Add error message for unbound wildcard type.

Author: jarrodj

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -676,7 +676,7 @@ object Parsers {
       val t = typ()
       findWildcardType(t) match {
         case Some(wildcardPos) =>
-          syntaxError("unbound wildcard type", wildcardPos)
+          syntaxError(UnboundWildcardType(), wildcardPos)
           scalaAny
         case None => t
       }

compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -923,4 +923,61 @@ object messages {
            |"""
   }
 
+  case class UnboundWildcardType()(implicit ctx: Context) extends Message(35) {
+    val kind = "Syntax"
+    val msg = "Unbound wildcard type"
+    val explanation = {
+
+      val paramListWildcard = """def foo(x: _) = ..."""
+      val paramList = """def foo(x: Any) = ..."""
+
+      val typeArgsWildcard = """val foo = List[_](1, 2)"""
+      val typeArgs = """val foo = List[Int](1, 2)"""
+
+      val typeBoundsWildcard = """def foo[T <: _](x: T) = ..."""
+      val typeBounds = """def foo[T](x: T) = ..."""
+
+      val valTypeWildcard = """val foo: _ = 3"""
+      val valType = """val foo: Int = 3"""
+
+      hl"""|The wildcard type syntax (`_`) was used where it could not be bound.
+           |Replace `_` with a non-wildcard type. If the type doesn't matter,
+           |try replacing `_` with ${"Any"}.
+           |
+           |Examples:
+           |
+           |- Parameter lists
+           |
+           |  Instead of:
+           |    $paramListWildcard
+           |
+           |  Use ${"Any"} if the type doesn't matter:
+           |    $paramList
+           |
+           |- Type arguments
+           |
+           |  Instead of:
+           |    $typeArgsWildcard
+           |
+           |  Use:
+           |    $typeArgs
+           |
+           |- Type bounds
+           |
+           |  Instead of:
+           |    $typeBoundsWildcard
+           |
+           |  Remove the bounds if the type doesn't matter:
+           |    $typeBounds
+           |
+           |- ${"val"} and ${"def"} types
+           |
+           |  Instead of:
+           |    $valTypeWildcard
+           |
+           |  Use:
+           |    $valType
+           |"""
+    }
+  }
 }