checker: error if assigning to a function (#6581)

vlib/v/checker/checker.v

@@ -1944,6 +1944,8 @@ pub fn (mut c Checker) assign_stmt(mut assign_stmt ast.AssignStmt) {
 					if assign_stmt.op !in [.assign, .decl_assign] {
 						c.error('cannot modify blank `_` identifier', left.pos)
 					}
+				} else if left.info !is ast.IdentVar {
+					c.error('cannot assign to $left.kind `$left.name`', left.pos)
 				} else {
 					if is_decl {
 						c.check_valid_snake_case(left.name, 'variable name', left.pos)

vlib/v/checker/tests/assign_multi_immutable_err.out

@@ -5,3 +5,17 @@ vlib/v/checker/tests/assign_multi_immutable_err.vv:4:2: error: `a` is immutable,
       |     ^
     5 |
     6 |     println('$a, $b')
+vlib/v/checker/tests/assign_multi_immutable_err.vv:18:5: error: cannot assign to function `error`
+   16 |
+   17 | fn assign_fn() {
+   18 |     _, error = g()
+      |        ~~~~~
+   19 |     g = f()
+   20 | }
+vlib/v/checker/tests/assign_multi_immutable_err.vv:19:2: error: cannot assign to function `g`
+   17 | fn assign_fn() {
+   18 |     _, error = g()
+   19 |     g = f()
+      |     ^
+   20 | }
+   21 |

vlib/v/checker/tests/assign_multi_immutable_err.vv

@@ -5,3 +5,17 @@ fn main() {
 
 	println('$a, $b')
 }
+
+fn f() int {
+	return 2
+}
+
+fn g() (int, int) {
+  return 1, 2
+}
+
+fn assign_fn() {
+	_, error = g()
+	g = f()
+}
+