c++: wrong error with static constexpr var in tmpl [PR109876]
Commit Message
Since r8-509, we'll no longer create a static temporary var for
the initializer '{ 1, 2 }' for num in the attached test because
the code in finish_compound_literal is now guarded by
'&& fcl_context == fcl_c99' but it's fcl_functional here. This
causes us to reject num as non-constant when evaluating it in
a template.
Jason's idea was to treat num as value-dependent even though it
actually isn't. This patch implements that suggestion.
The is_really_empty_class check is sort of non-obvious but the
comment should explain why I added it.
Co-authored-by: Jason Merrill <jason@redhat.com>
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
PR c++/109876
gcc/cp/ChangeLog:
* pt.cc (value_dependent_expression_p) <case VAR_DECL>: Treat a
constexpr-declared non-constant variable as value-dependent.
gcc/testsuite/ChangeLog:
* g++.dg/cpp0x/constexpr-template12.C: New test.
* g++.dg/cpp1z/constexpr-template1.C: New test.
---
gcc/cp/pt.cc | 12 ++++++
.../g++.dg/cpp0x/constexpr-template12.C | 38 +++++++++++++++++++
.../g++.dg/cpp1z/constexpr-template1.C | 25 ++++++++++++
3 files changed, 75 insertions(+)
create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-template12.C
create mode 100644 gcc/testsuite/g++.dg/cpp1z/constexpr-template1.C
base-commit: 8d6bd830f5f9c939e8565c0341a0c6c588834484
Comments
On 5/26/23 19:18, Marek Polacek wrote:
> Since r8-509, we'll no longer create a static temporary var for
> the initializer '{ 1, 2 }' for num in the attached test because
> the code in finish_compound_literal is now guarded by
> '&& fcl_context == fcl_c99' but it's fcl_functional here. This
> causes us to reject num as non-constant when evaluating it in
> a template.
>
> Jason's idea was to treat num as value-dependent even though it
> actually isn't. This patch implements that suggestion.
>
> The is_really_empty_class check is sort of non-obvious but the
> comment should explain why I added it.
>
> Co-authored-by: Jason Merrill <jason@redhat.com>
>
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>
> PR c++/109876
>
> gcc/cp/ChangeLog:
>
> * pt.cc (value_dependent_expression_p) <case VAR_DECL>: Treat a
> constexpr-declared non-constant variable as value-dependent.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp0x/constexpr-template12.C: New test.
> * g++.dg/cpp1z/constexpr-template1.C: New test.
> ---
> gcc/cp/pt.cc | 12 ++++++
> .../g++.dg/cpp0x/constexpr-template12.C | 38 +++++++++++++++++++
> .../g++.dg/cpp1z/constexpr-template1.C | 25 ++++++++++++
> 3 files changed, 75 insertions(+)
> create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-template12.C
> create mode 100644 gcc/testsuite/g++.dg/cpp1z/constexpr-template1.C
>
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 7fb3e75bceb..38fd8070705 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -27969,6 +27969,18 @@ value_dependent_expression_p (tree expression)
> else if (TYPE_REF_P (TREE_TYPE (expression)))
> /* FIXME cp_finish_decl doesn't fold reference initializers. */
> return true;
> + /* We have a constexpr variable and we're processing a template. When
> + there's lifetime extension involved (for which finish_compound_literal
> + used to create a temporary), we'll not be able to evaluate the
> + variable until instantiating, so pretend it's value-dependent. */
> + else if (DECL_DECLARED_CONSTEXPR_P (expression)
> + && !TREE_CONSTANT (expression)
> + /* When there's nothing to initialize, we'll never mark the
> + VAR_DECL TREE_CONSTANT, therefore it would remain
> + value-dependent and we wouldn't instantiate. */
Interesting. Can we change that (i.e. mark it TREE_CONSTANT) rather
than work around it here?
