Completeness of View Patterns With a Complete Set of Output Patterns (#15884) · Issues · Glasgow Haskell Compiler / GHC

Completeness of View Patterns With a Complete Set of Output Patterns

For example, the code

{-# OPTIONS_GHC -Wall #-}
{-# LANGUAGE ViewPatterns #-}
f :: Maybe a -> Bool
f (id->Nothing)  = False
f (id->(Just _)) = True

mistakenly returns the warning

warning: [-Wincomplete-patterns]
    Pattern match(es) are non-exhaustive
    In an equation for ‘f’: Patterns not matched: _
  |
4 | f (id->Nothing)  = False
  | ^^^^^^^^^^^^^^^^^^^^^^^^...

Trac metadata

Trac field	Value
Version	8.6.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	alanz, bgamari
Operating system
Architecture

For example, the code

```hs
{-# OPTIONS_GHC -Wall #-}
{-# LANGUAGE ViewPatterns #-}
f :: Maybe a -> Bool
f (id->Nothing)  = False
f (id->(Just _)) = True
```

mistakenly returns the warning

```
warning: [-Wincomplete-patterns]
    Pattern match(es) are non-exhaustive
    In an equation for ‘f’: Patterns not matched: _
  |
4 | f (id->Nothing)  = False
  | ^^^^^^^^^^^^^^^^^^^^^^^^...
```

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

| Trac field             | Value          |
| ---------------------- | -------------- |
| Version                | 8.6.2          |
| Type                   | Bug            |
| TypeOfFailure          | OtherFailure   |
| Priority               | normal         |
| Resolution             | Unresolved     |
| Component              | Compiler       |
| Test case              |                |
| Differential revisions |                |
| BlockedBy              |                |
| Related                |                |
| Blocking               |                |
| CC                     | alanz, bgamari |
| Operating system       |                |
| Architecture           |                |

</details>