checker: fix for _, mut j in array (#8785)

vlib/v/ast/ast.v

@@ -1537,12 +1537,20 @@ pub struct Table {
 }
 
 pub fn (expr Expr) is_mut_ident() bool {
-	if expr is Ident {
-		if expr.obj is Var {
-			if expr.obj.is_auto_deref {
+	match expr {
+		Ident {
+			if expr.obj is Var {
+				if expr.obj.is_auto_deref {
+					return true
+				}
+			}
+		}
+		PrefixExpr {
+			if expr.op == .amp && expr.right.is_mut_ident() {
 				return true
 			}
 		}
+		else {}
 	}
 	return false
 }

vlib/v/checker/checker.v

@@ -1406,7 +1406,11 @@ pub fn (mut c Checker) call_method(mut call_expr ast.CallExpr) table.Type {
 			// need to return `array_xxx` instead of `array`
 			// in ['clone', 'str'] {
 			call_expr.receiver_type = left_type.to_ptr()
-			call_expr.return_type = call_expr.receiver_type.set_nr_muls(0)
+			if call_expr.left.is_mut_ident() {
+				call_expr.return_type = left_type.deref()
+			} else {
+				call_expr.return_type = call_expr.receiver_type.set_nr_muls(0)
+			}
 		} else if method_name == 'sort' {
 			call_expr.return_type = table.void_type
 		} else if method_name == 'contains' {
@@ -1422,7 +1426,11 @@ pub fn (mut c Checker) call_method(mut call_expr ast.CallExpr) table.Type {
 				if method_name[0] == `m` {
 					c.fail_if_immutable(call_expr.left)
 				}
-				ret_type = left_type
+				if call_expr.left.is_mut_ident() {
+					ret_type = left_type.deref()
+				} else {
+					ret_type = left_type
+				}
 			}
 			'keys' {
 				info := left_type_sym.info as table.Map
@@ -2849,7 +2857,8 @@ pub fn (mut c Checker) assign_stmt(mut assign_stmt ast.AssignStmt) {
 				}
 			}
 		}
-		if !is_blank_ident && right_sym.kind != .placeholder && left_sym.kind != .interface_ {
+		if !is_blank_ident && !right.is_mut_ident() && right_sym.kind != .placeholder
+			&& left_sym.kind != .interface_ {
 			// Dual sides check (compatibility check)
 			c.check_expected(right_type_unwrapped, left_type_unwrapped) or {
 				c.error('cannot assign to `$left`: $err', right.position())

vlib/v/tests/mut_test.v

@@ -254,3 +254,97 @@ fn test_mut_12() {
     mut arr := [[0, 0]]
 	foo3(mut arr)
 }
+
+struct Foo {
+mut:
+	foo int
+}
+
+fn foo4(mut f Foo) {
+	f2 := &f
+	f.foo = 100
+	println(f.foo)
+	println(f2.foo)
+	assert f.foo == 100
+	assert f2.foo == 100
+}
+
+fn test_mut_13() {
+	mut f := Foo{foo: 1}
+	foo4(mut f)
+}
+
+fn foo5(mut arr []int) {
+	arr2 := &arr
+	arr[0] = 0
+	println(arr[0]) // 0
+	assert arr[0] == 0
+	unsafe {
+		println(arr2[0]) // 0
+		assert arr2[0] == 0
+	}
+}
+
+fn test_mut_14() {
+	mut arr := [1,2,3]
+	foo5(mut arr)
+}
+
+fn foo6(mut arr [3]int) {
+	arr2 := &arr
+	arr[0] = 0
+	println(arr[0]) // 0
+	assert arr[0] == 0
+	unsafe {
+		println(arr2[0]) // 0
+		assert arr2[0] == 0
+	}
+}
+
+fn test_mut_15() {
+	mut arr := [1,2,3]!
+	foo6(mut arr)
+}
+
+fn foo7(mut m map[string]int) {
+	m2 := &m
+	m['one'] = 1
+	println(m['one']) // 1
+	assert m['one'] == 1
+	unsafe {
+		println(m2['one']) // 1
+		assert m2['one'] == 1
+	}
+}
+
+fn test_mut_16() {
+	mut m := map{'one': 100, 'two': 2}
+	foo7(mut m)
+}
+
+fn test_mut_17() {
+	mut arr := [map{'foo':1}]
+	for _, mut j in arr {
+		mut k := j.clone()
+		j['foo'] = 0
+		unsafe {k['foo'] = 10}
+		println(j)
+		println(k)
+		assert j == {'foo': 0}
+		assert k == {'foo': 10}
+	}
+}
+
+fn foo8(mut a [1]int) {
+	a2 := a
+	a[0] = 100
+	println(a)
+	println(a2)
+	assert '$a' == '[100]'
+	assert '$a2' == '[1]'
+}
+
+fn test_mut_18() {
+	mut a := [1]!
+	foo8(mut a)
+}