Re: [PATCH] c++: excessive satisfaction in check_methods [PR108579]

[Jason Merrill via Gcc-patches]

On 1/30/23 14:10, Patrick Palka wrote:
In check_methods we're unnecessarily checking satisfaction for all
constructors and assignment operators, even those that don't look like
copy/move special members.  In the testcase below this manifests as an
unstable satisfaction error because the satisfaction result is first
determined to be false during check_methods (since A<int> is incomplete
at this point) and later true after completion of A<int>.
This patch fixes this simply by swapping the order of the
constraint_satisfied_p and copy_fn_p / move_fn_p tests.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look
OK for trunk?  This doesn't fix the regression completely, since
we get a similar unstable satisfaction error if one of the constrained
members is actually a copy/move special member.  I suppose we need to
rearrange things in finish_struct_1 so that check_methods gets called in
a complete class context?

I think the way to make that work, if indeed that's desirable, would be 
to determine those properties lazily instead of at finish_struct time.

The patch is OK.

        PR c++/108579

gcc/cp/ChangeLog:

        * class.cc (check_methods): Test constraints_satisfied_p after
        testing copy_fn_p / move_fn_p instead of beforehand.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp2a/concepts-pr108579.C: New test.
---
  gcc/cp/class.cc                                | 16 ++++++++--------
  gcc/testsuite/g++.dg/cpp2a/concepts-pr108579.C | 14 ++++++++++++++
  2 files changed, 22 insertions(+), 8 deletions(-)
  create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-pr108579.C

diff --git a/gcc/cp/class.cc b/gcc/cp/class.cc
index 351de6c5419..d3ce8532d56 100644
--- a/gcc/cp/class.cc
+++ b/gcc/cp/class.cc
@@ -4822,11 +4822,11 @@ check_methods (tree t)
        /* Might be trivial.  */;
        else if (TREE_CODE (fn) == TEMPLATE_DECL)
        /* Templates are never special members.  */;
-      else if (!constraints_satisfied_p (fn))
-       /* Not eligible.  */;
-      else if (copy_fn_p (fn))
+      else if (copy_fn_p (fn)
+              && constraints_satisfied_p (fn))
        TYPE_HAS_COMPLEX_COPY_CTOR (t) = true;
-      else if (move_fn_p (fn))
+      else if (move_fn_p (fn)
+              && constraints_satisfied_p (fn))
        TYPE_HAS_COMPLEX_MOVE_CTOR (t) = true;
      }
  
@@ -4836,11 +4836,11 @@ check_methods (tree t)
        /* Might be trivial.  */;
        else if (TREE_CODE (fn) == TEMPLATE_DECL)
        /* Templates are never special members.  */;
-      else if (!constraints_satisfied_p (fn))
-       /* Not eligible.  */;
-      else if (copy_fn_p (fn))
+      else if (copy_fn_p (fn)
+              && constraints_satisfied_p (fn))
        TYPE_HAS_COMPLEX_COPY_ASSIGN (t) = true;
-      else if (move_fn_p (fn))
+      else if (move_fn_p (fn)
+              && constraints_satisfied_p (fn))
        TYPE_HAS_COMPLEX_MOVE_ASSIGN (t) = true;
      }
  }
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-pr108579.C 
b/gcc/testsuite/g++.dg/cpp2a/concepts-pr108579.C
new file mode 100644
index 00000000000..bc7d709f889
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-pr108579.C
@@ -0,0 +1,14 @@
+// PR c++/108579
+// { dg-do compile { target c++20 } }
+
+template<class T>
+struct A {
+  A(double, char);
+  A(int) requires requires { A(0.0, 'c'); };
+  A& operator=(int) requires requires { A(1.0, 'd'); };
+};
+
+int main() {
+  A<int> x(3);
+  x = 5;
+}