From 2d9133b35a404a017b4b5d4052a3904dad528d8d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Maximilian=20B=C3=B6sing?=
 <2189546+boesing@users.noreply.github.com>
Date: Sun, 12 Jun 2022 00:32:36 +0200
Subject: [PATCH] bugfix: overriding types based on assertions have to pass
 more checks
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Signed-off-by: Maximilian Bösing <2189546+boesing@users.noreply.github.com>
---
 .../Statements/Expression/CallAnalyzer.php    | 24 ++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/src/Psalm/Internal/Analyzer/Statements/Expression/CallAnalyzer.php b/src/Psalm/Internal/Analyzer/Statements/Expression/CallAnalyzer.php
index c5c922e10..1111dab5b 100644
--- a/src/Psalm/Internal/Analyzer/Statements/Expression/CallAnalyzer.php
+++ b/src/Psalm/Internal/Analyzer/Statements/Expression/CallAnalyzer.php
@@ -763,7 +763,9 @@ class CallAnalyzer
                             $codebase
                         );
 
-                        if ($union->isSingle() || $union->allLiterals()) {
+                        $arg_var_type = $context->vars_in_scope[$assertion_var_id];
+
+                        if (self::isNewTypeNarrowingDownOldType($arg_var_type, $union)) {
                             foreach ($union->getAtomicTypes() as $atomic_type) {
                                 if ($assertion_type instanceof TTemplateParam
                                     && $assertion_type->as->getId() === $atomic_type->getId()
@@ -1117,4 +1119,24 @@ class CallAnalyzer
             }
         }
     }
+
+    /**
+     * This method should detect if the new type narrows down the old type.
+     */
+    private static function isNewTypeNarrowingDownOldType(Union $old_type, Union $new_type): bool
+    {
+        if ($new_type->isSingle()) {
+            return true;
+        }
+
+        if ($old_type->hasMixed() && !$new_type->hasMixed()) {
+            return true;
+        }
+
+        if ($old_type->isSingleAndMaybeNullable() && !$new_type->isNullable()) {
+            return true;
+        }
+
+        return false;
+    }
 }