Wrong Result Generated by findArgChanges

[helloqiu]

Reporting issues with GraphQL.js

Hi there. The function findArgChanges may generate wrong result if default value of the field is an Array:

const oldSchema = buildSchema(`
    type Type1 {
      field1(name: [String!] = []): String
    }

    type Query {
      field1: String
    }
  `);

const newSchema = buildSchema(`
    type Type1 {
      field1(name: [String!] = []): String
    }

    type Query {
      field1: String
    }
  `);
findDangerousChanges(oldSchema, newSchema);

Result:

[ { type: 'ARG_DEFAULT_VALUE_CHANGE',
    description: 'Type1.field1 arg name has changed defaultValue' } ]

Possible Solution

In graphql-js/src/utilities/findBreakingChanges.js:

          } else if (
            oldArgDef.defaultValue !== undefined &&
            oldArgDef.defaultValue !== newArgDef.defaultValue
          ) {
            dangerousChanges.push({
              type: DangerousChangeType.ARG_DEFAULT_VALUE_CHANGE,
              description:
                `${oldType.name}.${fieldName} arg ` +
                `${oldArgDef.name} has changed defaultValue`,
            });
          }

If the default value is an array, oldArgDef.defaultValue !== newArgDef.defaultValue is always true.

I also create a PR which can reproduce this bug: #1566.