Re: [PATCH] c++: argument pack expansion inside constraint [PR100138]

[Jason Merrill via Gcc-patches]

On 7/15/21 12:56 PM, Patrick Palka wrote:

On Sat, May 8, 2021 at 8:42 AM Jason Merrill <ja...@redhat.com> wrote:

On 5/7/21 12:33 PM, Patrick Palka wrote:

This PR is about CTAD but the underlying problems are more general;
CTAD is a good trigger for them because of the necessary substitution
into constraints that deduction guide generation entails.

In the testcase below, when generating the implicit deduction guide for
the constrained constructor template for A, we substitute the generic
flattening map 'tsubst_args' into the constructor's constraints.  During
this substitution, tsubst_pack_expansion returns a rebuilt pack
expansion for sizeof...(xs), but it's neglecting to carry over the
PACK_EXPANSION_LOCAL_P (and PACK_EXPANSION_SIZEOF_P) flag from the
original tree to the rebuilt one.  The flag is otherwise unset on the
original tree[1] but set for the rebuilt tree from make_pack_expansion
only because we're doing the CTAD at function scope (inside main).  This
leads us to crash when substituting into the pack expansion during
satisfaction because we don't have local_specializations set up (it'd be
set up for us if PACK_EXPANSION_LOCAL_P is unset)

Similarly, when substituting into a constraint we need to set
cp_unevaluated since constraints are unevaluated operands.  This avoids
a crash during CTAD for C below.

[1]: Although the original pack expansion is in a function context, I
guess it makes sense that PACK_EXPANSION_LOCAL_P is unset for it because
we can't rely on local specializations (which are formed when
substituting into the function declaration) during satisfaction.

Bootstrapped and regtested on x86_64-pc-linux-gnu, also tested on
cmcstl2 and range-v3, does this look OK for trunk?

OK.

Would it be ok to backport this patch to the 11 branch given its
impact on concepts (or perhaps backport only part of it, say all but
the PACK_EXPANSION_LOCAL_P propagation since that part just avoids
ICEing on the invalid portions of the testcase)?

The whole patch, I think.

gcc/cp/ChangeLog:

       PR c++/100138
       * constraint.cc (tsubst_constraint): Set up cp_unevaluated.
       (satisfy_atom): Set up iloc_sentinel before calling
       cxx_constant_value.
       * pt.c (tsubst_pack_expansion): When returning a rebuilt pack
       expansion, carry over PACK_EXPANSION_LOCAL_P and
       PACK_EXPANSION_SIZEOF_P from the original pack expansion.

gcc/testsuite/ChangeLog:

       PR c++/100138
       * g++.dg/cpp2a/concepts-ctad4.C: New test.
---
   gcc/cp/constraint.cc                        |  6 ++++-
   gcc/cp/pt.c                                 |  2 ++
   gcc/testsuite/g++.dg/cpp2a/concepts-ctad4.C | 25 +++++++++++++++++++++
   3 files changed, 32 insertions(+), 1 deletion(-)
   create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-ctad4.C

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 0709695fd08..30fccc46678 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -2747,6 +2747,7 @@ tsubst_constraint (tree t, tree args, tsubst_flags_t 
complain, tree in_decl)
     /* We also don't want to evaluate concept-checks when substituting the
        constraint-expressions of a declaration.  */
     processing_constraint_expression_sentinel s;
+  cp_unevaluated u;
     tree expr = tsubst_expr (t, args, complain, in_decl, false);
     return expr;
   }
@@ -3005,7 +3006,10 @@ satisfy_atom (tree t, tree args, sat_info info)

     /* Compute the value of the constraint.  */
     if (info.noisy ())
-    result = cxx_constant_value (result);
+    {
+      iloc_sentinel ils (EXPR_LOCATION (result));
+      result = cxx_constant_value (result);
+    }
     else
       {
         result = maybe_constant_value (result, NULL_TREE,
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 36a8cb5df5d..0d27dd1af65 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -13203,6 +13203,8 @@ tsubst_pack_expansion (tree t, tree args, 
tsubst_flags_t complain,
         else
       result = tsubst (pattern, args, complain, in_decl);
         result = make_pack_expansion (result, complain);
+      PACK_EXPANSION_LOCAL_P (result) = PACK_EXPANSION_LOCAL_P (t);
+      PACK_EXPANSION_SIZEOF_P (result) = PACK_EXPANSION_SIZEOF_P (t);
         if (PACK_EXPANSION_AUTO_P (t))
       {
         /* This is a fake auto... pack expansion created in add_capture with
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-ctad4.C 
b/gcc/testsuite/g++.dg/cpp2a/concepts-ctad4.C
new file mode 100644
index 00000000000..95a3a22dd04
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-ctad4.C
@@ -0,0 +1,25 @@
+// PR c++/100138
+// { dg-do compile { target c++20 } }
+
+template <class T>
+struct A {
+  A(T, auto... xs) requires (sizeof...(xs) != 0) { }
+};
+
+constexpr bool f(...) { return true; }
+
+template <class T>
+struct B {
+  B(T, auto... xs) requires (f(xs...)); // { dg-error "constant expression" }
+};
+
+template <class T>
+struct C {
+  C(T, auto x) requires (f(x)); // { dg-error "constant expression" }
+};
+
+int main() {
+  A x{1, 2}; // { dg-bogus "" }
+  B y{1, 2}; // { dg-error "deduction|no match" }
+  C z{1, 2}; // { dg-error "deduction|no match" }
+}