[C++ PATCH] [PR c++/87814] undefer deferred noexcept on tsubst if request

[Alexandre Oliva <aoliva@redhat.com>]

tsubst_expr and tsubst_copy_and_build are not expected to handle
DEFERRED_NOEXCEPT exprs, but if tsubst_exception_specification takes a
DEFERRED_NOEXCEPT expr with !defer_ok, it just passes the expr on for
tsubst_copy_and_build to barf.

This patch arranges for tsubst_exception_specification to combine the
incoming args with those already stored in a DEFERRED_NOEXCEPT, and
then substitute them into the pattern, when retaining a deferred
noexcept is unacceptable.

Regstrapped on x86_64- and i686-linux-gnu, mistakenly along with a patch
with a known regression, and got only that known regression.  Retesting
without it.  Ok to install?


for  gcc/cp/ChangeLog

	PR c++/87814
	* pt.c (tsubst_exception_specification): Handle
	DEFERRED_NOEXCEPT with !defer_ok.

for  gcc/testsuite/ChangeLog

	PR c++/87814
	* g++.dg/cpp1z/pr87814.C: New.
---
 gcc/cp/pt.c                          |   14 +++++++++++---
 gcc/testsuite/g++.dg/cpp1z/pr87814.C |   26 ++++++++++++++++++++++++++
 2 files changed, 37 insertions(+), 3 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp1z/pr87814.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 8560e5885933..72ae7173d92c 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -14150,9 +14150,17 @@ tsubst_exception_specification (tree fntype,
 	    }
 	}
       else
-	new_specs = tsubst_copy_and_build
-	  (expr, args, complain, in_decl, /*function_p=*/false,
-	   /*integral_constant_expression_p=*/true);
+	{
+	  if (DEFERRED_NOEXCEPT_SPEC_P (specs))
+	    {
+	      args = add_to_template_args (DEFERRED_NOEXCEPT_ARGS (expr),
+					   args);
+	      expr = DEFERRED_NOEXCEPT_PATTERN (expr);
+	    }
+	  new_specs = tsubst_copy_and_build
+	    (expr, args, complain, in_decl, /*function_p=*/false,
+	     /*integral_constant_expression_p=*/true);
+	}
       new_specs = build_noexcept_spec (new_specs, complain);
     }
   else if (specs)
diff --git a/gcc/testsuite/g++.dg/cpp1z/pr87814.C b/gcc/testsuite/g++.dg/cpp1z/pr87814.C
new file mode 100644
index 000000000000..37034bb58cd6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/pr87814.C
@@ -0,0 +1,26 @@
+// { dg-do compile { target c++17 } }
+
+template<class Element>
+struct box {
+    template<class E>
+    constexpr box(E && e)
+        noexcept(noexcept(Element(e)))
+    {}
+};
+
+template<class... Ts>
+struct compressed_tuple_ : box<Ts> ... {
+    template<typename... Args>
+    constexpr compressed_tuple_(Args &&... args)
+        noexcept((noexcept(box<Ts>(args)) && ...))
+      : box<Ts>(args)...
+    {}
+};
+
+struct adaptor_cursor : compressed_tuple_<int*> {
+    using compressed_tuple_::compressed_tuple_;
+};
+
+int main() {
+    (void)noexcept(adaptor_cursor{(int*)0});
+}

-- 
Alexandre Oliva, freedom fighter   https://FSFLA.org/blogs/lxo
Be the change, be Free!         FSF Latin America board member
GNU Toolchain Engineer                Free Software Evangelist
Hay que enGNUrecerse, pero sin perder la terGNUra jamás-GNUChe