panic on unboxed string literal in pattern (#13260) · Issues · Glasgow Haskell Compiler / GHC

panic on unboxed string literal in pattern

I don't think this program is meaningful, but it shouldn't cause a panic.

{-# LANGUAGE MagicHash #-}
g y = case y of
  "a"# -> True
  _ -> False

[1 of 1] Compiling CaseOnString     ( x/CaseOnString.hs, x/CaseOnString.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.0.1 for x86_64-unknown-linux):
	litValue "a"#

The panic is also in at least 7.8, 7.10, and HEAD.

"a"# has type Addr#, and I suppose that if we allow case on Addr# then it ought to use pointer equality. But the literal "a"# has no particular address, so matching against it in a case doesn't make sense. So I think we should just disallow unboxed string literals as patterns.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

I don't think this program is meaningful, but it shouldn't cause a panic.

```hs
{-# LANGUAGE MagicHash #-}
g y = case y of
  "a"# -> True
  _ -> False
```

```
[1 of 1] Compiling CaseOnString     ( x/CaseOnString.hs, x/CaseOnString.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.0.1 for x86_64-unknown-linux):
	litValue "a"#
```

The panic is also in at least 7.8, 7.10, and HEAD.

`"a"#` has type `Addr#`, and I suppose that if we allow `case` on `Addr#` then it ought to use pointer equality. But the literal `"a"#` has no particular address, so matching against it in a `case` doesn't make sense. So I think we should just disallow unboxed string literals as patterns.

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

| Trac field             | Value        |
| ---------------------- | ------------ |
| Version                | 8.1          |
| Type                   | Bug          |
| TypeOfFailure          | OtherFailure |
| Priority               | normal       |
| Resolution             | Unresolved   |
| Component              | Compiler     |
| Test case              |              |
| Differential revisions |              |
| BlockedBy              |              |
| Related                |              |
| Blocking               |              |
| CC                     |              |
| Operating system       |              |
| Architecture           |              |

</details>