apply is overloaded, so we need to typecheck the inner one
+    //    without an expected prototype
+    //
+    // 2. The inner Array.apply needs a ClassTag, so we need to instantiate
+    //    its type variable, and the best instantiation is Nothing.
+    //
+    // To prevent this, we'd need to do several things:
+    //
+    // 1. Pass argument types lazily into the isApplicable call in resolveOverloaded,
+    //    so that we can call constrainResult before any arguments are evaluated.
+    //
+    // 2. This is still not enough because the result type is initially an IgnoredProto.
+    //    (because an implicit might have to be inserted around the call, so we cannot
+    //     automatically assume that the call result is a subtype of the expected type).
+    //    Hence, we need to somehow create a closure in constrainResult that does the
+    //    comparison with the real expected result type "on demand".
+    //
+    // 3. When instantiating a type variable we need to categorize that some instantiations
+    //    are suspicous (e.g. scalac avoids instantiating to Nothing). In these
+    //    circumstances we should try to excute the delayed constrainResult closures
+    //    in order to get a better instance type.
+    //
+    // Quite a lot of work. It's looking really complicated to fix this.
+
+
+  ys = Array(Map(), Map())
+
+  val zs = Array(Map())
+}