Re: [PATCH] c++: constexpr folding in unevaluated context [PR105931]

[Jason Merrill via Gcc-patches]

On 6/23/22 11:06, Patrick Palka wrote:

Changing the type of N from int to unsigned in decltype82.C from
r12-8472-g47ea22015c90df reveals another spot where we perform
constexpr evaluation in an unevaluated context for sake of diagnostics,
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 diagnostics
when in an unevaluated context, but there's probably other spots
that have the same bug.  So this patch takes the approach of suppressing
fold_for_warn more generally when in an unevaluated context.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk/12?

OK.

        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.
---
  gcc/cp/expr.cc                           |  5 +++++
  gcc/testsuite/g++.dg/cpp0x/decltype82a.C | 11 +++++++++++
  2 files changed, 16 insertions(+)
  create mode 100644 gcc/testsuite/g++.dg/cpp0x/decltype82a.C

diff --git a/gcc/cp/expr.cc b/gcc/cp/expr.cc
index 31fe0980337..b71245f937d 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>();