CompileCheck test and extra note on how it works in docs

docs/programmers/api-variables/variable-compile-check.md

@@ -13,6 +13,9 @@ changing), the compile check step is no longer needed. Be sure to set
 change this to FALSE and a template file is changed, you will \*not\*
 see the change since the template will not get recompiled.
 
+Note that up to Smarty 4.x, Smarty will check for the existence of 
+the source template even if `$compile_check` is disabled.
+
 If [`$caching`](#variable.caching) is enabled and `$compile_check` is
 enabled, then the cache files will get regenerated if an involved
 template file or config file was updated.

tests/UnitTests/SmartyMethodsTests/CompileCheck/CompileCheckTest.php

@@ -0,0 +1,123 @@
+<?php
+/**
+ * Smarty PHPunit tests for compile check
+ */
+class CompileCheckTest extends PHPUnit_Smarty
+{
+    public $methodName = null;
+    public function setUp(): void
+    {
+        $this->setUpSmarty(__DIR__);
+        $this->smarty->addTemplateDir('./templates_tmp');
+        $this->cleanDirs();
+    }
+
+    /**
+     * generate templates
+     */
+    protected function makeFiles()
+    {
+        file_put_contents('./templates_tmp/t1.tpl', 'TPL1');
+        file_put_contents('./templates_tmp/t2.tpl', 'TPL2');
+        file_put_contents('./templates_tmp/base.tpl', '{include file="t1.tpl"}{include file="t2.tpl"}');
+    }
+    /**
+     * remove generated templates
+     */
+    protected function removeFiles()
+    {
+        unlink('./templates_tmp/t1.tpl');
+        unlink('./templates_tmp/t2.tpl');
+        unlink('./templates_tmp/base.tpl');
+    }
+
+    /**
+     * reset, but leave the files alone
+     * @return void
+     */
+    private function softResetSmarty() {
+        $this->smarty = new Smarty();
+        $this->smarty->addTemplateDir('./templates_tmp');
+    }
+
+    /**
+     * @group slow
+     */
+    public function testCompileCheckOn0()
+    {
+        $this->makeFiles();
+        $this->assertEquals('TPL1TPL2', $this->smarty->fetch('base.tpl'));
+
+        $this->softResetSmarty();
+        $this->smarty->setCompileCheck(Smarty::COMPILECHECK_ON);
+
+        $this->assertEquals('TPL1TPL2', $this->smarty->fetch('base.tpl'));
+    }
+
+    /**
+     * @group slow
+     */
+    public function testCompileCheckOn1()
+    {
+        $this->makeFiles();
+        $this->assertEquals('TPL1TPL2', $this->smarty->fetch('base.tpl'));
+
+        $this->softResetSmarty();
+        $this->smarty->setCompileCheck(Smarty::COMPILECHECK_ON);
+
+        unlink('./templates_tmp/base.tpl');
+        sleep(1);
+
+        $this->expectException(Exception::class);
+        $this->smarty->fetch('base.tpl');
+    }
+
+    /**
+     * @group slow
+     */
+    public function testCompileCheckOn2()
+    {
+        $this->makeFiles();
+        $this->assertEquals('TPL1TPL2', $this->smarty->fetch('base.tpl'));
+
+        $this->softResetSmarty();
+        $this->smarty->setCompileCheck(Smarty::COMPILECHECK_ON);
+
+        sleep(1);
+        file_put_contents('./templates_tmp/base.tpl', 'hello');
+
+        $this->assertEquals('hello', $this->smarty->fetch('base.tpl'));
+    }
+
+    /**
+     * @group slow
+     */
+    public function testCompileCheckOff0()
+    {
+        $this->makeFiles();
+        $this->assertEquals('TPL1TPL2', $this->smarty->fetch('base.tpl'));
+
+        $this->softResetSmarty();
+        $this->smarty->setCompileCheck(Smarty::COMPILECHECK_OFF);
+
+        $this->assertEquals('TPL1TPL2', $this->smarty->fetch('base.tpl'));
+    }
+
+    /**
+     * @group slow
+     */
+    public function testCompileCheckOff2()
+    {
+        $this->makeFiles();
+        $this->assertEquals('TPL1TPL2', $this->smarty->fetch('base.tpl'));
+
+        $this->softResetSmarty();
+        $this->smarty->setCompileCheck(Smarty::COMPILECHECK_OFF);
+
+        sleep(1);
+        file_put_contents('./templates_tmp/base.tpl', 'hello');
+
+        $this->assertEquals('TPL1TPL2', $this->smarty->fetch('base.tpl'));
+    }
+
+}