[eqn]: Fix Savannah #64105.

* src/preproc/eqn/main.cpp (do_file): Increment the line number if a
  malformed `lf` request was encountered, instead of decrementing it if
  a valid one was.  Increment it when processing an end eqn macro call
  (".EN"), not just a start one.  Drop an incrementation of the line
  number that applied too generally.

This ensures we don't throw the line numbers off in normal input
processing.  Fixes a problem introduced by me in commit dc98a8b09e, 31
October.

Fixes <https://savannah.gnu.org/bugs/?64105>.

2 files changed, 25 insertions, 6 deletions

diff --git a/ChangeLog b/ChangeLog
index fa1081442..e7c1ac245 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,22 @@
 2023-04-30  G. Branden Robinson <g.branden.robinson@gmail.com>
 
+	[eqn]: Fix Savannah #64105.
+
+	* src/preproc/eqn/main.cpp (do_file): Increment the line number
+	if a malformed `lf` request was encountered, instead of
+	decrementing it if a valid one was.  Increment it when
+	processing an end eqn macro call (".EN"), not just a start one.
+	Drop an incrementation of the line number that applied too
+	generally.
+
+	This ensures we don't throw the line numbers off in normal input
+	processing.  Fixes a problem introduced by me in commit
+	dc98a8b09e, 31 October.
+
+	Fixes <https://savannah.gnu.org/bugs/?64105>.
+
+2023-04-30  G. Branden Robinson <g.branden.robinson@gmail.com>
+
 	[eqn]: Improve tests of line number accuracy.
 
 	* src/preproc/eqn/tests/\
diff --git a/src/preproc/eqn/main.cpp b/src/preproc/eqn/main.cpp
index 324e2b4fb..f2678c2fa 100644
--- a/src/preproc/eqn/main.cpp
+++ b/src/preproc/eqn/main.cpp
@@ -109,9 +109,10 @@ void do_file(FILE *fp, const char *filename)
     {
       put_string(linebuf, stdout);
       linebuf += '\0';
-      // In GNU roff, `lf` assigns the number of the _next_ line.
-      if (interpret_lf_args(linebuf.contents() + 3))
-	current_lineno--;
+      // In GNU roff, `lf` assigns the number of the _next_ line.  If it
+      // is malformed, it is nevertheless an input line.
+      if (!(interpret_lf_args(linebuf.contents() + 3)))
+	current_lineno++;
     }
     else if (linebuf.length() >= 4
 	     && linebuf[0] == '.'
@@ -132,12 +133,14 @@ void do_file(FILE *fp, const char *filename)
 	    && linebuf[1] == 'E') {
 	  if (linebuf[2] == 'N'
 	      && (linebuf.length() == 3 || linebuf[3] == ' '
-		  || linebuf[3] == '\n' || compatible_flag))
+		  || linebuf[3] == '\n' || compatible_flag)) {
+	    current_lineno++;
 	    break;
+	  }
 	  else if (linebuf[2] == 'Q' && linebuf.length() > 3
 		   && (linebuf[3] == ' ' || linebuf[3] == '\n'
 		       || compatible_flag)) {
-	    current_lineno++; // We just read another line.
+	    current_lineno++;
 	    fatal("equations cannot be nested (.EQ within .EQ)");
 	  }
 	}
@@ -170,7 +173,6 @@ void do_file(FILE *fp, const char *filename)
       ;
     else
       put_string(linebuf, stdout);
-    current_lineno++;
   }
   current_filename = 0;
   current_lineno = 0;