Initializing record with similarly named field from a different record results in warning rather than error

When a record is initialized using a similarly named field from another record, the compiler ignores the initialization and generates a warning instead of throwing an error. I believe this is a bug introduced in GHC 8.0.1 (and probably related to DuplicateRecordFields), since earlier versions of GHC would refuse to compile the code.

Consider the following example:

-- A.hs
module A where
  data A = A { a :: (), b :: () }

-- B.hs
module B where
  data B = B { a :: (), b :: () }

-- Main.hs
module Main where
  import A hiding (a)
  import B

  x = A { a = (), b = () }

  main = case x of
    A () () -> return ()

On GHC 8.0.1, this compiles (with a warning), and the program throws an exception when run:

$ stack ghc --compiler ghc-8.0.1 -- --make Main.hs
[1 of 3] Compiling B                ( B.hs, B.o )
[2 of 3] Compiling A                ( A.hs, A.o )
[3 of 3] Compiling Main             ( Main.hs, Main.o )

Main.hs:5:7: warning: [-Wmissing-fields]
    • Fields of ‘A’ not initialised: a
    • In the expression: A {a = (), b = ()}
      In an equation for ‘x’: x = A {a = (), b = ()}
Linking Main ...

$ ./Main 
Main: Main.hs:5:7-26: Missing field in record construction a

On GHC 7.10.3, the program will not compile:

$ stack ghc --compiler ghc-7.10.3 -- --make Main.hs
[1 of 3] Compiling B                ( B.hs, B.o )
[2 of 3] Compiling A                ( A.hs, A.o )
[3 of 3] Compiling Main             ( Main.hs, Main.o )

Main.hs:5:7:
    Constructor ‘A’ does not have field ‘a’
    In the expression: A {a = (), b = ()}
    In an equation for ‘x’: x = A {a = (), b = ()}

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import labels

Attached file A.hs ($1357).

Attached file B.hs ($1358).

Attached file Main.hs ($1359).

added OverloadedRecordFields label

closed

changed milestone to %8.2.3

Thanks for the report! This bug exists in 8.0.x and 8.2.x but is fixed in the 8.4.x series.

Trac metadata

Trac field	Value
TypeOfFailure	OtherFailure → InvalidProgramAccepted
Resolution	Unresolved → ResolvedDuplicate
Related	- → #13847 (closed)

added records label

added Pnormal label