> + && !is_really_empty_class (TREE_TYPE (expression),
> + /*ignore_vptr*/false))
> + return true;
> return false;
>
> case DYNAMIC_CAST_EXPR:
> diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-template12.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-template12.C
> new file mode 100644
> index 00000000000..a9e065320c8
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-template12.C
> @@ -0,0 +1,38 @@
> +// PR c++/109876
> +// { dg-do compile { target c++11 } }
> +
> +using size_t = decltype(sizeof 0);
> +
> +namespace std {
> +template <class> struct initializer_list {
> + const int *_M_array;
> + size_t _M_len;
> + constexpr size_t size() const { return _M_len; }
> +};
> +} // namespace std
> +
> +constexpr std::initializer_list<int> gnum{2};
> +
> +template <int> struct Array {};
> +template <int> void g()
> +{
> + static constexpr std::initializer_list<int> num{2};
> + static_assert(num.size(), "");
> + Array<num.size()> ctx;
> +
> + constexpr Array<1> num1{};
> +}
> +
> +template <int N>
> +struct Foo
> +{
> + static constexpr std::initializer_list<int> num = { 1, 2 };
> + static_assert(num.size(), "");
> + Array<num.size()> ctx;
> +};
> +
> +void
> +f (Foo<5>)
> +{
> + g<0>();
> +}
> diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-template1.C b/gcc/testsuite/g++.dg/cpp1z/constexpr-template1.C
> new file mode 100644
> index 00000000000..58be046fd36
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-template1.C
> @@ -0,0 +1,25 @@
> +// PR c++/109876
> +// { dg-do compile { target c++17 } }
> +
> +struct Foo {};
> +template <const Foo&> struct X {};
> +
> +void g()
> +{
> + static constexpr Foo foo;
> + X<foo> x;
> +}
> +
> +template<int>
> +void f()
> +{
> + static constexpr Foo foo;
> + X<foo> x;
> +}
> +
> +void
> +h ()
> +{
> + f<0>();
> + f<1>();
> +}
>
> base-commit: 8d6bd830f5f9c939e8565c0341a0c6c588834484
@@ -27969,6 +27969,18 @@ value_dependent_expression_p (tree expression)
else if (TYPE_REF_P (TREE_TYPE (expression)))
/* FIXME cp_finish_decl doesn't fold reference initializers. */
return true;
+ /* We have a constexpr variable and we're processing a template. When
+ there's lifetime extension involved (for which finish_compound_literal
+ used to create a temporary), we'll not be able to evaluate the
+ variable until instantiating, so pretend it's value-dependent. */
+ else if (DECL_DECLARED_CONSTEXPR_P (expression)
+ && !TREE_CONSTANT (expression)
+ /* When there's nothing to initialize, we'll never mark the
+ VAR_DECL TREE_CONSTANT, therefore it would remain
+ value-dependent and we wouldn't instantiate. */
+ && !is_really_empty_class (TREE_TYPE (expression),
+ /*ignore_vptr*/false))
+ return true;
return false;
case DYNAMIC_CAST_EXPR:
new file mode 100644
@@ -0,0 +1,38 @@
+// PR c++/109876
+// { dg-do compile { target c++11 } }
+
+using size_t = decltype(sizeof 0);
+
+namespace std {
+template <class> struct initializer_list {
+ const int *_M_array;
+ size_t _M_len;
+ constexpr size_t size() const { return _M_len; }
+};
+} // namespace std
+
+constexpr std::initializer_list<int> gnum{2};
+
+template <int> struct Array {};
+template <int> void g()
+{
+ static constexpr std::initializer_list<int> num{2};
+ static_assert(num.size(), "");
+ Array<num.size()> ctx;
+
+ constexpr Array<1> num1{};
+}
+
+template <int N>
+struct Foo
+{
+ static constexpr std::initializer_list<int> num = { 1, 2 };
+ static_assert(num.size(), "");
+ Array<num.size()> ctx;
+};
+
+void
+f (Foo<5>)
+{
+ g<0>();
+}
new file mode 100644
@@ -0,0 +1,25 @@
+// PR c++/109876
+// { dg-do compile { target c++17 } }
+
+struct Foo {};
+template <const Foo&> struct X {};
+
+void g()
+{
+ static constexpr Foo foo;
+ X<foo> x;
+}
+
+template<int>
+void f()
+{
+ static constexpr Foo foo;
+ X<foo> x;
+}
+
+void
+h ()
+{
+ f<0>();
+ f<1>();
+}