From d470903722cfcbc1cd04744c5491d3e6d13ec3d9 Mon Sep 17 00:00:00 2001
From: Brown <github@muglug.com>
Date: Mon, 13 Apr 2020 08:47:11 -0400
Subject: [PATCH] Make Algebra::negateFormula a little more safe

---
 src/Psalm/Type/Algebra.php | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/src/Psalm/Type/Algebra.php b/src/Psalm/Type/Algebra.php
index 5e1bdb89309..12c85fe6ae9 100644
--- a/src/Psalm/Type/Algebra.php
+++ b/src/Psalm/Type/Algebra.php
@@ -546,7 +546,7 @@ function ($possible_type) {
     }
 
     /**
-     * @param  array<int, Clause>  $clauses
+     * @param  non-empty-array<int, Clause>  $clauses
      *
      * @return array<int, Clause>
      */
@@ -759,11 +759,21 @@ public static function combineOredClauses(array $left_clauses, array $right_clau
      */
     public static function negateFormula(array $clauses)
     {
+        if (!$clauses) {
+            return [new Clause([], true)];
+        }
+
         foreach ($clauses as $clause) {
             self::calculateNegation($clause);
         }
 
-        $negated = self::simplifyCNF(self::groupImpossibilities($clauses));
+        $impossible_clauses = self::groupImpossibilities($clauses);
+
+        if (!$impossible_clauses) {
+            return [new Clause([], true)];
+        }
+
+        $negated = self::simplifyCNF($impossible_clauses);
 
         if (!$negated) {
             return [new Clause([], true)];