Fix null coalescing on nullable output

src/Psalm/Checker/Statements/ExpressionChecker.php

@@ -1026,7 +1026,20 @@ class ExpressionChecker
             $lhs_type = null;
 
             if (isset($stmt->left->inferredType)) {
-                $lhs_type = $stmt->left->inferredType;
+                $if_return_type_reconciled = TypeChecker::reconcileTypes(
+                    '!null',
+                    $stmt->left->inferredType,
+                    '',
+                    $statements_checker,
+                    new CodeLocation($statements_checker->getSource(), $stmt),
+                    $statements_checker->getSuppressedIssues()
+                );
+
+                if ($if_return_type_reconciled === false) {
+                    return false;
+                }
+
+                $lhs_type = $if_return_type_reconciled;
             }
 
             if (!$lhs_type || !isset($stmt->right->inferredType)) {

tests/Php70Test.php

@@ -58,6 +58,17 @@ class Php70Test extends TestCase
                     '$a' => 'string|null',
                 ],
             ],
+            'nullCoalesceWithNullableOnLeft' => [
+                '<?php
+                    /** @return ?string */
+                    function foo() {
+                        return rand(0, 10) > 5 ? "hello" : null;
+                    }
+                    $a = foo() ?? "goodbye";',
+                'assertions' => [
+                    '$a' => 'string',
+                ],
+            ],
             'nullCoalesceWithReference' => [
                 '<?php
                     $var = 0;