c++: fn parm pack expansion inside constraint [PR100138]

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 doesn't 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 but gets set for the rebuilt tree from make_pack_expansion
since at_function_scope_p() is true (we're inside main).  This leads to
a crash during satisfaction when substituting into the pack expansion
because we don't have local_specializations set up (and it'd be set up
for us if PACK_EXPANSION_LOCAL_P is unset)

Similarly, tsubst_constraint needs to set cp_unevaluated so that the
substitution performed therein doesn't rely on local_specializations.
This avoids a crash during CTAD for C below.

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.

(cherry picked from commit e7a9f085ffd34b0d7bc4b803c182b41494f609aa)

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 27e22d52939646e8924f6eef20623b8fe2f803c0..4af7018f1a57e167c7f956ac70f1499f1ba45bcb 100644 (file)
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -2748,6 +2748,7 @@ 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;
 }
@@ -3006,7 +3007,10 @@ 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 072901d4eb97c3574eb3986a9120e2c2502daa97..b53b1fd9a977407663a60b0380a257a89392cc81 100644 (file)
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -13186,6 +13186,8 @@ 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 (file)
index 0000000..4f04cb3
--- /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" }
+}