[gcc r11-4680] c++: Make extern-C mismatch an error

[gcc r11-4680] c++: Make extern-C mismatch an error

[Nathan Sidwell]

https://gcc.gnu.org/g:770ec066b8b5d70921dfab9fcd1bbb87abeb0dc5

commit r11-4680-g770ec066b8b5d70921dfab9fcd1bbb87abeb0dc5
Author: Nathan Sidwell <nathan@acm.org>
Date:   Tue Nov 3 08:35:53 2020 -0800

    c++: Make extern-C mismatch an error
    
    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.

Diff:
---
 gcc/cp/decl.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 114e8d0cb01..9428fa05258 100644
--- a/gcc/cp/decl.c
+++ b/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.  */