[gcc r11-9350] c++: Fix bogus error with __integer_pack [PR94490]

[gcc r11-9350] c++: Fix bogus error with __integer_pack [PR94490]

[Marek Polacek]

https://gcc.gnu.org/g:5746f9199c2b8f0d679f13e3a48d6f8546a974f6

commit r11-9350-g5746f9199c2b8f0d679f13e3a48d6f8546a974f6
Author: Marek Polacek <polacek@redhat.com>
Date:   Sat Nov 6 18:10:39 2021 -0400

    c++: Fix bogus error with __integer_pack [PR94490]
    
    Here we issue a bogus:
    
    error: '(0 ? fake_tuple_size_v<int> : fake_tuple_size_v<int>)' is not a constant expression
    
    because cxx_constant_value in expand_integer_pack gets
    
    *(0 ? VIEW_CONVERT_EXPR<const int>(fake_tuple_size_v) : VIEW_CONVERT_EXPR<const int>(fake_tuple_size_v))
    
    which is a REFERENCE_REF_P and we evaluate its operand to 3, so we end
    up with *3 and that fails.  Sounds like we need to get rid of the
    REFERENCE_REF_P then.  That is what tsubst_copy_and_build/INDIRECT_REF
    will do:
    
            if (REFERENCE_REF_P (t))
              {
                /* A type conversion to reference type will be enclosed in
                   such an indirect ref, but the substitution of the cast
                   will have also added such an indirect ref.  */
                r = convert_from_reference (r);
              }
    
    so I think it's reasonable to call instantiate_non_dependent_expr_sfinae.
    
            PR c++/94490
    
    gcc/cp/ChangeLog:
    
            * pt.c (expand_integer_pack): Call
            instantiate_non_dependent_expr_sfinae.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/ext/integer-pack5.C: New test.
    
    (cherry picked from commit 9af081003f9c19f33457e0ed1aa14a764f462c3c)

Diff:
---
 gcc/cp/pt.c                              |  1 +
 gcc/testsuite/g++.dg/ext/integer-pack5.C | 29 +++++++++++++++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 5454bfca827..6ff8d7840f8 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -3820,6 +3820,7 @@ expand_integer_pack (tree call, tree args, tsubst_flags_t complain,
     }
   else
     {
+      hi = instantiate_non_dependent_expr_sfinae (hi, complain);
       hi = cxx_constant_value (hi);
       int len = valid_constant_size_p (hi) ? tree_to_shwi (hi) : -1;
 
diff --git a/gcc/testsuite/g++.dg/ext/integer-pack5.C b/gcc/testsuite/g++.dg/ext/integer-pack5.C
new file mode 100644
index 00000000000..84938649d31
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/integer-pack5.C
@@ -0,0 +1,29 @@
+// PR c++/94490
+// { dg-do compile { target c++14 } }
+
+template<class T>
+constexpr int fake_tuple_size_v = 3;
+template<int...> struct intseq {};
+
+// So that it compiles with clang++.
+#if __has_builtin(__make_integer_seq)
+using size_t = decltype(sizeof(1));
+template<typename, size_t... _Indices>
+using _IdxTuple = intseq<_Indices...>;
+
+template<int N> using genseq = __make_integer_seq<_IdxTuple, size_t, N>;
+#else
+template<int N> using genseq = intseq<__integer_pack(N)...>;
+#endif
+
+template<int A, class S = genseq<0 ? A : A>>
+struct arith_result
+{ };
+
+template<typename T>
+auto Mul(const T&)
+{
+    return [](auto) { return arith_result<fake_tuple_size_v<T>> { }; }(0);
+}
+
+auto x = Mul(0);