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.

(cherry picked from commit 206db06ee380f490db0293af4ea7a4d590abd78c)

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 0709695fd08fc295212afad788041b68ab85e402..6baaa0650b03873b85de25a75a52c8060dbd5e2e 100644 (file)
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -3088,6 +3088,15 @@ 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 (file)
index 0000000..a546c64
--- /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" }