[MLton-commit] [MLton/mlton] cb6504: Update `TypeEnv.Type.canUnify` to check equality s...

['Matthew F. v. MLton-c. <mlt...@ml...>]

  Branch: refs/heads/master
  Home:   https://github.com/MLton/mlton
  Commit: cb650477f133978903e2b75fc2ae0afcac848adc
      https://github.com/MLton/mlton/commit/cb650477f133978903e2b75fc2ae0afcac848adc
  Author: Matthew Fluet <mat...@gm...>
  Date:   2026-09-17 (Thu, 17 Sep 2026)

  Changed paths:
    M mlton/elaborate/type-env.fun

  Log Message:
  -----------
  Update `TypeEnv.Type.canUnify` to check equality status of `Unknown`s

Fixes MLton/mlton#651


  Commit: c08b15196b9acbb832875235dd15b2035ea74bd4
      https://github.com/MLton/mlton/commit/c08b15196b9acbb832875235dd15b2035ea74bd4
  Author: Matthew Fluet <mat...@gm...>
  Date:   2026-09-17 (Thu, 17 Sep 2026)

  Changed paths:
    M mlton/elaborate/elaborate-core.fun

  Log Message:
  -----------
  Eliminate unnecessary `? option` in handling of overload resolution


  Commit: 2c9006bf80486693d1e55121438965f0152cac07
      https://github.com/MLton/mlton/commit/2c9006bf80486693d1e55121438965f0152cac07
  Author: Matthew Fluet <mat...@gm...>
  Date:   2026-09-17 (Thu, 17 Sep 2026)

  Changed paths:
    M CHANGELOG.adoc

  Log Message:
  -----------
  Update CHANGELOG.adoc


  Commit: fa4e23cd3907bc0ad81704d26a5faad82875c83e
      https://github.com/MLton/mlton/commit/fa4e23cd3907bc0ad81704d26a5faad82875c83e
  Author: Matthew Fluet <Mat...@gm...>
  Date:   2026-09-17 (Thu, 17 Sep 2026)

  Changed paths:
    M CHANGELOG.adoc
    M mlton/elaborate/elaborate-core.fun
    M mlton/elaborate/type-env.fun

  Log Message:
  -----------
  Merge pull request #652 from MatthewFluet/overload-ice

Fix handling of overload resolution with respect to equality types

Closes #651.

Previously, a use of / (overloaded only for real types) that was
constrained to equality types would lead to an internal compiler
error:

```
$ cat test.sml
fun f (x,y) = (x/y, x=y)
$ mlton test.sml
MLton 20241230 raised: Fail: ElaborateCore.elabExp: Var:overload unify
```

As noted by @YawarRaza7349 in #651, this is due to a mismatch in
`TypeEnv.Type.canUnify` and `TypeEnv.Type.unify`. The former did not
check the required equality status of `Unknown` types. The suggested
fix does not quite work, because in the case of checking possible
overloads, the candidates types are ground types, so
`TypeEnv.Type.canUnify` is checking a type with equality-type unknowns
(e.g., `''?a * ''?a -> ''?a`) against a ground type (e.g.,
`real32 * real32 -> real32`). Therefore, checking the equality status
is required if either of the arguments to `TypeEnv.Type.canUnify` are
`Unknown`.

Now, a proper type-checking error is reported:

```
$ mlton test.sml
Error: test.sml 1.18-1.18.
  Variable not overloaded at type: /.
    type: ??? * ??? -> ???
Warning: test.sml 1.5-1.5.
  Type of variable was not inferred and could not be generalized: f.
    type: ??? * ??? -> ??? * bool
    in: fun f (x, y) = (x / y, x = y)
```


Compare: https://github.com/MLton/mlton/compare/a65f71fdb875...fa4e23cd3907

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