c++: Make extern-C mismatch an error

c++: Make extern-C mismatch an error

[Nathan Sidwell]

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duplicate_decls	was being lenient about	extern-c mismatches, allowing
you to have two declarations in the symbol table after emitting an
error.	This resulted in duplicate error messages in modules, when we
find the same problem multiple times.  Let's just not let that happen.

         gcc/cp/
         * decl.c (duplicate_decls): Return error_mark_node fo extern-c
         mismatch.

pushing to trunk
-- 
Nathan Sidwell

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diff --git i/gcc/cp/decl.c w/gcc/cp/decl.c
index 114e8d0cb01..9428fa05258 100644
--- i/gcc/cp/decl.c
+++ w/gcc/cp/decl.c
@@ -1776,7 +1776,7 @@ duplicate_decls (tree newdecl, tree olddecl, bool hiding, bool was_hidden)
 			newdecl);
 	      inform (olddecl_loc,
 		      "previous declaration %q#D", olddecl);
-	      return NULL_TREE;
+	      return error_mark_node;
 	    }
 	  /* For function versions, params and types match, but they
 	     are not ambiguous.  */