[gcc r10-9636] c++: Fix folding of non-dependent BASELINKs [PR95468]

[Patrick Palka]

https://gcc.gnu.org/g:78e6c55b0d0d2d49f489c581cf8d5a8125b28563

commit r10-9636-g78e6c55b0d0d2d49f489c581cf8d5a8125b28563
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Feb 23 09:40:09 2021 -0500

    c++: Fix folding of non-dependent BASELINKs [PR95468]
    
    Here, the problem ultimately seems to be that tsubst_copy_and_build,
    when called with empty args as we do during non-dependent expression
    folding, doesn't touch BASELINKs at all: it delegates to tsubst_copy
    which then immediately exits early due to the empty args.  This means
    that the CAST_EXPR int(1) in the BASELINK A::condition<int(1)> never
    gets folded (as part of folding of the overall CALL_EXPR), which later
    causes us to crash when performing overload resolution of the rebuilt
    CALL_EXPR (which is still in terms of this templated BASELINK).
    
    This doesn't happen when condition() is a namespace-scope function
    because then condition<int(1)> is represented by a TEMPLATE_ID_EXPR
    rather than by a BASELINK, which does get handled directly from
    tsubst_copy_and_build.
    
    This patch fixes this issue by having tsubst_copy_and_build handle
    BASELINK directly rather than delegating to tsubst_copy, so that it
    processes BASELINKs even when args is empty.
    
    gcc/cp/ChangeLog:
    
            PR c++/95468
            * pt.c (tsubst_copy_and_build) <case BASELINK>: New case, copied
            over from tsubst_copy.
    
    gcc/testsuite/ChangeLog:
    
            PR c++/95468
            * g++.dg/template/non-dependent15.C: New test.
    
    (cherry picked from commit 5bd7afb71fca3a5a6e9f8586d86903bae1849193)

Diff:
---
 gcc/cp/pt.c                                     |  5 +++++
 gcc/testsuite/g++.dg/template/non-dependent15.C | 12 ++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index e74e45b64c3..a6ea6737d99 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -19624,6 +19624,11 @@ 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);
diff --git a/gcc/testsuite/g++.dg/template/non-dependent15.C b/gcc/testsuite/g++.dg/template/non-dependent15.C
new file mode 100644
index 00000000000..00dfe26d6ba
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/non-dependent15.C
@@ -0,0 +1,12 @@
+// PR c++/95468
+// { dg-do compile { target c++11 } }
+
+struct A {
+  template <int N>
+  static constexpr int condition() { return N; }
+};
+
+template <int> struct B {};
+
+template <class>
+using T = B<A::condition<int(1)>()>;