Add implies as a logical assertion.

This assertion can be used to easily assert stuff like:

> Assert::implies($hasDog, $leashAttached);

Which will assert that $leashAttached is true, if $hasDog is true.

This is a shorthand for `Assert::true(!$hasDog || $leashAttached)`.

README.md

@@ -132,6 +132,8 @@ Method                                          | Description
 `range($value, $min, $max, $message = '')`      | Check that a value is within a range
 `inArray($value, array $values, $message = '')` | Check that a value is one of a list of values
 `oneOf($value, array $values, $message = '')`   | Check that a value is one of a list of values (alias of `inArray`)
+`implies($p, $q, $message = '')`                | Check that `$p` logically implies `$q` (i.e. _if_ `$p` is true, than `$q` must be true).
+
 
 ### String Assertions
 

src/Assert.php

@@ -658,6 +658,20 @@ public static function notFalse($value, $message = '')
         }
     }
 
+    /**
+     * @psalm-pure
+     *
+     * @param mixed $p
+     * @param mixed $q
+     * @param string $message
+     *
+     * @throws InvalidArgumentException
+     */
+    public static function implies($p, $q, $message = '')
+    {
+        self::true(!$p || $q, $message ?: 'Logical implication $p => $q did not hold.');
+    }
+
     /**
      * @param mixed  $value
      * @param string $message

src/Mixin.php

@@ -677,6 +677,28 @@ public static function nullOrNotFalse($value, $message = '');
      */
     public static function allNotFalse($value, $message = '');
 
+    /**
+     * @psalm-pure
+     *
+     * @param mixed  $p
+     * @param mixed  $q
+     * @param string $message
+     *
+     * @throws InvalidArgumentException
+     */
+    public static function nullOrImplies($p, $q, $message = '');
+
+    /**
+     * @psalm-pure
+     *
+     * @param mixed  $p
+     * @param mixed  $q
+     * @param string $message
+     *
+     * @throws InvalidArgumentException
+     */
+    public static function allImplies($p, $q, $message = '');
+
     /**
      * @param mixed  $value
      * @param string $message

tests/AssertTest.php

@@ -226,6 +226,10 @@ public function getTests()
             array('range', array(2, 1, 2), true),
             array('range', array(0, 1, 2), false),
             array('range', array(3, 1, 2), false),
+            array('implies', array(true, true), true),
+            array('implies', array(true, false), false),
+            array('implies', array(false, true), true),
+            array('implies', array(false, false), true),
             array('oneOf', array(1, array(1, 2, 3)), true),
             array('oneOf', array(1, array('1', '2', '3')), false),
             array('inArray', array(1, array(1, 2, 3)), true),

tests/static-analysis/assert-implies.php

@@ -0,0 +1,44 @@
+<?php
+
+namespace Webmozart\Assert\Tests\StaticAnalysis;
+
+use Webmozart\Assert\Assert;
+
+/**
+ * @param mixed $p
+ * @param mixed $q
+ *
+ * @return mixed
+ */
+function implies($p, $q)
+{
+    Assert::implies($p, $q);
+
+    return $p;
+}
+
+/**
+ * @param mixed $p
+ * @param mixed $q
+ *
+ * @return mixed
+ */
+function nullOrImplies($p, $q)
+{
+    Assert::nullOrImplies($p, $q);
+
+    return $p;
+}
+
+/**
+ * @param mixed $p
+ * @param mixed $q
+ *
+ * @return mixed
+ */
+function allImplies($p, $q)
+{
+    Assert::allImplies($p, $q);
+
+    return $p;
+}