fix: keep async flag when instantiating generic function aliases

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`

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 = {},
         }

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
+]]

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