Fix #373

Author: xuhuanzy

crates/emmylua_code_analysis/src/compilation/test/flow.rs

@@ -590,6 +590,34 @@ end
         assert_eq!(b, b_expected);
     }
 
+    #[test]
+    fn test_issue_373() {
+        let mut ws = VirtualWorkspace::new();
+
+        ws.def(
+            r#"
+            --- @alias myalias string|string[]
+
+            --- @param x myalias
+            function foo(x)
+                if type(x) == 'string' then
+                    a = x
+                elseif type(x) == 'table' then
+                    b = x
+                end
+            end
+        "#,
+        );
+
+        let a = ws.expr_ty("a");
+        let a_expected = ws.ty("string");
+        assert_eq!(a, a_expected);
+
+        let b = ws.expr_ty("b");
+        let b_expected = ws.ty("string[]");
+        assert_eq!(b, b_expected);
+    }
+
     #[test]
     fn test_issue_405() {
         let mut ws = VirtualWorkspace::new();

crates/emmylua_code_analysis/src/db_index/type/type_ops/mod.rs

@@ -47,3 +47,16 @@ impl TypeOps {
         }
     }
 }
+
+pub fn get_real_type<'a>(db: &'a DbIndex, compact_type: &'a LuaType) -> Option<&'a LuaType> {
+    match compact_type {
+        LuaType::Ref(type_decl_id) => {
+            let type_decl = db.get_type_index().get_type_decl(type_decl_id)?;
+            if type_decl.is_alias() {
+                return get_real_type(db, type_decl.get_alias_ref()?);
+            }
+            Some(compact_type)
+        }
+        _ => Some(compact_type),
+    }
+}

crates/emmylua_code_analysis/src/db_index/type/type_ops/narrow_type.rs

@@ -1,5 +1,7 @@
 use crate::{DbIndex, LuaType, LuaUnionType};
 
+use super::get_real_type;
+
 // need to be optimized
 pub fn narrow_down_type(db: &DbIndex, source: LuaType, target: LuaType) -> Option<LuaType> {
     if source == target {
@@ -45,9 +47,15 @@ pub fn narrow_down_type(db: &DbIndex, source: LuaType, target: LuaType) -> Optio
             LuaType::Ref(type_decl_id) | LuaType::Def(type_decl_id) => {
                 let type_decl = db.get_type_index().get_type_decl(type_decl_id)?;
                 // enum 在实际使用时实际上是 enum.field, 并不等于 table
-                if !type_decl.is_enum() {
-                    return Some(source);
+                if type_decl.is_enum() {
+                    return None;
+                }
+                if type_decl.is_alias() {
+                    if let Some(alias_ref) = get_real_type(db, &source) {
+                        return narrow_down_type(db, alias_ref.clone(), target);
+                    }
                 }
+                return Some(source);
             }
             _ => {}
         },

crates/emmylua_code_analysis/src/db_index/type/type_ops/remove_type.rs

@@ -1,5 +1,7 @@
 use crate::{DbIndex, LuaType, LuaUnionType};
 
+use super::get_real_type;
+
 pub fn remove_type(db: &DbIndex, source: LuaType, removed_type: LuaType) -> Option<LuaType> {
     if source == removed_type {
         match source {
@@ -163,16 +165,3 @@ pub fn remove_type(db: &DbIndex, source: LuaType, removed_type: LuaType) -> Opti
 
     Some(source)
 }
-
-fn get_real_type<'a>(db: &'a DbIndex, compact_type: &'a LuaType) -> Option<&'a LuaType> {
-    match compact_type {
-        LuaType::Ref(type_decl_id) => {
-            let type_decl = db.get_type_index().get_type_decl(type_decl_id)?;
-            if type_decl.is_alias() {
-                return get_real_type(db, type_decl.get_alias_ref()?);
-            }
-            Some(compact_type)
-        }
-        _ => Some(compact_type),
-    }
-}