#15523: GHC Panic on malformed newtype and StrictData -------------------------------------+------------------------------------- Reporter: rdnetto | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following source causes a panic: {{{#!hs {-# LANGUAGE StrictData #-} module Main where newtype Duration = Foo data Literal = LitDuration Duration }}} Result: {{{ [1 of 1] Compiling Main ( src/Main.hs, .stack-work/dist/x86_64 -linux-tinfo6/Cabal-2.2.0.1/build/tcalc/tcalc-tmp/Main.o ) ghc: panic! (the 'impossible' happened) (GHC version 8.4.3 for x86_64-unknown-linux): mkNewTyConRhs Foo [] Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in ghc:Outputable pprPanic, called at compiler/iface/BuildTyCl.hs:77:27 in ghc:BuildTyCl Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} If we remove the StrictData pragma, then we get a syntax error as expected: {{{ /home/reuben/tcalc/src/Main.hs:3:20: error: • The constructor of a newtype must have exactly one field but ‘Foo’ has none • In the definition of data constructor ‘Foo’ In the newtype declaration for ‘Duration’ | 3 | newtype Duration = Foo | ^^^ }}} I was able to reproduce this with GHC 8.4.3 and 8.2.2 (using Stack LTS snapshots). GHC 8.0.2 is not affected. A test case buildable with Stack can be found at https://github.com/rdnetto/tcalc/tree/ghc-bug (i.e. on the ghc-bug branch). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15523 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler