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c++: constexpr virtual and vbase thunk

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C++20 allows virtual functions to be constexpr.  I don't think that calling
through a pointer to a vbase subobject is supposed to work in a constant
expression, since an object with virtual bases can't be constant, but the
call shouldn't ICE.

gcc/cp/ChangeLog:

	* constexpr.c (cxx_eval_thunk_call): Error instead of ICE
	on vbase thunk to constexpr function.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/constexpr-virtual20.C: New test.
This commit is contained in:
Jason Merrill 2021-11-10 16:42:04 -05:00
parent adcfd2c45c
commit 267318a285
2 changed files with 32 additions and 5 deletions
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15
gcc/cp/constexpr.c
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@ -2246,15 +2246,20 @@ cxx_eval_thunk_call (const constexpr_ctx *ctx, tree t, tree thunk_fndecl,
{
tree function = THUNK_TARGET (thunk_fndecl);
/* virtual_offset is only set in the presence of virtual bases, which make
the class non-literal, so we don't need to handle it here. */
if (THUNK_VIRTUAL_OFFSET (thunk_fndecl))
{
gcc_assert (!DECL_DECLARED_CONSTEXPR_P (function));
if (!ctx->quiet)
{
error ("call to non-%<constexpr%> function %qD", function);
explain_invalid_constexpr_fn (function);
if (!DECL_DECLARED_CONSTEXPR_P (function))
{
error ("call to non-%<constexpr%> function %qD", function);
explain_invalid_constexpr_fn (function);
}
else
/* virtual_offset is only set for virtual bases, which make the
class non-literal, so we don't need to handle it here. */
error ("calling constexpr member function %qD through virtual "
"base subobject", function);
}
*non_constant_p = true;
return t;

22
gcc/testsuite/g++.dg/cpp2a/constexpr-virtual20.C Normal file
View file

@ -0,0 +1,22 @@
// Test for constexpr call through vbase thunk.
// { dg-do compile { target c++20 } }
class Rep {
public:
constexpr virtual int foo() { return 1; }
};
class VBase {
public:
constexpr virtual int foo() { return 2; }
};
class Main : public Rep, virtual public VBase {
public:
constexpr virtual int foo() { return 5; }
};
int main() {
Main m;
static_assert(static_cast<VBase*>(&m)->foo() == 5); // { dg-error "" }
}