Failed production build from Webpack errors after 7.0.2-canary.11 -> 7.0.2-canary.18

[4cm4k1]

Bug report

Describe the bug

Upon upgrading next from 7.0.2-canary.11 to 7.0.2-canary.18 in my project here, production builds fail with the following error. However, running it in development mode has no issues.

> Failed to build
Error: Webpack errors
    at /Users/4cm4k1/Code/website/node_modules/next/dist/build/index.js:239:33
    at finalCallback (/Users/4cm4k1/Code/website/node_modules/webpack/lib/MultiCompiler.js:247:12)
    at runWithDependencies.err (/Users/4cm4k1/Code/website/node_modules/webpack/lib/MultiCompiler.js:270:6)
    at done (/Users/4cm4k1/Code/website/node_modules/neo-async/async.js:2928:13)
    at runCompilers (/Users/4cm4k1/Code/website/node_modules/webpack/lib/MultiCompiler.js:174:48)
    at err (/Users/4cm4k1/Code/website/node_modules/webpack/lib/MultiCompiler.js:181:7)
    at compiler.run (/Users/4cm4k1/Code/website/node_modules/webpack/lib/MultiCompiler.js:263:7)
    at finalCallback (/Users/4cm4k1/Code/website/node_modules/webpack/lib/Compiler.js:204:39)
    at hooks.done.callAsync.err (/Users/4cm4k1/Code/website/node_modules/webpack/lib/Compiler.js:253:14)
    at AsyncSeriesHook.eval [as callAsync] (eval at create (/Users/4cm4k1/Code/website/node_modules/tapable/lib/HookCodeFactory.js:32:10), <anonymous>:42:1)
    at AsyncSeriesHook.lazyCompileHook (/Users/4cm4k1/Code/website/node_modules/tapable/lib/Hook.js:154:20)
    at emitRecords.err (/Users/4cm4k1/Code/website/node_modules/webpack/lib/Compiler.js:251:22)
    at /Users/4cm4k1/Code/website/node_modules/graceful-fs/graceful-fs.js:43:10
    at FSReqCallback.oncomplete (fs.js:148:20)

To Reproduce

Steps to reproduce the behavior, please provide code snippets or a repository:

1. git clone https://github.com/4cm4k1/website.git
2. cd website
3. yarn
4. yarn add next@canary
5. yarn build

Expected behavior

I expect the production build to complete successfully, but it does not.

Screenshots

N/A

System information

• OS: macOS 10.14.2
• Browser: N/A
• Version of Next.js: 7.0.2-canary.18
• Node: 11.1.0

Additional context

I'm guessing this may be related to this work-in-progress feature, but if it isn't I want to give this issue visibility.