IntelliJ "The SDK is not specified for modules .."

Original post on StackOverflow: IntelliJ "The SDK is not specified for modules .."

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My org uses custom IntelliJ plugins through which we perform 'sync' from remote. Recently I ran into troubles with few symbols not being recognized after my change because of either code auto-generation (from XML schemas) not working correctly or the IDE resolving a wrong classpath for auto-generated code.

That's when I tried a bunch of fixes including deleting .idea/ directory and un-installing / re-installing our custom plugins. After that when I'm trying to perform a sync, it fails with following error

The SDK is not specified for modules {module_1}, {module_2} ..

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To clarify

1. Project Settings > (Project) SDK is defined and most of the symbols are getting recognized
2. Even under Project Settings > Modules > Dependencies, Module SDK is defined for all modules. But for half of those it is still showing as <No SDK> in that list of dependencies (see 3rd screenshot below)
   • and when I right click on those <No SDK> list items, in the popup the options to Edit, Remove etc are all greyed out

At the same time I'm getting some strange red-wavy line errors like

Required type: String
Provided: java.lang.String

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I've tried

• restarting IDE multiple times
• upgrade to latest IDE version, downgraded to penultimate version
• (already mentioned above) removing .idea/ directory
• (already mentioned above) un-installing + re-installing custom plugins

What else could I try?

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I'm on

• IntelliJ IDEA 2023.2.5 (Ultimate Edition) Build #IU-232.10227.8, built on November 9, 2023
  • I have also tried v2023.3 but some plugins were incompatible so had to downgrade
• macBook Pro with Apple M1 Prod running macOS Ventura 13.6.2