From 84fdb5c975683606a66b968de9f40961e764e380 Mon Sep 17 00:00:00 2001
From: Stephen Heumann <stephenheumann@gmail.com>
Date: Thu, 22 Feb 2024 21:55:46 -0600
Subject: [PATCH] Fix handling of empty macro arguments as ## operands.

Previously, there were a couple problems:

*If the parameter that was passed an empty argument appeared directly after the ##, the ## would permanently be removed from the macro record, affecting subsequent uses of the macro even if the argument was not empty.
*If the parameter that was passed an empty argument appeared between two ## operators, both would effectively be skipped, so the tokens to the left of the first ## and to the right of the second would not be combined.

This example illustrates both issues (not expected to compile; just check preprocessor output):

#pragma expand 1
#define x(a,b,c) a##b##c
x(1, ,3)
x(a,b,c)
---
 Scanner.pas | 10 +++++++---
 cc.notes    |  2 ++
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/Scanner.pas b/Scanner.pas
index c38f6e8..ccb767a 100644
--- a/Scanner.pas
+++ b/Scanner.pas
@@ -1882,6 +1882,8 @@ procedure Expand (macro: macroRecordPtr);
 { Globals:                                                      }
 {       macroList - scanner putback buffer                      }
 
+label 1;
+
 type
    parameterPtr = ^parameterRecord;
    parameterRecord = record             {parameter list element}
@@ -2160,8 +2162,10 @@ else begin
                tcPtr := pptr^.tokens;
                if tcPtr = nil then begin
                   if tlPtr^.next <> nil then
-                     if tlPtr^.next^.token.kind = poundpoundop then
-                        tlPtr^.next := tlPtr^.next^.next;                  
+                     if tlPtr^.next^.token.kind = poundpoundop then begin
+                        tlPtr := tlPtr^.next;
+                        goto 1;
+                        end; {if}
                   if lastPtr <> nil then
                      if lastPtr^.token.kind = poundpoundop then
                         if tokenList <> nil then
@@ -2213,7 +2217,7 @@ else begin
          tokenEnd := tlPtr^.tokenEnd;
          PutBackToken(tlPtr^.token, expandEnabled, false);
          end; {else}
-      lastPtr := tlPtr;
+1:    lastPtr := tlPtr;
       tlPtr := tlPtr^.next;
       end; {while}
    end; {else}
diff --git a/cc.notes b/cc.notes
index eb0700e..6cfda7f 100644
--- a/cc.notes
+++ b/cc.notes
@@ -1614,6 +1614,8 @@ If you use #pragma debug 0x0010 to enable stack check debug code, the compiler w
 
 12. If a numeric token formed using a ## preprocessor operator was an operand for another ## preprocessor operator, the resulting token would be incorrect.
 
+13. If an empty argument was passed for a macro parameter that was used as an operand of the ## preprocessing operator, the result would likely be incorrect, and subsequent uses of the same macro also might not be expanded correctly.
+
 -- Bugs from C 2.1.1 B3 that have been fixed in C 2.2.0 ---------------------
 
 1.  There were various bugs that could cause incorrect code to be generated in certain cases.  Some of these were specific to certain optimization passes, alone or in combination.