parser: allow map cast syntax map[k]v(expr) (#23401)

vlib/v/parser/parser.v

@@ -2719,10 +2719,23 @@ fn (mut p Parser) name_expr() ast.Expr {
 		if is_option {
 			map_type = map_type.set_flag(.option)
 		}
-		return ast.MapInit{
+		node = ast.MapInit{
 			typ: map_type
 			pos: pos
 		}
+		if p.tok.kind == .lpar {
+			// ?map[int]int(none) cast expr
+			p.check(.lpar)
+			expr := p.expr(0)
+			p.check(.rpar)
+			return ast.CastExpr{
+				typ:     map_type
+				typname: p.table.sym(map_type).name
+				expr:    expr
+				pos:     pos.extend(p.tok.pos())
+			}
+		}
+		return node
 	}
 	// `chan typ{...}`
 	if p.tok.lit == 'chan' {

vlib/v/tests/map_cast_test.v

@@ -0,0 +1,25 @@
+fn foo() map[int]int {
+	return {
+		1: 2
+	}
+}
+
+fn test_main() {
+	a := ?map[int]int(none)
+	assert a == none
+
+	b := ?map[int]int({
+		1: 2
+	})
+	assert b?[1] == 2
+
+	c := ?map[int]map[string]string({
+		1: {
+			'foo': 'bar'
+		}
+	})
+	assert c?[1]['foo'] == 'bar'
+
+	d := ?map[int]int(foo())
+	assert d?[1] == 2
+}