Follow-up from #6676:

A single catch-all error is currently reported for problems relating to a module manifest's nonempty RootModule entry:

• The RootModule entry has no filename extension Update: specifying .psm1 should not be a requirement - see Test-ModuleManifest erroneously requires a script-module RootModule entry to be specified with filename extension #8388
• It does have extension .psm1, but the referenced file doesn't exist.
• The *.psd1 file is not located in a folder named for its base name (e.g., test.psd1 must be inside a folder named test) Update: Such a test isn't actually being performed - see Test-ModuleManifest should check the manifest / root-module file base name for naming consistency with the enclosing directory #8391

The unhelpful current error message:

Test-ModuleManifest : The module manifest 'foo.psd1' could not be processed because it is not a valid PowerShell module manifest file. Remove the elements that are not permitted: /path/to/foo.psd1

It's probably worth checking if all error scenarios have specific, helpful error messages.

Additionally, the behavior regarding requiring the extension to be specified may have changed compared to Windows PowerShell.

Environment data

PowerShell Core v6.0.2 on macOS 10.13.4
PowerShell Core v6.0.2 on Ubuntu 16.04.4 LTS
PowerShell Core v6.0.2 on Microsoft Windows 10 Pro (64-bit; Version 1709, OS Build: 16299.371)