parser: trigger declaration of array element as error

vlib/v/checker/tests/array_declare_element_a.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/array_declare_element_a.v:2:9: error: non-name `arr[0]` on left side of `:=` 
+    1 | fn main() {
+    2 |     arr[0] := 2
+      |            ~~
+    3 | }

vlib/v/checker/tests/array_declare_element_a.vv

@@ -0,0 +1,3 @@
+fn main() {
+	arr[0] := 2
+}

vlib/v/checker/tests/array_declare_element_b.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/array_declare_element_b.v:3:9: error: non-name `arr[1]` on left side of `:=` 
+    1 | fn main() {
+    2 |     arr := [1, 2]
+    3 |     arr[1] := 1
+      |            ~~
+    4 | }

vlib/v/checker/tests/array_declare_element_b.vv

@@ -0,0 +1,4 @@
+fn main() {
+	arr := [1, 2]
+	arr[1] := 1
+}

vlib/v/checker/tests/array_declare_element_c.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/array_declare_element_c.v:3:12: error: non-name `arr[1][0]` on left side of `:=` 
+    1 | fn main() {
+    2 |     arr := [[1, 2], [0, 3]]
+    3 |     arr[1][0] := 1
+      |               ~~
+    4 | }

vlib/v/checker/tests/array_declare_element_c.vv

@@ -0,0 +1,4 @@
+fn main() {
+	arr := [[1, 2], [0, 3]]
+	arr[1][0] := 1
+}

vlib/v/parser/assign.v

@@ -111,6 +111,9 @@ fn (mut p Parser) partial_assign_stmt(left []ast.Expr) ast.Stmt {
 				}
 			}
 			ast.IndexExpr {
+				if op == .decl_assign {
+					p.error_with_pos('non-name `$lx.left[$lx.index]` on left side of `:=`', lx.pos)
+				}
 				lx.is_setter = true
 			}
 			ast.ParExpr {}