checker: check empty map invalid syntax (fix #9162) (#9186)

vlib/v/checker/checker.v

@@ -5501,6 +5501,9 @@ pub fn (mut c Checker) map_init(mut node ast.MapInit) table.Type {
 			node.key_type = info.key_type
 			node.value_type = info.value_type
 			return node.typ
+		} else {
+			c.error('invalid empty map initilization syntax, use e.g. map[string]int{} instead',
+				node.pos)
 		}
 	}
 	// `x := map[string]string` - set in parser
@@ -5512,6 +5515,7 @@ pub fn (mut c Checker) map_init(mut node ast.MapInit) table.Type {
 		node.value_type = info.value_type
 		return node.typ
 	}
+	if node.keys.len > 0 && node.vals.len > 0 {
 		// `{'age': 20}`
 		mut key0_type := c.table.mktyp(c.expr(node.keys[0]))
 		if node.keys[0].is_auto_deref_var() {
@@ -5551,6 +5555,8 @@ pub fn (mut c Checker) map_init(mut node ast.MapInit) table.Type {
 		node.value_type = val0_type
 		return map_type
 	}
+	return node.typ
+}
 
 // call this *before* calling error or warn
 pub fn (mut c Checker) add_error_detail(s string) {

vlib/v/checker/tests/map_init_invalid_syntax.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/map_init_invalid_syntax.vv:2:10: error: invalid empty map initilization syntax, use e.g. map[string]int{} instead
+    1 | fn main() {
+    2 |     a := map{}
+      |             ~~
+    3 |     println(a)
+    4 | }

vlib/v/checker/tests/map_init_invalid_syntax.vv

@@ -0,0 +1,4 @@
+fn main() {
+	a := map{}
+	println(a)
+}