checker: disallow (x) := 10 (#20695)

vlib/v/checker/assign.v

@@ -267,6 +267,11 @@ fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 			}
 		}
 		node.left_types << left_type
+
+		if left is ast.ParExpr && is_decl {
+			c.error('parentheses are not supported on the left side of `:=`', left.pos())
+		}
+
 		for left is ast.ParExpr {
 			left = (left as ast.ParExpr).expr
 		}

vlib/v/checker/tests/par_expr_decl_assign_err.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/par_expr_decl_assign_err.vv:2:2: error: parentheses are not supported on the left side of `:=`
+    1 | fn main() {
+    2 |     (x) := 5
+      |     ~~~
+    3 | }

vlib/v/checker/tests/par_expr_decl_assign_err.vv

@@ -0,0 +1,3 @@
+fn main() {
+	(x) := 5
+}

vlib/v/checker/tests/prefix_expr_decl_assign_err.out

@@ -10,6 +10,12 @@ vlib/v/checker/tests/prefix_expr_decl_assign_err.vv:2:5: error: non-name on the
       |     ^
     3 |     (*d) := 14
     4 | }
+vlib/v/checker/tests/prefix_expr_decl_assign_err.vv:3:5: error: parentheses are not supported on the left side of `:=`
+    1 | fn main() {
+    2 |     &a := 12
+    3 |     (*d) := 14
+      |     ~~~~
+    4 | }
 vlib/v/checker/tests/prefix_expr_decl_assign_err.vv:3:10: error: modifying variables via dereferencing can only be done in `unsafe` blocks
     1 | fn main() {
     2 |     &a := 12