[gcc r12-1373] c++: normalization of non-templated return-type-req [PR100946]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:206db06ee380f490db0293af4ea7a4d590abd78c

commit r12-1373-g206db06ee380f490db0293af4ea7a4d590abd78c
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Jun 10 18:31:18 2021 -0400

    c++: normalization of non-templated return-type-req [PR100946]
    
    Here the satisfaction cache is conflating the satisfaction value of the
    two return-type-requirements because the corresponding constrained
    'auto's have level 2, but they capture an empty current_template_parms.
    This ultimately causes the satisfaction cache to think the type
    constraint doesn't depend on the deduced type of the expression.
    
    When normalizing the constraints on an 'auto', the assumption made by
    normalize_placeholder_type_constraints is that the level of the 'auto'
    is one greater than the depth of the captured current_template_parms, an
    assumption which is not holding here.  So this patch just makes n_p_t_c
    adjust the normalization context appropriately in this situation.
    
            PR c++/100946
    
    gcc/cp/ChangeLog:
    
            * constraint.cc (normalize_placeholder_type_constraints): When
            normalizing a non-templated return-type-requirement, add a dummy
            level to initial_parms.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp2a/concepts-return-req3.C: New test.

Diff:
---
 gcc/cp/constraint.cc                              | 9 +++++++++
 gcc/testsuite/g++.dg/cpp2a/concepts-return-req3.C | 6 ++++++
 2 files changed, 15 insertions(+)

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 03ce8eb9ff2..74b16d27101 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -3065,6 +3065,15 @@ normalize_placeholder_type_constraints (tree t, bool diag)
      scope for this placeholder type; use them as the initial template
      parameters for normalization.  */
   tree initial_parms = TREE_PURPOSE (ci);
+
+  if (!initial_parms && TEMPLATE_TYPE_LEVEL (t) == 2)
+    /* This is a return-type-requirement of a non-templated requires-expression,
+       which are parsed under processing_template_decl == 1 and empty
+       current_template_parms; hence the 'auto' has level 2 and initial_parms
+       is empty.  Fix up initial_parms to be consistent with the value of
+       processing_template_decl whence the 'auto' was created.  */
+    initial_parms = build_tree_list (size_int (1), make_tree_vec (0));
+
   /* The 'auto' itself is used as the first argument in its own constraints,
      and its level is one greater than its template depth.  So in order to
      capture all used template parameters, we need to add an extra level of
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-return-req3.C b/gcc/testsuite/g++.dg/cpp2a/concepts-return-req3.C
new file mode 100644
index 00000000000..a546c6457be
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-return-req3.C
@@ -0,0 +1,6 @@
+// PR c++/100946
+// { dg-do compile { target c++20 } }
+
+template<class T> concept C = __is_same(T, int);
+static_assert(requires { { 0 } -> C; });
+static_assert(requires { { true } -> C; }); // { dg-error "failed" }