My composer.json:

{
    "name": "example/example",
    "description": "",
    "license": "MIT",
    "autoload": {
        "psr-4": {
            "Foo\\": "src/"
        }
    }
}

Output of composer diagnose:

When I run this command:

composer du --strict-psr --optimize

With the two following classes:

src/Bar.php

<?php
class Bar {}

src/Boop.php

<?php
namespace Boop;
class Boop {}

I get the following output:

Generating optimized autoload files
Generated optimized autoload files containing 1 classes

And I expected this to happen:

Generating optimized autoload files
+ Class Bar located in ./src/Bar.php does not comply with psr-4 autoloading standard. Skipping.
+ Class Boop\Boop located in ./src/Boop.php does not comply with psr-4 autoloading standard. Skipping.
Generated optimized autoload files containing 1 classes

Only classes that are outside the configured for a folder get ignored. So if src/ is configured with namespace Foo\\, if the class misses a namespace or is in a totally different namespace like Bar\\, it doesn't trigger any errors when running with --strict-psr.