c++: Diagnose self-recursive satisfaction

This patch further extends the satisfaction_cache class to diagnose
self-recursive satisfaction.

gcc/cp/ChangeLog:

	* constraint.cc (sat_entry::evaluating): New member.
	(satisfaction_cache::get): If entry->evaluating, diagnose
	self-recursive satisfaction.  Otherwise, set entry->evaluating
	if we're not reusing a cached satisfaction result.
	(satisfaction_cache::save): Clear entry->evaluating.
	(satisfy_atom): Set up diagnosing_failed_constraint before the
	first call to get().

gcc/testsuite/ChangeLog:

	PR c++/96840
	* g++.dg/cpp2a/concepts-pr88395.C: Adjust to expect the
	self-recursive satisfaction to get directly diagnosed.
	* g++.dg/cpp2a/concepts-recursive-sat2.C: Likewise.
	* g++.dg/cpp2a/concepts-recursive-sat4.C: New test.

3 files changed, 18 insertions, 9 deletions

diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-pr88395.C b/gcc/testsuite/g++.dg/cpp2a/concepts-pr88395.C
index 1c25252..e1792e1 100644
--- a/gcc/testsuite/g++.dg/cpp2a/concepts-pr88395.C
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-pr88395.C
@@ -1,9 +1,9 @@
 // { dg-do compile { target c++20 } }
 
 template <class T, class U>
-concept Concept2 = requires (T t, U u)
+concept Concept2 = requires (T t, U u) // { dg-error "depends on itself" }
 {
-    t += u; // { dg-error "template instantiation depth" }
+    t += u;
 };
 
 template <class T>
@@ -17,7 +17,5 @@ struct S
 
 constexpr S operator * (S a, S b)
 {
-    return a += b;
+    return a += b; // { dg-error "no match" }
 }
-
-// { dg-prune-output "compilation terminated" }
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-recursive-sat2.C b/gcc/testsuite/g++.dg/cpp2a/concepts-recursive-sat2.C
index 992fcbb..9bc96f5 100644
--- a/gcc/testsuite/g++.dg/cpp2a/concepts-recursive-sat2.C
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-recursive-sat2.C
@@ -1,7 +1,7 @@
 // { dg-do compile { target c++20 } }
 
 template<typename T>
-concept Fooable = requires(T t) { foo(t); }; // { dg-error "template instantiation depth" }
+concept Fooable = requires(T t) { foo(t); }; // { dg-error "depends on itself" }
 
 template<Fooable T>
 void foo(T t) { }
@@ -9,7 +9,5 @@ void foo(T t) { }
 void test()
 {
   struct S {} s;
-  foo(s);
+  foo(s); // { dg-error "no match" }
 }
-
-// { dg-prune-output "compilation terminated" }
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-recursive-sat4.C b/gcc/testsuite/g++.dg/cpp2a/concepts-recursive-sat4.C
new file mode 100644
index 0000000..18d126e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-recursive-sat4.C
@@ -0,0 +1,13 @@
+// PR c++/96840
+// { dg-do compile { target c++20 } }
+
+template <class T, class U> concept C = requires(T t, U u) { t * u; };
+// { dg-message "required for the satisfaction of 'C<T, Rep>' .with T = Int<int>; Rep = int." "" { target *-*-* } .-1 }
+// { dg-error "depends on itself" "" { target *-*-* } .-2 }
+
+template <class Rep> struct Int {
+  template <class T> requires C<T, Rep> friend void operator*(T, Int) { }
+  template <class T> requires C<T, Rep> friend void operator*(Int, T) { }
+};
+
+void f() { 0 * Int<int>{}; }