Ping.
On Wed, Mar 20, 2019 at 04:12:13PM -0400, Marek Polacek wrote:
> The fix for 77656 caused us to call convert_nontype_argument even for
> value-dependent arguments, to perform the conversion in order to avoid
> a bogus warning.
>
> In this case, the argument is Pod{N}. The call to
> build_converted_constant_expr
> in convert_nontype_argument produces Pod::operator Enum(&{N}). It doesn't
> crash
> because we're in a template and build_address no longer crashes on
> CONSTRUCTORs
> in a template.
>
> Then when instantiating the function foo we substitute its argument: &{N}. So
> we're in tsubst_copy_and_build/ADDR_EXPR. The call to
> tsubst_non_call_postfix_expression turns {N} into TARGET_EXPR <D.2329,
> {.val=2}>.
> Then build_x_unary_op is supposed to put the ADDR_EXPR back. It calls
> cp_build_addr_expr_strict. But it's *strict*, so the prvalue of class type
> TARGET_EXPR <D.2329, {.val=2}> isn't allowed -> error.
>
> It's _strict since <https://gcc.gnu.org/ml/gcc-patches/2010-09/msg02144.html>,
> that seem like a desirable change, and we had a warning for taking the address
> of a TARGET_EXPR in build_x_unary_op even before that.
>
> So rather than messing with _strict, let's avoid this scenario altogether.
> I checked whether we have a case in the testsuite that results in convert_like
> getting a value-dependent CONSTRUCTOR, but found none.
>
> With this patch, we avoid it, and only call convert_nontype_argument after
> substitution, at which point maybe_constant_value will be able to evaluate
> the conversion to a constant.
>
> This problem doesn't occur when passing Pod{N} as an argument to a function,
> or using it as an array dimension; seems we avoid converting the argument if
> it's value-dependent.
>
> Bootstrapped/regtested on x86_64-linux, ok for trunk?
>
> 2019-03-20 Marek Polacek <pola...@redhat.com>
>
> PR c++/87145 - bogus error converting class type in template arg list.
> * pt.c (convert_template_argument): Don't call convert_nontype_argument
> if it could involve calling a conversion function with a value-dependent
> constructor as its argument.
>
> * g++.dg/cpp0x/constexpr-conv3.C: New test.
> * g++.dg/cpp0x/constexpr-conv4.C: New test.
>
> diff --git gcc/cp/pt.c gcc/cp/pt.c
> index 0acc16d1b92..6878583d99b 100644
> --- gcc/cp/pt.c
> +++ gcc/cp/pt.c
> @@ -8056,7 +8056,16 @@ convert_template_argument (tree parm,
> t = canonicalize_type_argument (t, complain);
>
> if (!type_dependent_expression_p (orig_arg)
> - && !uses_template_parms (t))
> + && !uses_template_parms (t)
> + /* This might trigger calling a conversion function with
> + a value-dependent argument, which could invoke taking
> + the address of a temporary representing the result of
> + the conversion. */
> + && !(COMPOUND_LITERAL_P (orig_arg)
> + && MAYBE_CLASS_TYPE_P (TREE_TYPE (orig_arg))
> + && TYPE_HAS_CONVERSION (TREE_TYPE (orig_arg))
> + && INTEGRAL_OR_ENUMERATION_TYPE_P (t)
> + && value_dependent_expression_p (orig_arg)))
> /* We used to call digest_init here. However, digest_init
> will report errors, which we don't want when complain
> is zero. More importantly, digest_init will try too
> @@ -8092,7 +8101,7 @@ convert_template_argument (tree parm,
> && TREE_CODE (TREE_TYPE (innertype)) == FUNCTION_TYPE
> && TREE_OPERAND_LENGTH (inner) > 0
> && reject_gcc_builtin (TREE_OPERAND (inner, 0)))
> - return error_mark_node;
> + return error_mark_node;
> }
>
> if (TREE_CODE (val) == SCOPE_REF)
> diff --git gcc/testsuite/g++.dg/cpp0x/constexpr-conv3.C
> gcc/testsuite/g++.dg/cpp0x/constexpr-conv3.C
> new file mode 100644
> index 00000000000..3f47c58cd2a
> --- /dev/null
> +++ gcc/testsuite/g++.dg/cpp0x/constexpr-conv3.C
> @@ -0,0 +1,25 @@
> +// PR c++/87145
> +// { dg-do compile { target c++11 } }
> +
> +template<typename T, T t> struct integral_constant {
> + static constexpr T value = t;
> +};
> +
> +enum class Enum : unsigned {};
> +
> +struct Pod {
> + unsigned val;
> +
> + constexpr operator Enum() const {
> + return static_cast<Enum>(val);
> + }
> +};
> +
> +template<unsigned N>
> +constexpr void foo() {
> + using Foo = integral_constant<Enum, Pod{N}>;
> +}
> +
> +int main() {
> + foo<2>();
> +}
> diff --git gcc/testsuite/g++.dg/cpp0x/constexpr-conv4.C
> gcc/testsuite/g++.dg/cpp0x/constexpr-conv4.C
> new file mode 100644
> index 00000000000..f4e3f00a585
> --- /dev/null
> +++ gcc/testsuite/g++.dg/cpp0x/constexpr-conv4.C
> @@ -0,0 +1,25 @@
> +// PR c++/87145
> +// { dg-do compile { target c++11 } }
> +
> +struct S {
> + int val;
> +
> + constexpr operator int() const {
> + return static_cast<int>(val);
> + }
> +};
> +
> +template<int N>
> +struct F { };
> +
> +template<unsigned N>
> +constexpr void foo() {
> + F<int{N}> f;
> + F<S{N}> f2;
> +}
> +
> +int
> +main()
> +{
> + foo<2>();
> +}
Marek
