cgen: fix fixed array ret with anon fn (#18279)

vlib/v/gen/c/cgen.v

@@ -1562,8 +1562,10 @@ pub fn (mut g Gen) write_fn_typesymbol_declaration(sym ast.TypeSymbol) {
 		} else {
 			''
 		}
-
-		g.type_definitions.write_string('typedef ${g.typ(func.return_type)} (${msvc_call_conv}*${fn_name})(')
+		ret_typ :=
+			if !func.return_type.has_flag(.option) && g.table.sym(func.return_type).kind == .array_fixed { '_v_' } else { '' } +
+			g.typ(func.return_type)
+		g.type_definitions.write_string('typedef ${ret_typ} (${msvc_call_conv}*${fn_name})(')
 		for i, param in func.params {
 			g.type_definitions.write_string(g.typ(param.typ))
 			if i < func.params.len - 1 {
@@ -2441,7 +2443,9 @@ fn (mut g Gen) expr_with_cast(expr ast.Expr, got_type_raw ast.Type, expected_typ
 	}
 	// no cast
 	g.expr(expr)
-	if expr is ast.CallExpr && exp_sym.kind == .array_fixed {
+	if expr is ast.CallExpr && !(expr as ast.CallExpr).is_fn_var && !expected_type.has_flag(.option)
+		&& exp_sym.kind == .array_fixed {
+		// it's non-option fixed array, requires accessing .ret_arr member to get the array
 		g.write('.ret_arr')
 	}
 }

vlib/v/parser/fn.v

@@ -755,7 +755,9 @@ fn (mut p Parser) anon_fn() ast.AnonFn {
 	if same_line {
 		if (p.tok.kind.is_start_of_type() && (same_line || p.tok.kind != .lsbr))
 			|| (same_line && p.tok.kind == .key_fn) {
+			p.inside_fn_return = true
 			return_type = p.parse_type()
+			p.inside_fn_return = false
 			return_type_pos = return_type_pos.extend(p.tok.pos())
 		} else if p.tok.kind != .lcbr {
 			p.error_with_pos('expected return type, not ${p.tok} for anonymous function',

vlib/v/tests/anon_fn_fixed_arr_test.v

@@ -0,0 +1,38 @@
+struct Struct {
+	f fn () ?[2]int
+	g fn () [2]int
+}
+
+fn test_struct_member() {
+	s := Struct{
+		f: fn () ?[2]int {
+			return [1, 2]!
+		}
+		g: fn () [2]int {
+			return [1, 2]!
+		}
+	}
+
+	mut a := s.f()
+	println(s.f())
+	dump(a)
+	mut b := s.g()
+	println(s.g())
+	dump(b)
+}
+
+fn test_fn_var() {
+	mut h := fn () [2]int {
+		return [1, 2]!
+	}
+	mut i := fn () ?[2]int {
+		return [1, 2]!
+	}
+
+	mut c := h()
+	println(h())
+	dump(c)
+	mut d := i()
+	println(i())
+	dump(d)
+}