checker: add a checker error for fn calls on left side of an assignment (#5993)

vlib/v/checker/checker.v

@@ -1619,6 +1619,10 @@ pub fn (mut c Checker) assign_stmt(mut assign_stmt ast.AssignStmt) {
 	//
 	is_decl := assign_stmt.op == .decl_assign
 	for i, left in assign_stmt.left {
+		if left is ast.CallExpr {
+			c.error('cannot call function `${left.name}()` on the left side of an assignment',
+				left.pos)
+		}
 		is_blank_ident := left.is_blank_ident()
 		mut left_type := table.void_type
 		if !is_decl && !is_blank_ident {

vlib/v/checker/tests/assign_fn_call_on_left_side_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/assign_fn_call_on_left_side_err.v:6:2: error: cannot call function `foo()` on the left side of an assignment
+    4 | 
+    5 | fn main() {
+    6 |     foo('s') = 1
+      |     ~~~~~~~~
+    7 | }

vlib/v/checker/tests/assign_fn_call_on_left_side_err.vv

@@ -0,0 +1,7 @@
+fn foo(s string) int {
+	return 1
+}
+
+fn main() {
+	foo('s') = 1
+}