[gcc r11-3714] c++: ICE in dependent_type_p with constrained auto [PR97052]

[gcc r11-3714] c++: ICE in dependent_type_p with constrained auto [PR97052]

[Patrick Palka]

https://gcc.gnu.org/g:c1c62aec6751678e958ab5c61b2d903a09d7efd9

commit r11-3714-gc1c62aec6751678e958ab5c61b2d903a09d7efd9
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Oct 8 00:05:41 2020 -0400

    c++: ICE in dependent_type_p with constrained auto [PR97052]
    
    This patch fixes an "unguarded" call to coerce_template_parms in
    build_standard_check: processing_template_decl could be zero if we
    get here during processing of the first 'auto' parameter of an
    abbreviated function template, or if we're processing the type
    constraint of a non-templated variable.  In the testcase below, this
    leads to an ICE when coerce_template_parms instantiates C's dependent
    default template argument.
    
    gcc/cp/ChangeLog:
    
            PR c++/97052
            * constraint.cc (build_type_constraint): Temporarily increment
            processing_template_decl before calling build_concept_check.
            * pt.c (make_constrained_placeholder_type): Likewise.
    
    gcc/testsuite/ChangeLog:
    
            PR c++/97052
            * g++.dg/cpp2a/concepts-defarg2.C: New test.

Diff:
---
 gcc/cp/constraint.cc                          |  2 ++
 gcc/cp/pt.c                                   |  2 ++
 gcc/testsuite/g++.dg/cpp2a/concepts-defarg2.C | 13 +++++++++++++
 3 files changed, 17 insertions(+)

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index d49957a6c4a..050b55ce092 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -1427,7 +1427,9 @@ tree
 build_type_constraint (tree decl, tree args, tsubst_flags_t complain)
 {
   tree wildcard = build_nt (WILDCARD_DECL);
+  ++processing_template_decl;
   tree check = build_concept_check (decl, wildcard, args, complain);
+  --processing_template_decl;
   if (check == error_mark_node)
     return error_mark_node;
   return unpack_concept_check (check);
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index fc4b9bb7c7f..9a83810b67c 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -27928,7 +27928,9 @@ make_constrained_placeholder_type (tree type, tree con, tree args)
   tree expr = tmpl;
   if (TREE_CODE (con) == FUNCTION_DECL)
     expr = ovl_make (tmpl);
+  ++processing_template_decl;
   expr = build_concept_check (expr, type, args, tf_warning_or_error);
+  --processing_template_decl;
 
   PLACEHOLDER_TYPE_CONSTRAINTS (type) = expr;
 
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-defarg2.C b/gcc/testsuite/g++.dg/cpp2a/concepts-defarg2.C
new file mode 100644
index 00000000000..a63ca4e133d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-defarg2.C
@@ -0,0 +1,13 @@
+// PR c++/97052
+// { dg-do compile { target c++20 } }
+
+template<typename T, typename U = typename T::type>
+concept C = true;
+
+constexpr bool f(C auto) {
+  return true;
+}
+
+static_assert(f(0));
+
+C auto x = 0;