Try introducing phpstan

[szepeviktor]

phpstan.neon.dist

#$ composer require --dev szepeviktor/phpstan-wordpress phpstan/phpstan-shim phpstan/phpstan-mockery
#$ vendor/bin/phpstan analyze

includes:
    - phar://phpstan.phar/conf/bleedingEdge.neon
    - vendor/phpstan/phpstan-mockery/extension.neon
    - vendor/szepeviktor/phpstan-wordpress/extension.neon
parameters:
    level: 4
#    level: max
    inferPrivatePropertyTypeFromConstructor: true
    paths:
        - %currentWorkingDirectory%/src/
        - %currentWorkingDirectory%/inc/
    ignoreErrors:

[szepeviktor]

the current output on Level 4 is as follows

 ------ --------------------------------------------------------------------------------
  Line   src/FunctionExpectations.php
 ------ --------------------------------------------------------------------------------
  45     Call to an undefined method Brain\Monkey\Expectation\Expectation::byDefault().
  60     Call to an undefined method Brain\Monkey\Expectation\Expectation::byDefault().
 ------ --------------------------------------------------------------------------------

 ------ --------------------------------------------------------------------------------------------------------------------------------------------------
  Line   src/MonkeyWpUser.php
 ------ --------------------------------------------------------------------------------------------------------------------------------------------------
  26     Method Brain\Faker\MonkeyWpUser::__monkeyMakeCurrent() should return Brain\Faker\MonkeyWpUser|Mockery\MockInterface|WP_User but return statement
         is missing.
 ------ --------------------------------------------------------------------------------------------------------------------------------------------------

 ------ ---------------------------------------------------------------------------------------------------------------
  Line   src/Provider/Post.php
 ------ ---------------------------------------------------------------------------------------------------------------
  285    PHPDoc tag @param has invalid value ($date): Unexpected token "$date", expected TOKEN_IDENTIFIER at offset 18
 ------ ---------------------------------------------------------------------------------------------------------------

 ------ ----------------------------------------------------
  Line   src/Provider/Site.php
 ------ ----------------------------------------------------
  28     Access to an undefined property WP_Site::$term_id.
  28     Access to an undefined property WP_Site::$term_id.
 ------ ----------------------------------------------------

 ------ ----------------------------------------------------------------------------------------------------------------------------
  Line   src/Provider/Taxonomy.php
 ------ ----------------------------------------------------------------------------------------------------------------------------
  330    PHPDoc tag @return contains unresolvable type.
  350    Method Brain\Faker\Provider\Taxonomy::__invoke() with return type void returns WP_Taxonomy but should not return anything.
  367    Method Brain\Faker\Provider\Taxonomy::__invoke() with return type void returns WP_Taxonomy but should not return anything.
 ------ ----------------------------------------------------------------------------------------------------------------------------

 ------ -----------------------------------------------------------------------------------------------------------------
  Line   src/Provider/Term.php
 ------ -----------------------------------------------------------------------------------------------------------------
  83     Access to an undefined property Mockery\MockInterface::$term_id.
  86     Method Brain\Faker\Provider\Term::__invoke() should return WP_Term but returns Mockery\MockInterface.
  126    PHPDoc tag @param has invalid value ($term): Unexpected token "$term", expected TOKEN_IDENTIFIER at offset 18
  162    PHPDoc tag @param has invalid value ($field): Unexpected token "$field", expected TOKEN_IDENTIFIER at offset 18
  162    PHPDoc tag @param has invalid value ($value): Unexpected token "$value", expected TOKEN_IDENTIFIER at offset 39
 ------ -----------------------------------------------------------------------------------------------------------------

 ------ ------------------------------------------------------------------------------------------------------
  Line   src/Provider/User.php
 ------ ------------------------------------------------------------------------------------------------------
  242    PHPDoc tag @return with type Brain\Faker\MonkeyWpUser|WP_User is not subtype of native type WP_User.
 ------ ------------------------------------------------------------------------------------------------------