Restrict constant values

Author: kocsismate

build/gen_stub.php

@@ -1847,6 +1847,9 @@ public function getDeclaration(iterable $allConstInfos): string
         }
 
         $value = EvaluatedValue::createFromExpression($this->value, $type, $this->cname, $allConstInfos);
+        if (!$value->isUnknownConstValue && !$value->originatingConst) {
+            throw new Exception("Constant " . $this->name->__toString() . " must have either UNKNOWN or a constant value");
+        }
 
         $code = "";