[gcc r12-7860] c++: constrained template friend matching ICE [PR105064]

[gcc r12-7860] c++: constrained template friend matching ICE [PR105064]

[Patrick Palka]

https://gcc.gnu.org/g:ecb4882e362e80a1bf172453ac9b366edbb4e89c

commit r12-7860-gecb4882e362e80a1bf172453ac9b366edbb4e89c
Author: Patrick Palka <ppalka@redhat.com>
Date:   Mon Mar 28 14:15:16 2022 -0400

    c++: constrained template friend matching ICE [PR105064]
    
    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 this patch rearranges maybe_substitute_reqs_for to gracefully
    handle missing DECL_TEMPLATE_INFO by just skipping the substitution.
    
            PR c++/105064
    
    gcc/cp/ChangeLog:
    
            * constraint.cc (maybe_substitute_reqs_for): Don't assume
            DECL_TEMPLATE_INFO is available.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp2a/concepts-friend9.C: New test.

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

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;
+};