[gcc r13-1225] c++: constexpr folding in unevaluated context [PR105931]

[gcc r13-1225] c++: constexpr folding in unevaluated context [PR105931]

[Patrick Palka]

https://gcc.gnu.org/g:b00b95198e6720eb23a2618870d67800f6180fdd

commit r13-1225-gb00b95198e6720eb23a2618870d67800f6180fdd
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Jun 23 16:36:43 2022 -0400

    c++: constexpr folding in unevaluated context [PR105931]
    
    Changing the type of N from int to unsigned in decltype82.C (from
    r13-986-g0ecb6b906f215e) reveals another spot where we perform constexpr
    evaluation in an unevaluated context for sake of warnings, this time
    from the call to shorten_compare in cp_build_binary_op, which calls
    fold_for_warn.
    
    We could (and probably should) suppress the shorten_compare warnings
    when in an unevaluated context, but there's probably other callers of
    fold_for_warn that are similarly affected.  So this patch takes the
    approach of directly suppressing fold_for_warn when in an unevaluated
    context.
    
            PR c++/105931
    
    gcc/cp/ChangeLog:
    
            * expr.cc (fold_for_warn): Don't fold when in an unevaluated
            context.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp0x/decltype82a.C: New test.

Diff:
---
 gcc/cp/expr.cc                           |  5 +++++
 gcc/testsuite/g++.dg/cpp0x/decltype82a.C | 11 +++++++++++
 2 files changed, 16 insertions(+)

diff --git a/gcc/cp/expr.cc b/gcc/cp/expr.cc
index 31fe0980337..56fc11f425b 100644
--- a/gcc/cp/expr.cc
+++ b/gcc/cp/expr.cc
@@ -397,6 +397,11 @@ fold_for_warn (tree x)
 {
   /* C++ implementation.  */
 
+  if (cp_unevaluated_operand)
+    /* In an unevaluated context we don't care about the reduced value
+       of an expression, so neither should any warnings.  */
+    return x;
+
   /* Prevent warning-dependent constexpr evaluation from changing
      DECL_UID (which breaks -fcompare-debug) and from instantiating
      templates.  */
diff --git a/gcc/testsuite/g++.dg/cpp0x/decltype82a.C b/gcc/testsuite/g++.dg/cpp0x/decltype82a.C
new file mode 100644
index 00000000000..cbbcdfd8e58
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/decltype82a.C
@@ -0,0 +1,11 @@
+// PR c++/105931
+// { dg-do compile { target c++11 } }
+// A version of decltype82.C where N is unsigned.
+
+template<unsigned N>
+void f() {
+  using ty1 = decltype((5 % N) == 0);
+  using ty2 = decltype((5 / N) == 0);
+}
+
+template void f<0>();