Overeager deferred type errors (#11197) · Issues · Glasgow Haskell Compiler / GHC

Overeager deferred type errors

With TypeInType, the solver now works in terms of unlifted equality. The only place this matters is in deferred type errors, which now are more eager than they were before. For example:

{-# OPTIONS_GHC -fdefer-type-errors #-}

module Main where

main = do
  putStrLn "Hi there."
  putStrLn True

This used to print

Hi there.
Defer: Defer.hs:7:12: error:
   ...

Now it prints

Defer: Defer.hs:7:12: error:
   ...

No more Hi there..

Thinking about Core, this change should be expected. GHC puts all evidence bindings for main at the top, and with unlifted equality in the solver, these bindings now contain case typeError ... of ..., which forces the type error eagerly. Previously, there was a lazy binding for a lifted equality witness, unpacked only at the last moment by magic in the desugarer, thus the lazier warning.

Simon has proposed customizing the !FloatIn pass to deal with this scenario. I do believe this would work nicely, but I have not yet implemented it.

Trac metadata

Trac field	Value
Version	7.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

With `TypeInType`, the solver now works in terms of unlifted equality. The only place this matters is in deferred type errors, which now are more eager than they were before. For example:

```
{-# OPTIONS_GHC -fdefer-type-errors #-}

module Main where

main = do
  putStrLn "Hi there."
  putStrLn True
```

This used to print

```
Hi there.
Defer: Defer.hs:7:12: error:
   ...
```

Now it prints

```
Defer: Defer.hs:7:12: error:
   ...
```

No more `Hi there.`.

Thinking about Core, this change should be expected. GHC puts all evidence bindings for `main` at the top, and with unlifted equality in the solver, these bindings now contain `case typeError ... of ...`, which forces the type error eagerly. Previously, there was a lazy binding for a lifted equality witness, unpacked only at the last moment by magic in the desugarer, thus the lazier warning.

Simon has proposed customizing the !FloatIn pass to deal with this scenario. I do believe this would work nicely, but I have not yet implemented it.

<details><summary>Trac metadata</summary>

| Trac field             | Value        |
| ---------------------- | ------------ |
| Version                | 7.11         |
| Type                   | Bug          |
| TypeOfFailure          | OtherFailure |
| Priority               | normal       |
| Resolution             | Unresolved   |
| Component              | Compiler     |
| Test case              |              |
| Differential revisions |              |
| BlockedBy              |              |
| Related                |              |
| Blocking               |              |
| CC                     |              |
| Operating system       |              |
| Architecture           |              |

</details>