[Bug c++/95468] [8/9/10/11 Regression] ICE in expression sfinae

["ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95468

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
The reason we fail to compile the testcase when 'condition' is at class scope
instead of at namespace scope is because in the former case, the template
argument 'c::condition<bool(true)>' is a CALL_EXPR to a BASELINK, and when
doing instantiate_non_dependent_expr_internal on this CALL_EXPR, we don't
semantically process the template argument 'bool(true)' within BASELINK because
we never call tsubst_baselink because tsubst_copy exits early when args is
NULL_TREE.

We don't have this problem in the latter case where 'condition' is at namespace
scope, because then the template argument 'condition<bool(true)>' is a
CALL_EXPR to a TEMPLATE_ID_EXPR, and TEMPLATE_ID_EXPRs are handled directly
from tsubst_copy_and_build which doesn't do an early exit when args is
NULL_TREE.

A potential untested fix therefore is to also handle BASELINK directly from
tsubst_copy_and_build:

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c                                          
index d7f61289621..e15fca54823 100644                                           
--- a/gcc/cp/pt.c                                                               
+++ b/gcc/cp/pt.c                                                               
@@ -19472,6 +19472,11 @@ tsubst_copy_and_build (tree t,                         
     case SCOPE_REF:                                                            
       RETURN (tsubst_qualified_id (t, args, complain, in_decl, /*done=*/true,  
                                  /*address_p=*/false));                        
+                                                                               
+    case BASELINK:                                                             
+      RETURN (tsubst_baselink (t, current_nonlambda_class_type (),             
+                              args, complain, in_decl));                       
+                                                                               
     case ARRAY_REF:                                                            
       op1 = tsubst_non_call_postfix_expression (TREE_OPERAND (t, 0),           
                                                args, complain, in_decl);       

Ideally we might want two versions of tsubst_baselink, one that does template
argument substitution and another than additionally performs semantic
processing?