Intellij will not recognize antlr generated source code

It turns out that my fiddling with preferences caused this issue. If anyone else has this problem:

As v0rl0n mentioned, first make sure the gen folder is listed as a source. Go to Module Settings -> Modules -> Sources.
Look to the right for the blue Source Folders. Ensure your generated source folders are listed (they should have a [generated] flag on the row).

If your generated source directory is listed there, and they are still not being recognized, go to Preferences -> File Types. Look in the Ignore files and folders field. Make sure you don't have anything listed that will match for your generated sources directory.

In my case, I had added target to this list, which caused my problem, and was a real pain to find.

My issue was somewhat similar to @spilymp's:

I was putting the *.g4 files directly in src/main/antlr4:

.
├── src/
|   └── main/
|       ├── antlr/
|       |   ├── Main.g4
|       |   └── imported.g4
|       └── java/
|           └── com/
|               └── test/
|                   └── Test.java

which led to source being generated in default package. I changed the package structure to match that of java:

.
├── src/
|   └── main/
|       ├── antlr/
|       |   ├── com/
|       |   |   └── test/
|       |   |       └── Main.g4            
|       |   └── imports/
|       |       └── imported.g4
|       └── java/
|           └── com/
|               └── test/
|                   └── Test.java

(Note that imports need to go directly in src/main/antlr4/imports)

After this I just run the antlr4 goal from the maven menu ([Module] > Plugins > antlr4 > antlr4:antlr4), which generated the sources in the default location (target/generated-sources/antlr4), where they were already marked with blue, generated-sources icon by Intellij, and MainParser can now be imported!

The problem

target/generated-sources/antlr4 is not automatically marked as source dir, instead its direct subdir com.example is. Intellij Idea fails to detect proper package for classes inside target/generated-sources/antlr4/com.example.

The cause

The source file *.g4 is in src/main/antlr4/com.example, but it actually it should be src/main/antlr4/com/example. Note the /. You probably forgot to mark src/main/antlr4 as source dir in Idea, and when you thought you are creating package structure, you actually just created single dir called com.example.

The fix

Mark src/main/antlr4 as source dir, create proper directory structure src/main/antlr4/com/example. Rebuild.

Alternative fix

Go to Project Structure - Modules - Source Folders and find the target/generated-sources/antlr4/com.example - click Edit properties and set Package prefix to com.example.

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Different but related problem here