[gcc r13-4390] c++: explicit specialization and trailing requirements [PR107864]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:36cabc257dfb7dd4f7625896891f6c5b195a0241

commit r13-4390-g36cabc257dfb7dd4f7625896891f6c5b195a0241
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Nov 29 09:55:21 2022 -0500

    c++: explicit specialization and trailing requirements [PR107864]
    
    Here we're crashing when using the explicit specialization of the
    function template g with trailing requirements ultimately because
    earlier decls_match (called indirectly from register_specialization) for
    for the explicit specialization returned false since the template has
    trailing requirements whereas the specialization doesn't.
    
    In r12-2230-gddd25bd1a7c8f4, we fixed a similar issue concerning template
    requirements instead of trailing requirements.  We could extend that fix
    to ignore trailing requirement mismatches for explicit specializations
    as well, but it seems cleaner to just propagate constraints from the
    specialized template to the specialization when declaring an explicit
    specialization so that decls_match will naturally return true in this
    case.  And it looks like determine_specialization already does this,
    albeit inconsistently (only when specializing a non-template member
    function of a class template as in cpp2a/concepts-explicit-spec4.C).
    
    So this patch makes determine_specialization consistently propagate
    constraints from the specialized template to the specialization, which
    in turn lets us get rid of the function_requirements_equivalent_p special
    case added by r12-2230.
    
            PR c++/107864
    
    gcc/cp/ChangeLog:
    
            * decl.cc (function_requirements_equivalent_p): Don't check
            DECL_TEMPLATE_SPECIALIZATION.
            * pt.cc (determine_specialization): Propagate constraints when
            specializing a function template too.  Simplify by using
            add_outermost_template_args.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/concepts/explicit-spec1a.C: New test.

Diff:
---
 gcc/cp/decl.cc                                  |  4 +---
 gcc/cp/pt.cc                                    | 19 ++++++++-----------
 gcc/testsuite/g++.dg/concepts/explicit-spec1a.C | 11 +++++++++++
 3 files changed, 20 insertions(+), 14 deletions(-)

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 544efdc9914..238e72f90da 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -956,9 +956,7 @@ static bool
 function_requirements_equivalent_p (tree newfn, tree oldfn)
 {
   /* In the concepts TS, the combined constraints are compared.  */
-  if (cxx_dialect < cxx20
-      && (DECL_TEMPLATE_SPECIALIZATION (newfn)
-	  <= DECL_TEMPLATE_SPECIALIZATION (oldfn)))
+  if (cxx_dialect < cxx20)
     {
       tree ci1 = get_constraints (oldfn);
       tree ci2 = get_constraints (newfn);
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index fbf498ad16a..2d8e4fdd4b5 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -2482,17 +2482,14 @@ determine_specialization (tree template_id,
     }
 
   /* It was a specialization of a template.  */
-  targs = DECL_TI_ARGS (DECL_TEMPLATE_RESULT (TREE_VALUE (templates)));
-  if (TMPL_ARGS_HAVE_MULTIPLE_LEVELS (targs))
-    {
-      *targs_out = copy_node (targs);
-      SET_TMPL_ARGS_LEVEL (*targs_out,
-			   TMPL_ARGS_DEPTH (*targs_out),
-			   TREE_PURPOSE (templates));
-    }
-  else
-    *targs_out = TREE_PURPOSE (templates);
-  return TREE_VALUE (templates);
+  tree tmpl = TREE_VALUE (templates);
+  *targs_out = add_outermost_template_args (tmpl, TREE_PURPOSE (templates));
+
+  /* Propagate the template's constraints to the declaration.  */
+  if (tsk != tsk_template)
+    set_constraints (decl, get_constraints (tmpl));
+
+  return tmpl;
 }
 
 /* Returns a chain of parameter types, exactly like the SPEC_TYPES,
diff --git a/gcc/testsuite/g++.dg/concepts/explicit-spec1a.C b/gcc/testsuite/g++.dg/concepts/explicit-spec1a.C
new file mode 100644
index 00000000000..ec678740cb8
--- /dev/null
+++ b/gcc/testsuite/g++.dg/concepts/explicit-spec1a.C
@@ -0,0 +1,11 @@
+// A version of explicit-spec1.C where the template g has trailing instead of
+// template requirements.
+// PR c++/107864
+// { dg-do compile { target concepts } }
+
+template<typename T> concept C = __is_class(T);
+struct Y { int n; } y;
+template<class T> void g(T) requires C<T> { }
+int called;
+template<> void g(Y) { called = 3; }
+int main() { g(y); }