Strategy to override a class in a library installed with Composer

In composer.json, under ["autoload"]["psr-4"], add an entry with namespace as the key and path as the value:

{
     "autoload": {

         "psr-4": {

             "BuggyVendor\\Namespace\\": "myfixes/BuggyVendor/Namespace"
         }
     }
}

Copy files you want to override under that path (keeping sub-namespace directory structure) and edit them there. They will be picked in preference to the library package's original "classpath". It would seem that namespace->path mappings added to composer.json in this manner are considered before those added by required packages. Note: I just tried it and it worked, though I don't know if it is an intended feature or what possible gotchas are.

EDIT: found a gotcha. Sometimes when you subsequently require another package with composer require vendor/package, you will "lose" the override. If this happens, you must issue composer dump-autoload manually. This will restore the correct autoload order honoring your override.

Adding these last 2 lines to the autoload section of my composer.json is what worked for me when I wanted to override just one file within the vendors directory:

"autoload": {        
    "classmap": [
        "database"
    ],
    "psr-4": {
        "App\\": "app/"
    },
    "exclude-from-classmap": ["vendor/somepackagehere/blah/Something.php"],
    "files": ["app/Overrides/Something.php"]
},

Remember that the namespace within app/Overrides/Something.php needs to match whatever the original was in vendor/somepackagehere/blah/Something.php.

Remember to run composer dump-autoload after editing the composer.json.

_{Docs: https://getcomposer.org/doc/04-schema.md#files}