fix(compiler): infer concat expressions as strings

phpunit/src/SsaAnalysisTest.php

@@ -1263,6 +1263,15 @@ class SsaAnalysisTest extends TestCase
         }
     }
 
+    public function testDetectTypeOfConcatExpressions(): void
+    {
+        $concat = new Expr\BinaryOp\Concat(new Scalar\LNumber(1), new Scalar\String_(''));
+        $concatAssign = new Expr\AssignOp\Concat(new Expr\Variable('value'), new Scalar\LNumber(2));
+
+        $this->assertSame(Type::STR, $this->invoke('detectTypeOfExpr', $concat));
+        $this->assertSame(Type::STR, $this->invoke('detectTypeOfExpr', $concatAssign));
+    }
+
     public function testDetectSsaDefTypeNull(): void
     {
         $ssaVar = new SsaVar(1, 'x');

src/CompilerBase.php

@@ -2299,6 +2299,9 @@ class CompilerBase implements PropertyAccessContext
             case 'Expr_Array':
             case 'Expr_Cast_Array':
                 return Type::ARRAY;
+            case 'Expr_BinaryOp_Concat':
+            case 'Expr_AssignOp_Concat':
+                return Type::STR;
             case 'Expr_BinaryOp_Plus':
             case 'Expr_BinaryOp_Minus':
             case 'Expr_BinaryOp_Mul':