Migration to Gatsby v2 - Variable "$slug" of required type "String!" was not provided

when you run gatsby develop, you received the all data without these errors, but when use gatsby build you received this error.

"Variable "$slug" of required type "String!" was not provided.".

solution

The issue is you should not put template files in src/pages folder.

you may be creating pages with gatsby page API.

result.data.allWordpressPost.edges.forEach(({ node }) => {
      createPage({
        // Decide URL structure
        path: node.slug,
        // path to template file do not put templates in pages
           folder.
        component: path.resolve("./src/templates/blog.js"),
        context: {
          slug: node.slug,
          $slug: node.slug
        },
      });
    });

It so happens that I have already moved my single-post.js, post.js and other CPT .js files into a new, custom folder (/src/templates).

If this didn't work then remove ! mark from ($slug: String!)

Thank you!