From 97607d5e7cf3811ade81390e169d221013f68d29 Mon Sep 17 00:00:00 2001
From: ybai08 <271969568+ybai08@users.noreply.github.com>
Date: Sun, 18 Oct 2026 03:38:59 -0500
Subject: [PATCH] fix: keep async flag when instantiating generic function
 aliases

cloneObject rebuilt doc.type.function without copying `async`/`asyncPos`,
so `---@alias Name<T> async fun(): T` lost its async flag once used as
`Name<string>`, and await-in-sync/hover treated it as a sync function.

Fixes #3469
---
 changelog.md                       |  1 +
 script/vm/generic.lua              |  2 ++
 test/diagnostics/await-in-sync.lua | 50 ++++++++++++++++++++++++++++++
 test/hover/init.lua                | 10 ++++++
 4 files changed, 63 insertions(+)

diff --git a/changelog.md b/changelog.md
index daefdb5ae..af1f653e4 100644
--- a/changelog.md
+++ b/changelog.md
@@ -2,6 +2,7 @@
 
 ## Unreleased
 <!-- Add all new changes here. They will be moved under a version at release -->
+* `FIX` Generic aliases of async functions (e.g. `---@alias Name<T> async fun(): T` used as `Name<string>`) keep the `async` flag, so `await-in-sync` and hover treat them as async [#3469](https://github.com/LuaLS/lua-language-server/issues/3469)
 
 ## 3.19.2
 `2026-10-13`
diff --git a/script/vm/generic.lua b/script/vm/generic.lua
index 0b171622c..db46a5eeb 100644
--- a/script/vm/generic.lua
+++ b/script/vm/generic.lua
@@ -113,6 +113,8 @@ local function cloneObject(source, resolved)
             start   = source.start,
             finish  = source.finish,
             parent  = source.parent,
+            async   = source.async,
+            asyncPos = source.asyncPos,
             args    = {},
             returns = {},
         }
diff --git a/test/diagnostics/await-in-sync.lua b/test/diagnostics/await-in-sync.lua
index 323c11137..1c8da1d8d 100644
--- a/test/diagnostics/await-in-sync.lua
+++ b/test/diagnostics/await-in-sync.lua
@@ -130,3 +130,53 @@ end
 
 return af
 ]]
+
+TEST [[
+---@alias AsyncGenericFunc<T> async fun(): T
+
+---@type AsyncGenericFunc<string>
+local f
+
+function F()
+    <!f!>()
+end
+]]
+
+TEST [[
+---@alias AsyncGenericFunc<T> async fun(x: T)
+
+---@type AsyncGenericFunc<integer>
+local f
+
+---@async
+function F()
+    f(1)
+end
+]]
+
+TEST [[
+---@alias AsyncGenericFunc<T> async fun(): T
+
+---@class AsyncGenericHolder
+---@field run AsyncGenericFunc<string>
+
+---@type AsyncGenericHolder
+local holder
+
+function F()
+    <!holder.run!>()
+end
+]]
+
+TEST [[
+---@alias AsyncGenericFunc<T> async fun(): T
+
+---@type AsyncGenericFunc<string>
+local function f()
+    return 'x'
+end
+
+function F()
+    <!f!>()
+end
+]]
diff --git a/test/hover/init.lua b/test/hover/init.lua
index dbe245ad4..415657e2c 100644
--- a/test/hover/init.lua
+++ b/test/hover/init.lua
@@ -1812,6 +1812,16 @@ local <?f?>
 local f: async fun()
 ]]
 
+TEST [[
+---@alias AsyncGenericFunc<T> async fun(): T
+
+---@type AsyncGenericFunc<string>
+local <?f?>
+]]
+[[
+local f: async AsyncGenericFunc<string>
+]]
+
 config.set(nil, 'Lua.runtime.nonstandardSymbol', { '//' })
 TEST [[
 local <?x?> = 1 // 2