[PATCH] c++: conditional noexcept-spec on defaulted comparison op [PR96242]

[Patrick Palka via Gcc-patches]

When synthesizing a defaulted comparison op from
maybe_instantiate_noexcept, we seem to be forgetting to instantiate the
noexcept-spec afterwards.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk and perhaps 11?

        PR c++/96242

gcc/cp/ChangeLog:

        * pt.cc (maybe_instantiate_noexcept): Keep going after
        successfully synthesizing a DECL_MAYBE_DELETED fn.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp2a/spaceship-synth15.C: New test.
---
 gcc/cp/pt.cc                                  |  3 ++-
 .../g++.dg/cpp2a/spaceship-synth15.C          | 22 +++++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/spaceship-synth15.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 6dd64101ced..03345ed3ed3 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -25986,7 +25986,8 @@ maybe_instantiate_noexcept (tree fn, tsubst_flags_t 
complain)
       ++function_depth;
       maybe_synthesize_method (fn);
       --function_depth;
-      return !DECL_MAYBE_DELETED (fn);
+      if (DECL_MAYBE_DELETED (fn))
+       return false;
     }
 
   tree fntype = TREE_TYPE (fn);
diff --git a/gcc/testsuite/g++.dg/cpp2a/spaceship-synth15.C 
b/gcc/testsuite/g++.dg/cpp2a/spaceship-synth15.C
new file mode 100644
index 00000000000..00ea6c10474
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/spaceship-synth15.C
@@ -0,0 +1,22 @@
+// PR c++/96242
+// { dg-do compile { target c++20 } }
+
+#include <compare>
+
+template<bool B>
+struct X {
+  auto operator<=>(const X&) const noexcept(B) = default;
+  bool operator==(const X&) const noexcept(!B) = default;
+};
+
+X<true> x_t;
+static_assert(noexcept(x_t <=> x_t));
+static_assert(noexcept(x_t < x_t));
+static_assert(!noexcept(x_t == x_t));
+static_assert(!noexcept(x_t != x_t));
+
+X<false> x_f;
+static_assert(!noexcept(x_f <=> x_f));
+static_assert(!noexcept(x_f < x_f));
+static_assert(noexcept(x_f == x_f));
+static_assert(noexcept(x_f != x_f));
-- 
2.35.0