infer parameter type by return

resolve #1201

Author: sumneko

changelog.md

@@ -28,7 +28,7 @@ server will generate `doc.json` and `doc.md` in `LOGPATH`.
   ---@type myClass
   local class
 
-  print(class.a.b.c.e.f.g) --> infered as integer
+  print(class.a.b.c.e.f.g) --> inferred as integer
   ```
 * `CHG` [#1582] the following diagnostics consider `overload`
   * `missing-return`
@@ -58,6 +58,14 @@ server will generate `doc.json` and `doc.md` in `LOGPATH`.
 
   local arr = x(cb) --> `arr` is inferred as `integer[]`
   ```
+* `CHG` [#1201] infer parameter type by expected returned function of parent function
+  ```lua
+  ---@return fun(x: integer)
+  local function f()
+      return function (x) --> `x` is inferred as `integer`
+      end
+  end
+  ```
 * `FIX` [#1567]
 * `FIX` [#1593]
 * `FIX` [#1595]
@@ -70,6 +78,7 @@ server will generate `doc.json` and `doc.md` in `LOGPATH`.
 
 [#1153]: https://github.com/sumneko/lua-language-server/issues/1153
 [#1177]: https://github.com/sumneko/lua-language-server/issues/1177
+[#1202]: https://github.com/sumneko/lua-language-server/issues/1202
 [#1458]: https://github.com/sumneko/lua-language-server/issues/1458
 [#1557]: https://github.com/sumneko/lua-language-server/issues/1557
 [#1558]: https://github.com/sumneko/lua-language-server/issues/1558

script/vm/compiler.lua

@@ -1059,6 +1059,7 @@ local function compileLocal(source)
     end
     if source.parent.type == 'funcargs' and not hasMarkDoc and not hasMarkParam then
         local func = source.parent.parent
+        -- local call ---@type fun(f: fun(x: number));call(function (x) end) --> x -> number
         local funcNode = vm.compileNode(func)
         local hasDocArg
         for n in funcNode:eachObject() do
@@ -1158,6 +1159,22 @@ local compilerSwitch = util.switch()
             local call = source.parent.parent
             vm.compileCallArg(source, call)
         end
+
+        -- function f() return function (<?x?>) end end
+        if source.parent.type == 'return' then
+            for i, ret in ipairs(source.parent) do
+                if ret == source then
+                    local func = guide.getParentFunction(source.parent)
+                    if func then
+                        local returnObj = vm.getReturnOfFunction(func, i)
+                        if returnObj then
+                            vm.setNode(source, vm.compileNode(returnObj))
+                        end
+                    end
+                    break
+                end
+            end
+        end
     end)
     : case 'paren'
     : call(function (source)

test/type_inference/init.lua

@@ -3858,3 +3858,11 @@ local cb
 
 local <?arr?> = x(cb)
 ]]
+
+TEST 'integer' [[
+---@return fun(x: integer)
+local function f()
+    return function (<?x?>)
+    end
+end
+]]