Allow `as` to determine a type even if the casted value doesn't have a type (yet)

spec/compiler/type_inference/cast_spec.cr

@@ -203,4 +203,20 @@ describe "Type inference: cast" do
       ),
       "can't cast Int32 to Bool"
   end
+
+  it "casts to target type even if can't infer casted value type" do
+    assert_type(%(
+      require "prelude"
+
+      class Foo
+        property! x
+      end
+
+      a = [1, 2, 3]
+      b = a.map { Foo.new.x as Int32 }
+
+      Foo.new.x = 1
+      b
+      )) { array_of(int32) }
+  end
 end

src/compiler/crystal/semantic/ast.cr

@@ -195,10 +195,15 @@ module Crystal
     end
 
     def update(from = nil)
+      to_type = to.type.instance_type
+
       obj_type = obj.type?
-      return unless obj_type
 
-      to_type = to.type.instance_type
+      # If we don't know what type we are casting from, leave it as the to_type
+      unless obj_type
+        self.type = to_type.virtual_type
+        return
+      end
 
       if obj_type.pointer? || to_type.pointer?
         self.type = to_type
@@ -214,6 +219,11 @@ module Crystal
           filtered_type = to_type.virtual_type
           @upcast = true
         end
+
+        # If we don't have a matching type, leave it as the to_type:
+        # later (in after type inference) we will check again.
+        filtered_type ||= to_type.virtual_type
+
         self.type = filtered_type
       end
     end