[PATCH] c++: normalization of non-templated return-type-req [PR100946]

[Patrick Palka]

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 captured current_template_parms, an assumption
which is not holding here.  To fix this, this patch adds a dummy level
to current_template_parms to match processing_template_decl when parsing
a non-templated return-type-requirement.  This patch also makes us
verify this assumption upon creation of a constrained 'auto'.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk/11?

	PR c++/100946

gcc/cp/ChangeLog:

	* parser.c (cp_parser_compound_requirement): When parsing a
	non-templated return-type-requirement, add a dummy level
	to current_template_parms.
	* pt.c (make_constrained_placeholder_type): Verify the depth
	of current_template_parms is consistent with the level of
	the 'auto'.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-return-req3.C: New test.
---
 gcc/cp/parser.c                                   | 12 ++++++++++++
 gcc/cp/pt.c                                       |  8 ++++++++
 gcc/testsuite/g++.dg/cpp2a/concepts-return-req3.C |  6 ++++++
 3 files changed, 26 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-return-req3.C

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index d59a829d0b9..8278a5608ae 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -29181,6 +29181,18 @@ cp_parser_compound_requirement (cp_parser *parser)
       cp_lexer_consume_token (parser->lexer);
       cp_token *tok = cp_lexer_peek_token (parser->lexer);
 
+      auto ctp = make_temp_override (current_template_parms);
+      if (!current_template_parms)
+	{
+	  /* We're parsing a return-type-requirement within a non-templated
+	     requires-expression.  Update current_template_parms to agree with
+	     processing_template_decl so that the normalization context that's
+	     captured by the corresponding constrained 'auto' is sensible.  */
+	  gcc_checking_assert (processing_template_decl == 1);
+	  current_template_parms
+	    = build_tree_list (size_int (1), make_tree_vec (0));
+	}
+
       bool saved_result_type_constraint_p = parser->in_result_type_constraint_p;
       parser->in_result_type_constraint_p = true;
       /* C++20 allows either a type-id or a type-constraint. Parsing
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index b0155a9c370..05679b12973 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -28131,6 +28131,14 @@ make_constrained_placeholder_type (tree type, tree con, tree args)
   expr = build_concept_check (expr, type, args, tf_warning_or_error);
   --processing_template_decl;
 
+  /* Verify the normalization context is consistent with the level of
+     this 'auto'.  */
+  if (TEMPLATE_TYPE_LEVEL (type) == 1)
+    gcc_checking_assert (!current_template_parms);
+  else
+    gcc_checking_assert (1 + TMPL_PARMS_DEPTH (current_template_parms)
+			 == TEMPLATE_TYPE_LEVEL (type));
+
   PLACEHOLDER_TYPE_CONSTRAINTS_INFO (type)
     = build_tree_list (current_template_parms, expr);
 
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" }
-- 
2.32.0.rc2