Handle union and mixed in plus operation

Author: VincentLanglet

src/Reflection/InitializerExprTypeResolver.php

@@ -1095,9 +1095,17 @@ public function getPlusType(Expr $left, Expr $right, callable $getTypeCallback):
 				new BooleanType(),
 			]);
 
-			if ($plusable->isSuperTypeOf($leftType)->yes() && $plusable->isSuperTypeOf($rightType)->yes()) {
+			$plusableSuperTypeOfLeft = $plusable->isSuperTypeOf($leftType)->yes();
+			$plusableSuperTypeOfRight = $plusable->isSuperTypeOf($rightType)->yes();
+			if ($plusableSuperTypeOfLeft && $plusableSuperTypeOfRight) {
 				return TypeCombinator::union($leftType, $rightType);
 			}
+			if ($plusableSuperTypeOfLeft && $rightType instanceof MixedType) {
+				return $leftType;
+			}
+			if ($plusableSuperTypeOfRight && $leftType instanceof MixedType) {
+				return $rightType;
+			}
 		}
 
 		return $this->resolveCommonMath(new BinaryOp\Plus($left, $right), $leftType, $rightType);

tests/PHPStan/Rules/Methods/ReturnTypeRuleTest.php

@@ -1039,4 +1039,9 @@ public function testBug11491(): void
 		$this->analyse([__DIR__ . '/data/bug-11491.php'], []);
 	}
 
+	public function testBug3759(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-3759.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Methods/data/bug-3759.php

@@ -0,0 +1,31 @@
+<?php declare(strict_types = 1);
+
+namespace Bug3759;
+
+class A
+{
+	/**
+	 * @return mixed[]
+	 */
+	function modules(): array
+	{
+		$modules = moduleList();
+		if (!$modules) {
+			return [];
+		}
+
+		return $modules['major'] + $modules['minor'] + $modules['patch'];
+	}
+
+	/**
+	 * @return mixed[]
+	 */
+	function moduleList(): array
+	{
+		return [
+			'major' => ['x' => 'x'],
+			'minor' => ['y' => 'y'],
+			'patch' => ['z' => 'z'],
+		];
+	}
+}