checker: check error for `json.decode` (fix #6732) (#14305)

vlib/v/checker/fn.v

@@ -496,8 +496,7 @@ pub fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool)
 			}
 		}
 		panic('unreachable')
-	}
-	if fn_name == 'json.encode' {
+	} else if fn_name == 'json.encode' {
 	} else if fn_name == 'json.decode' && node.args.len > 0 {
 		if node.args.len != 2 {
 			c.error("json.decode expects 2 arguments, a type and a string (e.g `json.decode(T, '')`)",
@@ -506,12 +505,20 @@ pub fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool)
 		}
 		expr := node.args[0].expr
 		if expr is ast.TypeNode {
-			sym := c.table.sym(expr.typ)
-			if !c.table.known_type(sym.name) {
+			sym := c.table.sym(c.unwrap_generic(expr.typ))
+			if c.table.known_type(sym.name) && sym.kind != .placeholder {
+				mut kind := sym.kind
+				if sym.info is ast.Alias {
+					kind = c.table.sym(sym.info.parent_type).kind
+				}
+				if kind !in [.struct_, .sum_type, .map, .array] {
+					c.error('json.decode: expected sum type, struct, map or array, found $kind',
+						expr.pos)
+				}
+			} else {
 				c.error('json.decode: unknown type `$sym.name`', node.pos)
 			}
 		} else {
-			// if expr !is ast.TypeNode {
 			typ := expr.type_name()
 			c.error('json.decode: first argument needs to be a type, got `$typ`', node.pos)
 			return ast.void_type

vlib/v/checker/tests/json_decode.out

@@ -0,0 +1,34 @@
+vlib/v/checker/tests/json_decode.vv:11:7: error: json.decode: unknown type `St2`
+    9 | fn main() {
+   10 |     json.decode(St, '{a: ""}') ? // OK
+   11 |     json.decode(St2, '{a: ""}') ? // BAD
+      |          ~~~~~~~~~~~~~~~~~~~~~~
+   12 |     json.decode(St) ? // BAD
+   13 |     json.decode(string, '""') ? // BAD
+vlib/v/checker/tests/json_decode.vv:12:7: error: json.decode expects 2 arguments, a type and a string (e.g `json.decode(T, '')`)
+   10 |     json.decode(St, '{a: ""}') ? // OK
+   11 |     json.decode(St2, '{a: ""}') ? // BAD
+   12 |     json.decode(St) ? // BAD
+      |          ~~~~~~~~~~
+   13 |     json.decode(string, '""') ? // BAD
+   14 |     json.decode(Num, '5') ? // BAD
+vlib/v/checker/tests/json_decode.vv:13:14: error: json.decode: expected sum type, struct, map or array, found string
+   11 |     json.decode(St2, '{a: ""}') ? // BAD
+   12 |     json.decode(St) ? // BAD
+   13 |     json.decode(string, '""') ? // BAD
+      |                 ~~~~~~
+   14 |     json.decode(Num, '5') ? // BAD
+   15 |     json.decode(St, 6) ? // BAD
+vlib/v/checker/tests/json_decode.vv:14:14: error: json.decode: expected sum type, struct, map or array, found u8
+   12 |     json.decode(St) ? // BAD
+   13 |     json.decode(string, '""') ? // BAD
+   14 |     json.decode(Num, '5') ? // BAD
+      |                 ~~~
+   15 |     json.decode(St, 6) ? // BAD
+   16 | }
+vlib/v/checker/tests/json_decode.vv:15:7: error: json.decode: second argument needs to be a string
+   13 |     json.decode(string, '""') ? // BAD
+   14 |     json.decode(Num, '5') ? // BAD
+   15 |     json.decode(St, 6) ? // BAD
+      |          ~~~~~~~~~~~~~
+   16 | }

vlib/v/checker/tests/json_decode.vv

@@ -0,0 +1,16 @@
+import json
+
+struct St {
+	a string
+}
+
+type Num = u8
+
+fn main() {
+	json.decode(St, '{a: ""}') ? // OK
+	json.decode(St2, '{a: ""}') ? // BAD
+	json.decode(St) ? // BAD
+	json.decode(string, '""') ? // BAD
+	json.decode(Num, '5') ? // BAD
+	json.decode(St, 6) ? // BAD
+}