cgen: fix casting sumtype as fntype (#15612)

vlib/v/gen/c/cgen.v

@@ -2103,9 +2103,12 @@ fn (mut g Gen) write_sumtype_casting_fn(fun SumtypeCastingFn) {
 	got, exp := fun.got, fun.exp
 	got_sym, exp_sym := g.table.sym(got), g.table.sym(exp)
 	mut got_cname, exp_cname := got_sym.cname, exp_sym.cname
+	mut type_idx := g.type_sidx(got)
 	if got_sym.info is ast.FnType {
 		if got_sym.info.is_anon {
+			got_name := 'fn ${g.table.fn_type_source_signature(got_sym.info.func)}'
 			got_cname = 'anon_fn_${g.table.fn_type_signature(got_sym.info.func)}'
+			type_idx = g.table.type_idxs[got_name].str()
 		}
 	}
 	mut sb := strings.new_builder(128)
@@ -2128,7 +2131,7 @@ fn (mut g Gen) write_sumtype_casting_fn(fun SumtypeCastingFn) {
 		// if the variable is not used, the C compiler will optimize it away
 		sb.writeln('\t$embed_cname* ${embed_name}_ptr = memdup($accessor, sizeof($embed_cname));')
 	}
-	sb.write_string('\treturn ($exp_cname){ ._$got_cname = ptr, ._typ = ${g.type_sidx(got)}')
+	sb.write_string('\treturn ($exp_cname){ ._$got_cname = ptr, ._typ = $type_idx')
 	for field in (exp_sym.info as ast.SumType).fields {
 		mut ptr := 'ptr'
 		mut type_cname := got_cname

vlib/v/tests/inout/sumtype_with_fntype.out

@@ -1,2 +1,6 @@
-Fn
-Fn
+Fn(fn () int)
+Fn(fn (int) int)
+123
+123
+321
+321

vlib/v/tests/inout/sumtype_with_fntype.vv

@@ -1,13 +1,32 @@
 type Fn = fn () int | fn (int) int
 
-fn main() {
-	f1 := Fn(fn (a int) int {
-		return a
-	})
-	println(typeof(f1).name)
-
-	f2 := Fn(fn () int {
-		return 22
-	})
-	println(typeof(f2).name)
+fn abc(n int) Fn {
+	if n == 1 {
+		return fn () int {
+			return 123
+		}
+	} else {
+		return fn (x int) int {
+			return x
+		}
+	}
+}
+
+fn main() {
+	println(abc(1))
+	println(abc(2))
+
+	x1 := abc(1)
+	y1 := x1 as fn () int
+	println(y1())
+	if x1 is fn () int {
+		println(x1())
+	}
+
+	x2 := abc(2)
+	y2 := x2 as fn (int) int
+	println(y2(321))
+	if x2 is fn (int) int {
+		println(x2(321))
+	}
 }