checker: protect against assigning to a typeless variable (#11094)

vlib/v/checker/checker.v

@@ -3834,6 +3834,7 @@ pub fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 			left_type = c.expr(left)
 			c.expected_type = c.unwrap_generic(left_type)
 			// `map = {}`
+			if left_type != 0 {
 				sym := c.table.get_type_symbol(left_type)
 				if sym.kind == .map && node.right[i] is ast.StructInit {
 					c.warn('assigning a struct literal to a map is deprecated - use `map{}` instead',
@@ -3841,6 +3842,7 @@ pub fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 					node.right[i] = ast.MapInit{}
 				}
 			}
+		}
 		if node.right_types.len < node.left.len { // first type or multi return types added above
 			old_inside_ref_lit := c.inside_ref_lit
 			if left is ast.Ident {
@@ -4038,14 +4040,16 @@ pub fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 			// right type was a generic `T`
 			continue
 		}
-		left_sym := c.table.get_type_symbol(left_type_unwrapped)
-		right_sym := c.table.get_type_symbol(right_type_unwrapped)
-
 		if c.pref.translated {
 			// TODO fix this in C2V instead, for example cast enums to int before using `|` on them.
 			// TODO replace all c.pref.translated checks with `$if !translated` for performance
 			continue
 		}
+		if left_type_unwrapped == 0 {
+			continue
+		}
+		left_sym := c.table.get_type_symbol(left_type_unwrapped)
+		right_sym := c.table.get_type_symbol(right_type_unwrapped)
 		if left_sym.kind == .array && !c.inside_unsafe && node.op in [.assign, .decl_assign]
 			&& right_sym.kind == .array && (left is ast.Ident && !left.is_blank_ident())
 			&& right is ast.Ident {

vlib/v/checker/tests/assign_to_typeless_variable_err.out

@@ -0,0 +1,12 @@
+vlib/v/checker/tests/assign_to_typeless_variable_err.vv:2:10: error: invalid empty map initilization syntax, use e.g. map[string]int{} instead
+    1 | fn main() {
+    2 |   val := {}
+      |          ~~
+    3 |   val = 1
+    4 | }
+vlib/v/checker/tests/assign_to_typeless_variable_err.vv:3:3: error: `val` is immutable, declare it with `mut` to make it mutable
+    1 | fn main() {
+    2 |   val := {}
+    3 |   val = 1
+      |   ~~~
+    4 | }

vlib/v/checker/tests/assign_to_typeless_variable_err.vv

@@ -0,0 +1,4 @@
+fn main() {
+  val := {}
+  val = 1
+}