diff --git a/src/Psalm/Internal/Analyzer/Statements/Expression/AssertionFinder.php b/src/Psalm/Internal/Analyzer/Statements/Expression/AssertionFinder.php
index 22323df3a..d4a46964a 100644
--- a/src/Psalm/Internal/Analyzer/Statements/Expression/AssertionFinder.php
+++ b/src/Psalm/Internal/Analyzer/Statements/Expression/AssertionFinder.php
@@ -1666,6 +1666,10 @@ class AssertionFinder
             if ($first_var_name) {
                 $if_types[$first_var_name] = [[$prefix . 'iterable']];
             }
+        } elseif (self::hasCountableCheck($expr)) {
+            if ($first_var_name) {
+                $if_types[$first_var_name] = [[$prefix . 'countable']];
+            }
         } elseif ($class_exists_check_type = self::hasClassExistsCheck($expr)) {
             if ($first_var_name) {
                 if ($class_exists_check_type === 2) {
@@ -2298,6 +2302,20 @@ class AssertionFinder
         return false;
     }
 
+    /**
+     * @param   PhpParser\Node\Expr\FuncCall    $stmt
+     *
+     * @return  bool
+     */
+    protected static function hasCountableCheck(PhpParser\Node\Expr\FuncCall $stmt)
+    {
+        if ($stmt->name instanceof PhpParser\Node\Name && strtolower($stmt->name->parts[0]) === 'is_countable') {
+            return true;
+        }
+
+        return false;
+    }
+
     /**
      * @param   PhpParser\Node\Expr\FuncCall    $stmt
      *
diff --git a/src/Psalm/Internal/Analyzer/TypeAnalyzer.php b/src/Psalm/Internal/Analyzer/TypeAnalyzer.php
index f572fac46..631c73d33 100644
--- a/src/Psalm/Internal/Analyzer/TypeAnalyzer.php
+++ b/src/Psalm/Internal/Analyzer/TypeAnalyzer.php
@@ -990,7 +990,7 @@ class TypeAnalyzer
                 return false;
             }
 
-            if ($input_type_part->isTraversable($codebase)) {
+            if ($input_type_part->hasTraversableInterface($codebase)) {
                 return true;
             }
         }
diff --git a/src/Psalm/Type/Atomic.php b/src/Psalm/Type/Atomic.php
index 48a760fb7..bdc22291f 100644
--- a/src/Psalm/Type/Atomic.php
+++ b/src/Psalm/Type/Atomic.php
@@ -249,7 +249,7 @@ abstract class Atomic
     public function isIterable(Codebase $codebase)
     {
         return $this instanceof TIterable
-            || $this->isTraversable($codebase)
+            || $this->hasTraversableInterface($codebase)
             || $this instanceof TArray
             || $this instanceof ObjectLike;
     }
@@ -257,7 +257,17 @@ abstract class Atomic
     /**
      * @return bool
      */
-    public function isTraversable(Codebase $codebase)
+    public function isCountable(Codebase $codebase)
+    {
+        return $this->hasCountableInterface($codebase)
+            || $this instanceof TArray
+            || $this instanceof ObjectLike;
+    }
+
+    /**
+     * @return bool
+     */
+    public function hasTraversableInterface(Codebase $codebase)
     {
         return $this instanceof TNamedObject
             && (strtolower($this->value) === 'traversable'
@@ -269,6 +279,40 @@ abstract class Atomic
                         $this->value,
                         'Traversable'
                     )))
+                || ($this->extra_types
+                    && array_filter(
+                        $this->extra_types,
+                        function (Atomic $a) use ($codebase) : bool {
+                            return $a->hasTraversableInterface($codebase);
+                        }
+                    )
+                )
+            );
+    }
+
+    /**
+     * @return bool
+     */
+    public function hasCountableInterface(Codebase $codebase)
+    {
+        return $this instanceof TNamedObject
+            && (strtolower($this->value) === 'countable'
+                || ($codebase->classOrInterfaceExists($this->value)
+                    && ($codebase->classExtendsOrImplements(
+                        $this->value,
+                        'Countable'
+                    ) || $codebase->interfaceExtends(
+                        $this->value,
+                        'Countable'
+                    )))
+                || ($this->extra_types
+                    && array_filter(
+                        $this->extra_types,
+                        function (Atomic $a) use ($codebase) : bool {
+                            return $a->hasCountableInterface($codebase);
+                        }
+                    )
+                )
             );
     }
 
