Issue warning for duplicate header identifiers.

As noted in the previous commit, an autogenerated identifier
may still coincide with an explicit identifier that is given
for a header later in the document, or with an identifier on
a div, span, link, or image. This commit adds a warning
in this case, so users can supply an explicit identifier.

* Added `DuplicateIdentifier` to LogMessage.
* Modified HTML, Org, MediaWiki readers so their custom
  state type is an instance of HasLogMessages.  This is necessary
  for `registerHeader` to issue warnings.

See #1745.

1 files changed, 8 insertions, 0 deletions

diff --git a/src/Text/Pandoc/Readers/Org/ParserState.hs b/src/Text/Pandoc/Readers/Org/ParserState.hs
index 0bbe27991..6bed2a547 100644
--- a/src/Text/Pandoc/Readers/Org/ParserState.hs
+++ b/src/Text/Pandoc/Readers/Org/ParserState.hs
@@ -60,7 +60,9 @@ import qualified Data.Set as Set
 import Text.Pandoc.Builder (Blocks, Inlines, trimInlines)
 import Text.Pandoc.Definition (Meta (..), nullMeta)
 import Text.Pandoc.Options (ReaderOptions (..))
+import Text.Pandoc.Logging
 import Text.Pandoc.Parsing (HasHeaderMap (..), HasIdentifierList (..),
+                            HasLogMessages (..),
                             HasLastStrPosition (..), HasQuoteContext (..),
                             HasReaderOptions (..), ParserContext (..),
                             QuoteContext (..), SourcePos)
@@ -104,6 +106,7 @@ data OrgParserState = OrgParserState
   , orgStateOptions              :: ReaderOptions
   , orgStateParserContext        :: ParserContext
   , orgStateTodoSequences        :: [TodoSequence]
+  , orgLogMessages               :: [LogMessage]
   }
 
 data OrgParserLocal = OrgParserLocal { orgLocalQuoteContext :: QuoteContext }
@@ -130,6 +133,10 @@ instance HasHeaderMap OrgParserState where
   extractHeaderMap = orgStateHeaderMap
   updateHeaderMap  f s = s{ orgStateHeaderMap = f (orgStateHeaderMap s) }
 
+instance HasLogMessages OrgParserState where
+  addLogMessage msg st = st{ orgLogMessages = msg : orgLogMessages st }
+  getLogMessages st = reverse $ orgLogMessages st
+
 instance Default OrgParserState where
   def = defaultOrgParserState
 
@@ -150,6 +157,7 @@ defaultOrgParserState = OrgParserState
   , orgStateOptions = def
   , orgStateParserContext = NullState
   , orgStateTodoSequences = []
+  , orgLogMessages = []
   }
 
 optionsToParserState :: ReaderOptions -> OrgParserState