checker: disallow iterating through .params if not a function

vlib/v/checker/comptime.v

@@ -326,11 +326,17 @@ fn (mut c Checker) comptime_for(mut node ast.ComptimeFor) {
 			c.pop_comptime_info()
 		}
 	} else if node.kind == .params {
-		c.push_new_comptime_info()
-		c.comptime.inside_comptime_for = true
-		c.comptime.comptime_for_method_param_var = node.val_var
-		c.stmts(mut node.stmts)
-		c.pop_comptime_info()
+		if sym.kind == .function {
+			c.push_new_comptime_info()
+			c.comptime.inside_comptime_for = true
+			c.comptime.comptime_for_method_param_var = node.val_var
+			c.stmts(mut node.stmts)
+			c.pop_comptime_info()
+		} else {
+			c.error('iterating over `.params` is supported only for functions, and `${sym.name}` is not a function',
+				node.typ_pos)
+			return
+		}
 	} else if node.kind == .variants {
 		if c.variant_data_type == 0 {
 			c.variant_data_type = ast.idx_to_type(c.table.find_type_idx('VariantData'))

vlib/v/checker/tests/comptime_param_not_fn_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/comptime_param_not_fn_err.vv:5:12: error: iterating over `.params` is supported only for functions, and `Test` is not a function
+    3 | 
+    4 | fn main() {
+    5 |     $for f in Test.params {
+      |               ~~~~
+    6 |         println(f)
+    7 |     }

vlib/v/checker/tests/comptime_param_not_fn_err.vv

@@ -0,0 +1,8 @@
+struct Test {
+}
+
+fn main() {
+	$for f in Test.params {
+		println(f)
+	}
+}