checker,cgen: allow alias to map type Dict = map[string]string (fix #24878) (#24883)

vlib/v/checker/checker.v

@@ -643,6 +643,7 @@ fn (mut c Checker) alias_type_decl(mut node ast.AliasTypeDecl) {
 			info := parent_typ_sym.info as ast.Map
 			c.check_alias_vs_element_type_of_parent(node, info.key_type, 'map key')
 			c.check_alias_vs_element_type_of_parent(node, info.value_type, 'map value')
+			c.markused_used_maps(c.table.used_features.used_maps == 0)
 		}
 		.sum_type {
 			// TODO: decide whether the following should be allowed. Note that it currently works,

vlib/v/checker/struct.v

@@ -523,16 +523,6 @@ fn (mut c Checker) struct_init(mut node ast.StructInit, is_field_zero_struct_ini
 			c.error('union `${struct_sym.name}` can have only one field initialised',
 				node.pos)
 		}
-	} else if struct_sym.info is ast.Alias {
-		parent_sym := c.table.sym(struct_sym.info.parent_type)
-		// e.g. ´x := MyMapAlias{}´, should be a cast to alias type ´x := MyMapAlias(map[...]...)´
-		if parent_sym.kind == .map {
-			alias_str := c.table.type_to_str(node.typ)
-			map_str := c.table.type_to_str(struct_sym.info.parent_type)
-			c.error('direct map alias init is not possible, use `${alias_str}(${map_str}{})` instead',
-				node.pos)
-			return ast.void_type
-		}
 	} else if struct_sym.info is ast.FnType {
 		c.error('functions must be defined, not instantiated like structs', node.pos)
 	}
@@ -649,8 +639,8 @@ fn (mut c Checker) struct_init(mut node ast.StructInit, is_field_zero_struct_ini
 					.struct {
 						info = sym.info as ast.Struct
 					}
-					.array, .array_fixed {
-						// we do allow []int{}, [10]int{}
+					.array, .array_fixed, .map {
+						// we do allow []int{}, [10]int{}, map[string]int{}
 					}
 					else {
 						c.error('alias type name: ${sym.name} is not struct type', node.pos)

vlib/v/checker/tests/direct_map_alias_init_err.out

@@ -1,6 +0,0 @@
-vlib/v/checker/tests/direct_map_alias_init_err.vv:4:7: error: direct map alias init is not possible, use `WordSet(map[string]bool{})` instead
-    2 |
-    3 | fn main() {
-    4 |     _ := WordSet{}
-      |          ~~~~~~~~~
-    5 | }

vlib/v/checker/tests/direct_map_alias_init_err.vv

@@ -1,5 +0,0 @@
-type WordSet = map[string]bool
-
-fn main() {
-	_ := WordSet{}
-}

vlib/v/gen/c/struct.v

@@ -48,6 +48,9 @@ fn (mut g Gen) struct_init(node ast.StructInit) {
 			g.write(g.type_default_sumtype(unwrapped_typ, sym))
 		}
 		return
+	} else if sym.kind == .map {
+		g.write(g.type_default(unwrapped_typ))
+		return
 	}
 	is_amp := g.is_amp
 	is_multiline := node.init_fields.len > 5

vlib/v/tests/aliases/alias_map_test.v

@@ -0,0 +1,9 @@
+module main
+
+type Dict = map[string]string
+
+fn test_main() {
+	mut x := Dict{}
+	x['foo'] = 'bar'
+	assert '${x}' == "Dict({'foo': 'bar'})"
+}