diff --git a/src/Psalm/Type/Reconciler.php b/src/Psalm/Type/Reconciler.php
index 0b1aeb379..e1933e4c0 100644
--- a/src/Psalm/Type/Reconciler.php
+++ b/src/Psalm/Type/Reconciler.php
@@ -728,7 +728,11 @@ class Reconciler
             return Type::getMixed();
         }
 
-        if ($new_var_type === 'iterable' && !$existing_var_type->hasMixed()) {
+        if ($new_var_type === 'iterable') {
+            if ($existing_var_type->hasMixed()) {
+                return new Type\Union([new Type\Atomic\TIterable]);
+            }
+
             $iterable_types = [];
             $did_remove_type = false;
 
@@ -736,7 +740,57 @@ class Reconciler
                 if ($type->isIterable($codebase)) {
                     $iterable_types[] = $type;
                 } elseif ($type instanceof TObject) {
-                    $iterable_types[] = new Type\Atomic\TCallableObject();
+                    $iterable_types[] = new Type\Atomic\TNamedObject('Traversable');
+                    $did_remove_type = true;
+                } else {
+                    $did_remove_type = true;
+                }
+            }
+
+            if ((!$iterable_types || !$did_remove_type) && !$is_equality) {
+                if ($key && $code_location) {
+                    self::triggerIssueForImpossible(
+                        $existing_var_type,
+                        $old_var_type_string,
+                        $key,
+                        $new_var_type,
+                        !$did_remove_type,
+                        $code_location,
+                        $suppressed_issues
+                    );
+                }
+            }
+
+            if ($iterable_types) {
+                return new Type\Union($iterable_types);
+            }
+
+            $failed_reconciliation = 2;
+
+            return Type::getMixed();
+        }
+
+        if ($new_var_type === 'countable') {
+            if ($existing_var_type->hasMixed()) {
+                return new Type\Union([
+                    new Type\Atomic\TArray([Type::getArrayKey(), Type::getMixed()]),
+                    new Type\Atomic\TNamedObject('Countable')
+                ]);
+            }
+
+            $iterable_types = [];
+            $did_remove_type = false;
+
+            foreach ($existing_var_atomic_types as $type) {
+                if ($type->isCountable($codebase)) {
+                    $iterable_types[] = $type;
+                } elseif ($type instanceof TObject) {
+                    $iterable_types[] = new TNamedObject('Countable');
+                    $did_remove_type = true;
+                } elseif ($type instanceof TNamedObject || $type instanceof Type\Atomic\TIterable) {
+                    $countable = new TNamedObject('Countable');
+                    $type->extra_types[$countable->getKey()] = $countable;
+                    $iterable_types[] = $type;
                     $did_remove_type = true;
                 } else {
                     $did_remove_type = true;
@@ -1850,7 +1904,7 @@ class Reconciler
             return $existing_var_type;
         }
 
-        if ($new_var_type === 'iterable') {
+        if ($new_var_type === 'iterable' || $new_var_type === 'countable') {
             $existing_var_type->removeType('array');
         }
 
diff --git a/tests/FunctionCallTest.php b/tests/FunctionCallTest.php
index 58280178a..8ed1397bb 100644
--- a/tests/FunctionCallTest.php
+++ b/tests/FunctionCallTest.php
@@ -1713,6 +1713,18 @@ class FunctionCallTest extends TestCase
                     /** @psalm-suppress TooFewArguments */
                     min(0);',
             ],
+            'PHP73-allowIsCountableToInformType' => [
+                '<?php
+                    function getObject() : iterable{
+                       return [];
+                    }
+
+                    $iterableObject = getObject();
+
+                    if (is_countable($iterableObject)) {
+                       if (count($iterableObject) === 0) {}
+                    }',
+            ],
         ];
     }