Re: [PATCH] c++: ICE during constrained template friend matching [PR105064]

[Jason Merrill via Gcc-patches]

On 3/28/22 09:29, Patrick Palka wrote:

Here during declaration matching for the two constrained template
friends, we crash from maybe_substitute_reqs_for because the second
friend doesn't yet have DECL_TEMPLATE_INFO set (we're being called
indirectly from push_template_decl).

As far as I can tell, this situation happens only when declaring a
constrained template friend within a non-template class (as in the
testcase), in which case the substitution would be a no-op anyway, so it
should be safe to just skip it.  To that end this patch rearranges
maybe_substitute_reqs_for to gracefully handle DECL_TEMPLATE_INFO being
empty.

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

OK.

        PR c++/105064

gcc/cp/ChangeLog:

        * constraint.cc (maybe_substitute_reqs_for): Don't assume
        DECL_TEMPLATE_INFO is set.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp2a/concepts-friend9.C: New test.
---
  gcc/cp/constraint.cc                          | 13 ++++---------
  gcc/testsuite/g++.dg/cpp2a/concepts-friend9.C | 12 ++++++++++++
  2 files changed, 16 insertions(+), 9 deletions(-)
  create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-friend9.C

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index e14578b1721..c5a991b9e71 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -1268,20 +1268,15 @@ remove_constraints (tree t)
     for declaration matching.  */

tree

-maybe_substitute_reqs_for (tree reqs, const_tree decl_)
+maybe_substitute_reqs_for (tree reqs, const_tree decl)
  {
    if (reqs == NULL_TREE)
      return NULL_TREE;

- tree decl = CONST_CAST_TREE (decl_);

-  tree result = STRIP_TEMPLATE (decl);
-
-  if (DECL_UNIQUE_FRIEND_P (result))
+  decl = STRIP_TEMPLATE (decl);
+  if (DECL_UNIQUE_FRIEND_P (decl) && DECL_TEMPLATE_INFO (decl))
      {
-      tree tmpl = decl;
-      if (TREE_CODE (decl) != TEMPLATE_DECL)
-       tmpl = DECL_TI_TEMPLATE (result);
+      tree tmpl = DECL_TI_TEMPLATE (decl);
        tree gargs = generic_targs_for (tmpl);
        processing_template_decl_sentinel s;
        if (uses_template_parms (gargs))
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-friend9.C 
b/gcc/testsuite/g++.dg/cpp2a/concepts-friend9.C
new file mode 100644
index 00000000000..09054d23d5d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-friend9.C
@@ -0,0 +1,12 @@
+// PR c++/105064
+// { dg-do compile { target c++20 } }
+
+struct A {
+  template<class T>
+  friend void f(T) requires true;
+};
+
+struct B {
+  template<class T>
+  friend void f(T) requires true;
+};