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c++: constexpr, empty base after non-empty [PR106369]

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Here the CONSTRUCTOR we were providing for D{} had an entry for the B base
subobject at offset 0 following the entry for the C base, causing
output_constructor_regular_field to ICE due to going backwards.  It might be
nice for that function to be more tolerant of empty fields, but it also
seems reasonable for the front end to prune the useless entry.

	PR c++/106369

gcc/cp/ChangeLog:

	* constexpr.cc (reduced_constant_expression_p): Return false
	if a CONSTRUCTOR initializes an empty field.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/constexpr-lambda27.C: New test.
This commit is contained in:
Jason Merrill 2022-07-26 11:02:21 -04:00
parent 9ef2c9aa5b
commit 9efe4e153d
2 changed files with 33 additions and 1 deletions
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8
gcc/cp/constexpr.cc
View file

@ -3081,7 +3081,13 @@ reduced_constant_expression_p (tree t)
element. */
if (!reduced_constant_expression_p (e.value))
return false;
/* Empty class field may or may not have an initializer. */
/* We want to remove initializers for empty fields in a struct to
avoid confusing output_constructor. */
if (is_empty_field (e.index)
&& TREE_CODE (TREE_TYPE (t)) == RECORD_TYPE)
return false;
/* Check for non-empty fields between initialized fields when
CONSTRUCTOR_NO_CLEARING. */
for (; field && e.index != field;
field = next_subobject_field (DECL_CHAIN (field)))
if (!is_really_empty_class (TREE_TYPE (field),

26
gcc/testsuite/g++.dg/cpp1z/constexpr-lambda27.C Normal file
View file

@ -0,0 +1,26 @@
// PR c++/106369
// { dg-do compile { target c++17 } }
struct A {
int a[256];
constexpr int &operator[] (int n) noexcept { return a[n]; }
constexpr const int &operator[] (int n) const noexcept { return a[n]; }
};
struct B {};
template <typename T>
struct C {
constexpr T &foo (const char x) noexcept { c = T::d[x]; return static_cast<T &>(*this); }
int c;
};
struct D : public C<D>, public B
{
D () noexcept = default;
static constexpr char e[9] { 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I' };
static constexpr A d = [] () constexpr {
A f {};
for (int i = 0; i < 9; ++i)
f[e[i]] = 1;
return f;
} ();
};
constexpr auto g = D{}.foo ('E');