Fix issue with undefined class constant keys

src/Psalm/Checker/StatementsChecker.php

@@ -779,7 +779,7 @@ class StatementsChecker extends SourceChecker implements StatementsSource
                 return Type::getClassString();
             }
 
-            return null;
+            return Type::getMixed();
         }
 
         if ($stmt instanceof PhpParser\Node\Scalar\String_) {

tests/ConstantTest.php

@@ -108,6 +108,19 @@ class ConstantTest extends TestCase
                 'assertions' => [],
                 'error_levels' => ['MixedAssignment'],
             ],
+            'hardToDefineClassConstant' => [
+                '<?php
+                    class A {
+                      const C = [
+                        self::B => 4,
+                        "name" => 3
+                      ];
+
+                      const B = 4;
+                    }
+
+                    echo A::C[4];',
+            ],
         ];
     }