Consider uninitialized typed properties as possibly undefined

`isset()` & friends can be used to check if a typed property has been
initialized. Phan does not consider the property as possibly undefined,
so we need to do that explicitly to avoid emitting an issue.

Bug: T378286
Change-Id: I6f9463550fd3eba63f25e8b4273bf4fe08fbab0e

src/Plugin/RedundantExistenceChecksVisitor.php

@@ -88,6 +88,15 @@ private function exprIsPossiblyUndefined( Node $expr ): bool {
 				// stdClass or another class with dynamic properties. These are always possibly undefined.
 				return true;
 			}
+
+			if ( !$property->getUnionType()->getRealUnionType()->isEmpty() ) {
+				// T378286: If it's a typed property without default value, do not emit an issue. `isset()` can be used
+				// to check if the property has been initialized. See also https://github.com/phan/phan/issues/4720
+				$defaultType = $property->getDefaultType();
+				if ( $defaultType && $defaultType->getRealUnionType()->isNull() ) {
+					return true;
+				}
+			}
 		}
 
 		$prevIgnoreUndefFlag = $expr->flags & PhanAnnotationAdder::FLAG_IGNORE_UNDEF;

tests/plugins/RedundantExistenceChecksPlugin/isset-typedprop/expectedResults.txt

@@ -0,0 +1,2 @@
+plugins/RedundantExistenceChecksPlugin/isset-typedprop/test.php:13 MediaWikiNoIssetIfDefined Found usage of isset() on expression $this->strWithDefault that appears to be always set. isset() should only be used to suppress errors. Check whether the expression is null instead. See https://w.wiki/98zs
+plugins/RedundantExistenceChecksPlugin/isset-typedprop/test.php:14 MediaWikiNoIssetIfDefined Found usage of isset() on expression $this->untypedWithDoc that appears to be always set. isset() should only be used to suppress errors. Check whether the expression is null instead. See https://w.wiki/98zs

tests/plugins/RedundantExistenceChecksPlugin/isset-typedprop/test.php

@@ -0,0 +1,19 @@
+<?php
+
+class TestTypedProperty {
+	private stdClass $obj1;
+	private string $str;
+	private string $strWithDefault = 'foo';
+	/** @var string */
+	private $untypedWithDoc;
+
+	function doTest() {
+		print_r( isset( $this->obj1 ) ); // No issue (to avoid false positives)
+		print_r( isset( $this->str ) ); // No issue (to avoid false positives)
+		print_r( isset( $this->strWithDefault ) ); // Redundant isset
+		print_r( isset( $this->untypedWithDoc ) ); // Redundant isset
+
+		$this->str = 'XcQ';
+		print_r( isset( $this->str ) ); // No issue (due to upstream limitation)
+	}
+}