v.checker: disallow assigning none to _ (vlang#20646)

vlib/v/checker/assign.v

@@ -283,6 +283,10 @@ fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 							right.right.pos)
 					}
 				} else if left.kind == .blank_ident {
+					if !is_decl && mut right is ast.None {
+						c.error('cannot assign a `none` value to blank `_` identifier',
+							right.pos)
+					}
 					left_type = right_type
 					node.left_types[i] = right_type
 					if node.op !in [.assign, .decl_assign] {

vlib/v/checker/tests/assign_blank_none_err.out

@@ -0,0 +1,12 @@
+vlib/v/checker/tests/assign_blank_none_err.vv:2:7: error: cannot assign a `none` value to a variable
+    1 | fn main() {
+    2 |     _ := none
+      |          ~~~~
+    3 |     _ = none
+    4 | }
+vlib/v/checker/tests/assign_blank_none_err.vv:3:6: error: cannot assign a `none` value to blank `_` identifier
+    1 | fn main() {
+    2 |     _ := none
+    3 |     _ = none
+      |         ~~~~
+    4 | }

vlib/v/checker/tests/assign_blank_none_err.vv

@@ -0,0 +1,4 @@
+fn main() {
+	_ := none
+	_ = none
+}