[commit: ghc] master: Don't prematurely force TyThing thunks with -ddump-if-trace. (da105ca)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/da105ca104f8382c33c01050dd04443c9c2d63ea/ghc

>---------------------------------------------------------------

commit da105ca104f8382c33c01050dd04443c9c2d63ea
Author: Edward Z. Yang <ezyang at cs.stanford.edu>
Date:   Tue May 10 00:14:05 2016 -0700

    Don't prematurely force TyThing thunks with -ddump-if-trace.
    
    Signed-off-by: Edward Z. Yang <ezyang at cs.stanford.edu>
    
    Test Plan: validate
    
    Reviewers: simonpj, austin, bgamari
    
    Subscribers: thomie
    
    Differential Revision: https://phabricator.haskell.org/D2187


>---------------------------------------------------------------

da105ca104f8382c33c01050dd04443c9c2d63ea
 compiler/iface/TcIface.hs | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/compiler/iface/TcIface.hs b/compiler/iface/TcIface.hs
index 25fa227..9a4a5c7 100644
--- a/compiler/iface/TcIface.hs
+++ b/compiler/iface/TcIface.hs
@@ -144,7 +144,10 @@ typecheckIface iface
 
                 -- Finished
         ; traceIf (vcat [text "Finished typechecking interface for" <+> ppr (mi_module iface),
-                         text "Type envt:" <+> ppr type_env])
+                         -- Careful! If we tug on the TyThing thunks too early
+                         -- we'll infinite loop with hs-boot.  See #10083 for
+                         -- an example where this would cause non-termination.
+                         text "Type envt:" <+> ppr (map fst names_w_things)])
         ; return $ ModDetails { md_types     = type_env
                               , md_insts     = insts
                               , md_fam_insts = fam_insts