checker: fix structinit validation on nested generic Map[K]V

vlib/v/checker/fn.v

@@ -1509,7 +1509,8 @@ fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool) ast.
 		if call_arg.expr is ast.MapInit && e_sym.kind == .struct {
 			c.error('cannot initialize a struct with a map', call_arg.pos)
 			continue
-		} else if call_arg.expr is ast.StructInit && e_sym.kind == .map {
+		} else if call_arg.expr is ast.StructInit && e_sym.kind == .map
+			&& !call_arg.expr.typ.has_flag(.generic) {
 			c.error('cannot initialize a map with a struct', call_arg.pos)
 			continue
 		}

vlib/x/json2/tests/decode_map_of_map_test.v

@@ -0,0 +1,13 @@
+import x.json2
+
+struct CrossVerifyResult {
+	confusion_matrix_map map[string]map[string]f64
+}
+
+fn test_main() {
+	x := json2.decode[CrossVerifyResult]('') or {
+		assert err.msg().contains('invalid token')
+		return
+	}
+	assert false
+}