checker: fix recursive define check is missing when defining sumtype. (fix #15684) (#15718)

vlib/v/checker/checker.v

@@ -519,6 +519,16 @@ pub fn (mut c Checker) sum_type_decl(node ast.SumTypeDecl) {
 					}
 				}
 			}
+		} else if sym.info is ast.FnType {
+			func := (sym.info as ast.FnType).func
+			if c.table.sym(func.return_type).name.ends_with('.$node.name') {
+				c.error('sum type `$node.name` cannot be defined recursively', variant.pos)
+			}
+			for param in func.params {
+				if c.table.sym(param.typ).name.ends_with('.$node.name') {
+					c.error('sum type `$node.name` cannot be defined recursively', variant.pos)
+				}
+			}
 		}
 
 		if sym.name.trim_string_left(sym.mod + '.') == node.name {

vlib/v/checker/tests/sumtype_define_recursively.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/sumtype_define_recursively.vv:1:13: error: sum type `Expr` cannot be defined recursively
+    1 | type Expr = fn ([]Expr) Expr | int
+      |             ~~~~~~~~~~~~~~~~
+    2 | 
+    3 | fn f(exprs []Expr) Expr {

vlib/v/checker/tests/sumtype_define_recursively.vv

@@ -0,0 +1,10 @@
+type Expr = fn ([]Expr) Expr | int
+
+fn f(exprs []Expr) Expr {
+	return 0
+}
+
+fn main() {
+	mut m := map[string]Expr{}
+	m['f'] = f
+}