Merge branch '3.4' into 4.3

* 3.4:
  [Yaml] Implement multiline string as scalar block for tagged values
  [HttpFoundation] Use `Cache-Control: must-revalidate` only if explicit lifetime has been given
  [FrameworkBundle] Use UserInterface to @return in getUser method
  [CI] Replace php7.4snapshot with php7.4 in Travis configuration
  [ExpressionLanguage][Node][BinaryNode] Process division by zero
  forward caught exception
  [Validator][ConstraintValidator] Stop passing unnecessary timezone argument to \DateTime
  add tags before processing them
  [MonologBridge] Fix debug processor datetime type

Author: nicolas-grekas

ResponseHeaderBag.php

@@ -267,13 +267,13 @@ public function makeDisposition($disposition, $filename, $filenameFallback = '')
      */
     protected function computeCacheControlValue()
     {
-        if (!$this->cacheControl && !$this->has('ETag') && !$this->has('Last-Modified') && !$this->has('Expires')) {
-            return 'no-cache, private';
-        }
-
         if (!$this->cacheControl) {
+            if ($this->has('Last-Modified') || $this->has('Expires')) {
+                return 'private, must-revalidate'; // allows for heuristic expiration (RFC 7234 Section 4.2.2) in the case of "Last-Modified"
+            }
+
             // conservative by default
-            return 'private, must-revalidate';
+            return 'no-cache, private';
         }
 
         $header = $this->getCacheControlHeader();

Tests/ResponseHeaderBagTest.php

@@ -51,9 +51,9 @@ public function testCacheControlHeader()
         $this->assertTrue($bag->hasCacheControlDirective('public'));
 
         $bag = new ResponseHeaderBag(['ETag' => 'abcde']);
-        $this->assertEquals('private, must-revalidate', $bag->get('Cache-Control'));
+        $this->assertEquals('no-cache, private', $bag->get('Cache-Control'));
         $this->assertTrue($bag->hasCacheControlDirective('private'));
-        $this->assertTrue($bag->hasCacheControlDirective('must-revalidate'));
+        $this->assertTrue($bag->hasCacheControlDirective('no-cache'));
         $this->assertFalse($bag->hasCacheControlDirective('max-age'));
 
         $bag = new ResponseHeaderBag(['Expires' => 'Wed, 16 Feb 2011 14:17:43 GMT']);