[PATCH] c++: return-type-req in constraint using only outer tparms [PR104527]

[Patrick Palka <ppalka@redhat.com>]

Here the template context for the atomic constraint has two levels of
template arguments, but since it depends only on the innermost argument
T we use a single-level argument vector during substitution into the
constraint (built by get_mapped_args).  We eventually pass this vector
to do_auto_deduction as part of checking the return-type-requirement
inside the atom, but do_auto_deduction expects outer_targs to be a full
set of arguments for sake of satisfaction.

do_auto_deduction has a workaround in place to compensate for callers
that pass only the innermost arguments as outer_targs, but here we're
passing the _outermost_ arguments.  Since the former situation should
now (after r12-7101) only occur with adc_unify callers and the latter
only with adc_requirement callers, this patch conditions the existing
workaround according to the auto_deduction_context: if the context is
adc_requirement, we add dummy innermost levels, otherwise we add dummy
outermost levels as before and also assert that the context is adc_unify.

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

	PR c++/104527

gcc/cp/ChangeLog:

	* pt.cc (do_auto_deduction): When template argument levels are
	missing from outer_targs, fill in the innermost rather than the
	outermost levels with dummy args if the context is
	adc_requirement, otherwise also assert that the context is
	adc_unify.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-return-req4.C: New test.
---
 gcc/cp/pt.cc                                  | 28 +++++++++++++------
 .../g++.dg/cpp2a/concepts-return-req4.C       | 24 ++++++++++++++++
 2 files changed, 44 insertions(+), 8 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-return-req4.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 1b18e2a7787..4ff2710b8ba 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -30215,20 +30215,32 @@ do_auto_deduction (tree type, tree init, tree auto_node,
 
       tree full_targs = add_to_template_args (outer_targs, targs);
 
-      /* HACK: Compensate for callers not always communicating all levels of
-	 outer template arguments by filling in the outermost missing levels
-	 with dummy levels before checking satisfaction.  We'll still crash
-	 if the constraint depends on a template argument belonging to one of
-	 these missing levels, but this hack otherwise allows us to handle a
-	 large subset of possible constraints (including all non-dependent
-	 constraints).  */
       if (int missing_levels = (TEMPLATE_TYPE_ORIG_LEVEL (auto_node)
 				- TMPL_ARGS_DEPTH (full_targs)))
 	{
 	  tree dummy_levels = make_tree_vec (missing_levels);
 	  for (int i = 0; i < missing_levels; ++i)
 	    TREE_VEC_ELT (dummy_levels, i) = make_tree_vec (0);
-	  full_targs = add_to_template_args (dummy_levels, full_targs);
+	  if (context == adc_requirement)
+	    /* We're checking a requires-expr's return-type-requirement that's
+	       part of an atomic constraint that doesn't depend on any innermost
+	       template arguments, so OUTER_TARGS (built by get_mapped_args) is
+	       missing at least one innermost level.  Fill in the innermost
+	       levels of OUTER_TARGS with dummy levels.  */
+	    full_targs = add_to_template_args
+	      (add_to_template_args (outer_targs, dummy_levels), targs);
+	  else
+	    {
+	      /* Otherwise, fill in the _outermost_ levels with dummy levels.
+		 This compensates for adc_unify callers that only pass the
+		 innermost level of template arguments as OUTER_TARGS.  We'll
+		 still crash if the constraint depends on a template argument
+		 belonging to one of these missing levels, but this hack
+		 otherwise allows us to handle a large subset of possible
+		 constraints (including all non-dependent constraints).  */
+	      gcc_checking_assert (context == adc_unify);
+	      full_targs = add_to_template_args (dummy_levels, full_targs);
+	    }
 	}
 
       if (!constraints_satisfied_p (auto_node, full_targs))
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-return-req4.C b/gcc/testsuite/g++.dg/cpp2a/concepts-return-req4.C
new file mode 100644
index 00000000000..471946bc8eb
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-return-req4.C
@@ -0,0 +1,24 @@
+// PR c++/104527
+// { dg-do compile { target c++20 } }
+
+template<class T, class U>
+concept is_same = __is_same(T, U);
+
+template<class T>
+struct A {
+  template<class...>
+    requires requires { { 0 } -> is_same<T>; }
+  struct B {};
+
+  template<class...>
+    requires requires { { 1 } -> is_same<T>; }
+  static void f();
+};
+
+A<int>::B<> a1;
+A<bool>::B<> a2; // { dg-error "constraint" }
+
+int main() {
+  A<int>::f();
+  A<bool>::f(); // { dg-error "no match" }
+}
-- 
2.35.1.102.g2b9c120970