[gcc r14-5898] libcpp: Fix unsigned promotion for unevaluated divide by zero [PR112701]

[Lewis Hyatt <lhyatt@gcc.gnu.org>]

https://gcc.gnu.org/g:ce52f1f7074d96c4d9ce63b1169c11087757e926

commit r14-5898-gce52f1f7074d96c4d9ce63b1169c11087757e926
Author: Lewis Hyatt <lhyatt@gmail.com>
Date:   Mon Nov 27 12:08:41 2023 -0500

    libcpp: Fix unsigned promotion for unevaluated divide by zero [PR112701]
    
    When libcpp encounters a divide by zero while processing a constant
    expression "x/y", it returns "x" as a fallback. The value of the fallback is
    not normally important, since an error will be generated anyway, but if the
    expression appears in an unevaluated context, such as "0 ? 0/0u : -1", then
    there will be no error, and the fallback value will be meaningful to the
    extent that it may cause promotion from signed to unsigned of an operand
    encountered later. As the PR notes, libcpp does not do the unsigned
    promotion correctly in this case; fix it by making the fallback return value
    unsigned as necessary.
    
    libcpp/ChangeLog:
    
            PR preprocessor/112701
            * expr.cc (num_div_op): Set unsignedp appropriately when returning a
            stub value for divide by 0.
    
    gcc/testsuite/ChangeLog:
    
            PR preprocessor/112701
            * gcc.dg/cpp/expr.c: Add additional tests to cover divide by 0 in an
            unevaluated context, where the unsignedness still matters.

Diff:
---
 gcc/testsuite/gcc.dg/cpp/expr.c | 22 ++++++++++++++++++++--
 libcpp/expr.cc                  |  1 +
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/gcc/testsuite/gcc.dg/cpp/expr.c b/gcc/testsuite/gcc.dg/cpp/expr.c
index 532bd681237..055e17ae753 100644
--- a/gcc/testsuite/gcc.dg/cpp/expr.c
+++ b/gcc/testsuite/gcc.dg/cpp/expr.c
@@ -1,6 +1,7 @@
 /* Copyright (C) 2000, 2001 Free Software Foundation, Inc.  */
 
 /* { dg-do preprocess } */
+/* { dg-additional-options "-Wall" } */
 
 /* Test we get signedness of ?: operator correct.  We would skip
    evaluation of one argument, and might therefore not transfer its
@@ -8,10 +9,27 @@
 
 /* Neil Booth, 19 Jul 2002.  */
 
-#if (1 ? -2: 0 + 1U) < 0
+#if (1 ? -2: 0 + 1U) < 0 /* { dg-warning {the left operand of ":" changes sign} } */
 #error				/* { dg-bogus "error" } */
 #endif
 
-#if (0 ? 0 + 1U: -2) < 0
+#if (0 ? 0 + 1U: -2) < 0 /* { dg-warning {the right operand of ":" changes sign} } */
 #error				/* { dg-bogus "error" } */
 #endif
+
+/* PR preprocessor/112701 */
+#if (0 ? 0/0u : -1) < 0 /* { dg-warning {the right operand of ":" changes sign} } */
+#error /* { dg-bogus "error" } */
+#endif
+
+#if (0 ? 0u/0 : -1) < 0 /* { dg-warning {the right operand of ":" changes sign} } */
+#error /* { dg-bogus "error" } */
+#endif
+
+#if (1 ? -1 : 0/0u) < 0 /* { dg-warning {the left operand of ":" changes sign} } */
+#error /* { dg-bogus "error" } */
+#endif
+
+#if (1 ? -1 : 0u/0) < 0 /* { dg-warning {the left operand of ":" changes sign} } */
+#error /* { dg-bogus "error" } */
+#endif
diff --git a/libcpp/expr.cc b/libcpp/expr.cc
index 825d2c2369d..4f4a9722ac7 100644
--- a/libcpp/expr.cc
+++ b/libcpp/expr.cc
@@ -2216,6 +2216,7 @@ num_div_op (cpp_reader *pfile, cpp_num lhs, cpp_num rhs, enum cpp_ttype op,
       if (!pfile->state.skip_eval)
 	cpp_error_with_line (pfile, CPP_DL_ERROR, location, 0,
 			     "division by zero in #if");
+      lhs.unsignedp = unsignedp;
       return lhs;
     }