False positives from -Wincomplete-record-updates

Summary

GHC will issue -Wincomplete-record-updates warnings even when the record update is guaranteed to succeed.

Steps to reproduce

Compile this code:

{-# OPTIONS -Wincomplete-record-updates #-}

data FooBar = Foo { w :: Int, x :: Char, y :: Float, z :: String } | Bar

f :: FooBar -> FooBar
f foo@Foo{} = foo{ w = 42 }
f Bar = Bar

Expected behavior

Since I specified foo@Foo{}, GHC should realize that foo must be a Foo and so not warn me that it could be a Bar.

Environment

• GHC version used: 8.10.2

Optional:

• Operating System: Linux
• System Architecture: x86_64