Support the $foo ?? false pattern when making sure variable exists

src/Analyser/TypeSpecifier.php

@@ -731,6 +731,16 @@ public function specifyTypesInCondition(
 			}
 
 			return $types;
+		} elseif (
+			$expr instanceof Expr\BinaryOp\Coalesce
+			&& $context->true()
+			&& ((new ConstantBooleanType(false))->isSuperTypeOf($scope->getType($expr->right))->yes())
+		) {
+			return $this->create(
+				$expr->left,
+				new NullType(),
+				TypeSpecifierContext::createFalse()
+			);
 		} elseif (
 			$expr instanceof Expr\Empty_ && $context->truthy()
 			&& (new ArrayType(new MixedType(), new MixedType()))->isSuperTypeOf($scope->getType($expr->expr))->yes()

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -5755,6 +5755,11 @@ public function dataTernarySpecifiedTypes(): array
 		return $this->gatherAssertTypes(__DIR__ . '/data/ternary-specified-types.php');
 	}
 
+	public function dataBug560(): array
+	{
+		return $this->gatherAssertTypes(__DIR__ . '/data/bug-560.php');
+	}
+
 	/**
 	 * @dataProvider dataArrayFunctions
 	 * @param string $description
@@ -11389,6 +11394,7 @@ private function gatherAssertTypes(string $file): array
 	 * @dataProvider dataBug2977
 	 * @dataProvider dataBug3190
 	 * @dataProvider dataTernarySpecifiedTypes
+	 * @dataProvider dataBug560
 	 * @param string $assertType
 	 * @param string $file
 	 * @param mixed ...$args

tests/PHPStan/Analyser/data/bug-560.php

@@ -0,0 +1,22 @@
+<?php
+
+namespace Bug560;
+
+use PHPStan\TrinaryLogic;
+use function PHPStan\Analyser\assertType;
+use function PHPStan\Analyser\assertVariableCertainty;
+
+assertVariableCertainty(TrinaryLogic::createMaybe(), $city);
+assertType('mixed', $city);
+
+if ($city ?? false) {
+	assertVariableCertainty(TrinaryLogic::createYes(), $city);
+	assertType('mixed~null', $city);
+}
+
+function (?string $s): void {
+	if ($s ?? false) {
+		assertVariableCertainty(TrinaryLogic::createYes(), $s);
+		assertType('string' ,$s);
+	}
+};