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c++: Don't reject calls through PMF during constant evaluation [PR102786]

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The following testcase incorrectly rejects the c initializer,
while in the s.*a case cxx_eval_* sees .__pfn reads etc.,
in the s.*&S::foo case get_member_function_from_ptrfunc creates
expressions which use INTEGER_CSTs with type of pointer to METHOD_TYPE.
And cxx_eval_constant_expression rejects any INTEGER_CSTs with pointer
type if they aren't 0.
Either we'd need to make sure we defer such folding till cp_fold but the
function and pfn_from_ptrmemfunc is used from lots of places, or
the following patch just tries to reject only non-zero INTEGER_CSTs
with pointer types if they don't point to METHOD_TYPE in the hope that
all such INTEGER_CSTs with POINTER_TYPE to METHOD_TYPE are result of
folding valid pointer-to-member function expressions.
I don't immediately see how one could create such INTEGER_CSTs otherwise,
cast of integers to PMF is rejected and would have the PMF RECORD_TYPE
anyway, etc.

2021-10-19  Jakub Jelinek  <jakub@redhat.com>

	PR c++/102786
	* constexpr.c (cxx_eval_constant_expression): Don't reject
	INTEGER_CSTs with type POINTER_TYPE to METHOD_TYPE.

	* g++.dg/cpp2a/constexpr-virtual19.C: New test.
This commit is contained in:
Jakub Jelinek 2021-10-19 09:24:57 +02:00
parent caab013976
commit f45610a452
2 changed files with 15 additions and 0 deletions
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4
gcc/cp/constexpr.c
View File

@ -6191,6 +6191,10 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
if (TREE_CODE (t) == INTEGER_CST
&& TYPE_PTR_P (TREE_TYPE (t))
/* INTEGER_CST with pointer-to-method type is only used
for a virtual method in a pointer to member function.
Don't reject those. */
&& TREE_CODE (TREE_TYPE (TREE_TYPE (t))) != METHOD_TYPE
&& !integer_zerop (t))
{
if (!ctx->quiet)

11
gcc/testsuite/g++.dg/cpp2a/constexpr-virtual19.C Normal file
View File

@ -0,0 +1,11 @@
// PR c++/102786
// { dg-do compile { target c++20 } }
struct S {
virtual constexpr int foo () const { return 42; }
};
constexpr S s;
constexpr auto a = &S::foo;
constexpr auto b = (s.*a) ();
constexpr auto c = (s.*&S::foo) ();