Missing validation of schema directive argument types

[vwkd]

It seems there is no validation for the argument types of custom schema directives.

Passing an argument with the wrong type to a custom schema directive doesn't throw an error during schema building, e.g. with buildASTSchema or buildSchema. Meanwhile, built-in schema directives like deprecated seem to validate their argument types correctly.

This results in the arguments of a custom schema directive possibly ending up with an unexpected type and your server breaking at runtime when you figure out the bug in your schema. Notably, the type ends up being EnumValue if it doesn't exist as this seems to be the default type.

Steps to reproduce

1. Run with deno, e.g. open deno repl and paste in, or run from file with deno run file.ts

import { buildSchema } from "npm:graphql@16.6.0";

const source1 = `
  type Query {
    # ups... forgot the quotes around the string FOOBAR
    baz: Boolean @foo(bar: FOOBAR)
  }

  directive @foo(bar: String!) on FIELD_DEFINITION
`;

// should throw but doesn't
const schema1 = buildSchema(source1);

console.log(schema1.getQueryType()!.getFields()["baz"].astNode!.directives![0].arguments![0].value.kind);
// EnumValue

const source2 = `
  type Query {
    # ups... forgot the quotes around the string FOOBAR
    baz: Boolean @deprecated(reason: FOOBAR)
  }
`;

// correctly throws
const schema2 = buildSchema(source2);
// error: Uncaught GraphQLError: Argument "reason" has invalid value FOOBAR.

Expected result

The buildSchema(source1) call throws like the buildSchema(source2) does.

Actual result

The buildSchema(source1) call doesn't throw.