explode() returns a non-empty array in certain scenarios

src/Type/Php/ExplodeFunctionDynamicReturnTypeExtension.php

@@ -6,13 +6,16 @@
 use PHPStan\Analyser\Scope;
 use PHPStan\Reflection\FunctionReflection;
 use PHPStan\Reflection\ParametersAcceptorSelector;
+use PHPStan\Type\Accessory\NonEmptyArrayType;
 use PHPStan\Type\ArrayType;
 use PHPStan\Type\Constant\ConstantBooleanType;
 use PHPStan\Type\Constant\ConstantStringType;
+use PHPStan\Type\IntegerRangeType;
 use PHPStan\Type\IntegerType;
 use PHPStan\Type\MixedType;
 use PHPStan\Type\StringType;
 use PHPStan\Type\Type;
+use PHPStan\Type\TypeCombinator;
 use PHPStan\Type\TypeUtils;
 
 class ExplodeFunctionDynamicReturnTypeExtension implements \PHPStan\Type\DynamicFunctionReturnTypeExtension
@@ -38,7 +41,15 @@ public function getTypeFromFunctionCall(
 		if ($isSuperset->yes()) {
 			return new ConstantBooleanType(false);
 		} elseif ($isSuperset->no()) {
-			return new ArrayType(new IntegerType(), new StringType());
+			$arrayType = new ArrayType(new IntegerType(), new StringType());
+			if (
+				!isset($functionCall->args[2])
+				|| IntegerRangeType::fromInterval(0, null)->isSuperTypeOf($scope->getType($functionCall->args[2]->value))->yes()
+			) {
+				return TypeCombinator::intersect($arrayType, new NonEmptyArrayType());
+			}
+
+			return $arrayType;
 		}
 
 		$returnType = ParametersAcceptorSelector::selectSingle($functionReflection->getVariants())->getReturnType();

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -10185,6 +10185,11 @@ public function dataClassConstantOnExpression(): array
 		return $this->gatherAssertTypes(__DIR__ . '/data/class-constant-on-expr.php');
 	}
 
+	public function dataBug3961(): array
+	{
+		return $this->gatherAssertTypes(__DIR__ . '/data/bug-3961.php');
+	}
+
 	/**
 	 * @dataProvider dataBug2574
 	 * @dataProvider dataBug2577
@@ -10265,6 +10270,7 @@ public function dataClassConstantOnExpression(): array
 	 * @dataProvider dataThrowExpression
 	 * @dataProvider dataNotEmptyArray
 	 * @dataProvider dataClassConstantOnExpression
+	 * @dataProvider dataBug3961
 	 * @param string $assertType
 	 * @param string $file
 	 * @param mixed ...$args

tests/PHPStan/Analyser/data/bug-3961.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace Bug3961;
+
+use function PHPStan\Analyser\assertType;
+
+class Foo
+{
+
+	public function doFoo(string $v, string $d, $m): void
+	{
+		assertType('array<int, string>&nonEmpty', explode('.', $v));
+		assertType('false', explode('', $v));
+		assertType('array<int, string>', explode('.', $v, -2));
+		assertType('array<int, string>&nonEmpty', explode('.', $v, 0));
+		assertType('array<int, string>&nonEmpty', explode('.', $v, 1));
+		assertType('array<int, string>|false', explode($d, $v));
+		assertType('(array<int, string>|false)', explode($m, $v));
+	}
+
+}