Implememt -fdefer-type-errors (Trac #5624)

This patch implements the idea of deferring (most) type errors to
runtime, instead emitting only a warning at compile time.  The
basic idea is very simple:

 * The on-the-fly unifier in TcUnify never fails; instead if it
   gets stuck it emits a constraint.

 * The constraint solver tries to solve the constraints (and is
   entirely unchanged, hooray).

 * The remaining, unsolved constraints (if any) are passed to
   TcErrors.reportUnsolved.  With -fdefer-type-errors, instead of
   emitting an error message, TcErrors emits a warning, AND emits
   a binding for the constraint witness, binding it
   to (error "the error message"), via the new form of evidence
   TcEvidence.EvDelayedError.  So, when the program is run,
   when (and only when) that witness is needed, the program will
   crash with the exact same error message that would have been
   given at compile time.

Simple really.  But, needless to say, the exercise forced me
into some major refactoring.

 * TcErrors is almost entirely rewritten

 * EvVarX and WantedEvVar have gone away entirely

 * ErrUtils is changed a bit:
     * New Severity field in ErrMsg
     * Renamed the type Message to MsgDoc (this change
       touches a lot of files trivially)

 * One minor change is that in the constraint solver we try
   NOT to combine insoluble constraints, like Int~Bool, else
   all such type errors get combined together and result in
   only one error message!

 * I moved some definitions from TcSMonad to TcRnTypes,
   where they seem to belong more

1 files changed, 5 insertions, 5 deletions

diff --git a/compiler/iface/LoadIface.lhs b/compiler/iface/LoadIface.lhs
index ec1205f83d..37379b5be4 100644
--- a/compiler/iface/LoadIface.lhs
+++ b/compiler/iface/LoadIface.lhs
@@ -167,7 +167,7 @@ loadInterfaceWithException doc mod_name where_from
 
 ------------------
 loadInterface :: SDoc -> Module -> WhereFrom
-              -> IfM lcl (MaybeErr Message ModIface)
+              -> IfM lcl (MaybeErr MsgDoc ModIface)
 
 -- loadInterface looks in both the HPT and PIT for the required interface
 -- If not found, it loads it, and puts it in the PIT (always). 
@@ -294,7 +294,7 @@ loadInterface doc_str mod from
     }}}}
 
 wantHiBootFile :: DynFlags -> ExternalPackageState -> Module -> WhereFrom
-               -> MaybeErr Message IsBootInterface
+               -> MaybeErr MsgDoc IsBootInterface
 -- Figure out whether we want Foo.hi or Foo.hi-boot
 wantHiBootFile dflags eps mod from
   = case from of
@@ -472,7 +472,7 @@ bumpDeclStats name
 findAndReadIface :: SDoc -> Module
                  -> IsBootInterface     -- True  <=> Look for a .hi-boot file
                                         -- False <=> Look for .hi file
-                 -> TcRnIf gbl lcl (MaybeErr Message (ModIface, FilePath))
+                 -> TcRnIf gbl lcl (MaybeErr MsgDoc (ModIface, FilePath))
         -- Nothing <=> file not found, or unreadable, or illegible
         -- Just x  <=> successfully found and parsed 
 
@@ -537,7 +537,7 @@ findAndReadIface doc_str mod hi_boot_file
 
 \begin{code}
 readIface :: Module -> FilePath -> IsBootInterface 
-          -> TcRnIf gbl lcl (MaybeErr Message ModIface)
+          -> TcRnIf gbl lcl (MaybeErr MsgDoc ModIface)
         -- Failed err    <=> file not found, or unreadable, or illegible
         -- Succeeded iface <=> successfully found and parsed 
 
@@ -794,7 +794,7 @@ badIfaceFile file err
   = vcat [ptext (sLit "Bad interface file:") <+> text file, 
           nest 4 err]
 
-hiModuleNameMismatchWarn :: Module -> Module -> Message
+hiModuleNameMismatchWarn :: Module -> Module -> MsgDoc
 hiModuleNameMismatchWarn requested_mod read_mod = 
   withPprStyle defaultUserStyle $
     -- we want the Modules below to be qualified with package names,