In a perfect world, I feel like passingself: self in the overload should trigger the same behaviour as a method definition, in that it allows injection (opposed to passingt: TestClass would treat it as a normal parameter and prevent injection)...

While I don't have any good ideas atm (since you said refactoring is not a solution due to tons of legacy code 😇)
I would like to reply to some of your points:

4. Unexpectedly (?) fields injected in other methods don't have any complaints.

You may already knew it, that you can only "inject" fields to variables that have@class attached to.
That's how you "fix" the situation of point3.
But for (4) this is actually as designed I believe

• theTestClass local variable has marked with@class TestClass in line 4
• then when declaring methods usingfunction TestClass:Bar(), the hidden variableself will also be marked with@class TestClass under the hood
• soself.field_meth is auto "injected" intoTestClass
• this is different from (1), because the server has no way to auto markself as@class TestClass
  and you also already knew that a variable marked with just@type behaves differently from a variable marked with@class

I suddenly have an idea that, maybe you can use theplugin feature to achieve your need 🤔
https://luals.github.io/wiki/plugins/

• a plugin is like a custom special script read by the server for your project
• one of its ability isOnSetText which cantransform the text buffer of a file when viewed by the server
• so in the script you can use some string pattern matching and replacing, to transform the code in yourideal case become the "awkward" form which is needed by the server to make it work 🤔 🤔

Here's my try

• plugin.lua

functionOnSetText(uri,text)-- print(uri, text)-- fix custom constructorlocaldiffs= {}forstart,overload,className,finishintext:gmatch'()(---@overload[^\n]+\n)local (%w+) = Class%(func[^\n]+\n()'do-- print(start, "|", overload, "|", className, "|", finish)diffs[#diffs+1]= {start=start,finish=start-1,text=overload:gsub("fun%(","fun(self: self,"),        }diffs[#diffs+1]= {start=finish,finish=finish-1,text= ("---@class %s\n---@diagnostic disable-next-line\nlocal self = {}\n"):format(className),        }endreturn#diffs>0anddiffsornilend

• what this plugin is doing
  • capture classname in (1) and the whole overload line (2b)
  • insert the captured overload line before (2b) but withself: self injected in it
  • finally add the---@class %s\n---@diagnostic disable-next-line\nlocal self = {}\n after location (1)

PS: duno why need to uselocal self = self instead of justself = self, otherwise the field inject won't take effect during workspace init 😕
and the---@diagnostic disable-next-line is used to silent theRedefined local `self` warning

demo

(diffed.lua is a debug file showing the transformed buffer, generated by the server when starting it with--develop, more details on official wiki page)

however...

I experienced some issues with this "plugin" system...

• the auto complete triggered when you typetest_inst. doesn't containfield_constr ...
  only if you pressesc and retrigger it, this field will show up
• and if the class definition is placed in one file, sayTestClass.lua
  and you create an instance of this class inanother file
  => thefield_constr is not recognized until you open thisTestClass.lua once ... 😇
• I have no idea if this is a bug related to the plugin system
  or if this is the designed behavior ofOnSetText (which only process files that are opened ?!)

edited:

‼️ I seems found a workaround forfield_constr is not recognized until open thisTestClass.lua once‼️

• somehow the trick of---@class TestClass\nself = self\n will not work on workspace init
  (somethings off in the type infer system I believe 😕 )
  because even without using plugin, the strange behavior persists
• I then found out that I can use---@class TestClass\nlocal self = self\n 😂
  This seems to work during workspace init (without openingTestClass.lua once)
• I have updated my demo script above 👀

edit2

I updated the string replace pattern again to"---@class %s\n---@diagnostic disable-next-line\nlocal self = {}\n"

• this will silent theRedefined local `self` warning
• also by usinglocal self = {}, thesetType inline hint won't show up for this virtuallocal self line