Improve error message for autocast error at Var assign

spec/compiler/semantic/automatic_cast_spec.cr

@@ -616,4 +616,18 @@ describe "Semantic: automatic cast" do
       Bar.new
       ))
   end
+
+  it "errors when autocast default value doesn't match enum member" do
+    assert_error <<-CR,
+      enum Foo
+        FOO
+      end
+
+      def foo(foo : Foo = :bar)
+      end
+
+      foo
+      CR
+      "can't autocast :bar to Foo: no matching enum member"
+  end
 end

src/compiler/crystal/semantic/main_visitor.cr

@@ -773,6 +773,10 @@ module Crystal
           if casted_value = check_automatic_cast(value, restriction_type, node)
             value = casted_value
           else
+            if value.is_a?(SymbolLiteral) && restriction_type.is_a?(EnumType)
+              node.raise "can't autocast #{value} to #{restriction_type}: no matching enum member"
+            end
+
             node.raise "can't restrict #{value.type} to #{restriction}"
           end
         end