tl;dr

• This issue was open before and marked as fixed Webpack isn't producing valid sourcemaps? #7616, but unfortunately, it's still reproducible with the latest version of webpack
• According to https://sourcemaps.io/ and sourcemap-validator, the sourcemaps produced by webpack are invalid
• using devtool: "source-map", a barebones webpack config, and webpack@4.23.1
• sample repro repo available at https://github.com/adaniliuk/webpack-sourcemap-testing
  (this might be an issue with sourcemap-validator, but the error that it's throwing (in the "current behavior" section below) seems pretty legit)

Bug report

What is the current behavior?

Running webpack with the following config:

const webpack = require('webpack');
const path = require('path');

module.exports = {
    entry: './src/index.js',
    output: {
        path: path.resolve(__dirname, 'dist'),
        filename: 'bundle.js'
    },
    devtool: 'source-map'
};

• produces dist/bundle.js and dist/bundle.js.map.
• running those files through sourcemap-validator (or https://sourcemaps.io/, and consequently Sentry) produces the following error:

/webpack-sourcemap-testing/node_modules/sourcemap-validator/index.js:146
          throw new Error(errMsg);
          ^
Error: Warning: mismatched names
Expected: installedModules || 'installedModules' || 'installedModules' || "installedModules" || "installedModules"
Got:  	var installedM ||  	var installedMod ||  	var installedMod ||  	var installedMod ||  	var installedMod
Original Line:  	var installedModules = {};
Mapping: 1:17→2:0 "installedModules" in webpack:///webpack/bootstrap
    at /webpack-sourcemap-testing/node_modules/sourcemap-validator/index.js:146:17
    at Array.forEach (<anonymous>)
    at SourceMapConsumer_eachMapping [as eachMapping] (/webpack-sourcemap-testing/node_modules/sourcemap-validator/node_modules/source-map/lib/source-map/source-map-consumer.js:570:10)
    at validate (/webpack-sourcemap-testing/node_modules/sourcemap-validator/index.js:83:12)
    at Object.<anonymous> (/webpack-sourcemap-testing/test-sourcemaps.js:6:1)
    at Module._compile (module.js:652:30)
    at Object.Module._extensions..js (module.js:663:10)
    at Module.load (module.js:565:32)
    at tryModuleLoad (module.js:505:12)
    at Function.Module._load (module.js:497:3)

If the current behavior is a bug, please provide the steps to reproduce.

Repro repo here: https://github.com/adaniliuk/webpack-sourcemap-testing

Repro instructions for that repo are on the readme.

If starting from scratch, use the following webpack config:

const webpack = require('webpack');
const path = require('path');

module.exports = {
    entry: './src/index.js',
    output: {
        path: path.resolve(__dirname, 'dist'),
        filename: 'bundle.js'
    },
    devtool: 'source-map'
};

and use sourcemap-validator to test dist/bundle.js for validity.

What is the expected behavior?

• webpack produces valid source maps
• so sourcemap-validator reads the sourcemap as valid (e.g., not throw an error).

It looks like it might have something to do with some misaligned whitespace, but I'm really not sure.

Other relevant information:

webpack version: 4.23.1
Node.js version: 8.11.3
Operating System: macOS 10.14 (18A391)
Additional tools: sourcemap-validator@1.1.0, yarn@1.9.2