Here we reject constexpr const int *p = nullptr; constexpr int *q = const_cast<int*>(p);
with "conversion of 'const int*' null pointer to 'int*' is not a constant expression", which seems bogus. This code has been rejected since r238909 which added the can_convert check when converting a null pointer. I'm not finding any standard rule that this check was supposed to enforce. The original discussion was here <https://gcc.gnu.org/legacy-ml/gcc-patches/2016-06/msg01447.html> and here <https://gcc.gnu.org/legacy-ml/gcc-patches/2016-07/msg00280.html>. Since can_convert never assumes a C-style cast, it rejects casting away constness as in the test above and in: constexpr int *q = (int *)(const int *) nullptr; Removing the check only breaks constexpr-nullptr-2.C by not giving any diagnostic for line 229: constexpr B *pb2 = static_cast<B*>(pa0); // { dg-error "not a constant expression" } but the cast seems to be valid: we do [expr.static.cast]/7, and [expr.const] only says that a reinterpreter_cast and converting from void* is invalid in constexpr. The can_convert check rejected convering from void *, but only when converting from a null pointer, so it's not good enough. So I've added a check to catch conversions from cv void*. I realize it's not a great time to be adding additional checking, but removing the can_convert check would then technically be a regression. (I could perhaps limit the new check to only trigger for integer_zerop and then remove it in GCC 12.) Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk? gcc/cp/ChangeLog: DR 1312 PR c++/99176 * constexpr.c (cxx_eval_constant_expression): Reject casting from void * as per DR 1312. Don't check can_convert. gcc/testsuite/ChangeLog: DR 1312 PR c++/99176 * g++.dg/cpp0x/constexpr-nullptr-2.C: Adjust dg-error. * g++.dg/cpp0x/constexpr-cast2.C: New test. * g++.dg/cpp0x/constexpr-cast3.C: New test. --- gcc/cp/constexpr.c | 49 ++++++++++++------- gcc/testsuite/g++.dg/cpp0x/constexpr-cast2.C | 16 ++++++ gcc/testsuite/g++.dg/cpp0x/constexpr-cast3.C | 14 ++++++ .../g++.dg/cpp0x/constexpr-nullptr-2.C | 4 +- 4 files changed, 64 insertions(+), 19 deletions(-) create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-cast2.C create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-cast3.C diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c index 377fe322ee8..adf575d3dc6 100644 --- a/gcc/cp/constexpr.c +++ b/gcc/cp/constexpr.c @@ -6653,6 +6653,37 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t, return t; } + /* [expr.const]: a conversion from type cv void* to a pointer-to-object + type cannot be part of a core constant expression as a resolution to + DR 1312. */ + if (TYPE_PTROB_P (type) + && TYPE_PTR_P (TREE_TYPE (op)) + && VOID_TYPE_P (TREE_TYPE (TREE_TYPE (op))) + /* Inside a call to std::construct_at or to + std::allocator<T>::{,de}allocate, we permit casting from void* + because that is compiler-generated code. */ + && !(ctx->call + && ctx->call->fundef + && (is_std_construct_at (ctx->call->fundef->decl) + || is_std_allocator_allocate (ctx->call->fundef->decl)))) + { + /* Likewise, don't error when casting from void* when OP is + &heap uninit and similar. */ + tree sop = tree_strip_nop_conversions (op); + if (TREE_CODE (sop) == ADDR_EXPR + && VAR_P (TREE_OPERAND (sop, 0)) + && DECL_ARTIFICIAL (TREE_OPERAND (sop, 0))) + /* OK */; + else + { + if (!ctx->quiet) + error_at (loc, "cast from %qT is not allowed", + TREE_TYPE (op)); + *non_constant_p = true; + return t; + } + } + if (TREE_CODE (op) == PTRMEM_CST && !TYPE_PTRMEM_P (type)) op = cplus_expand_constant (op); @@ -6671,26 +6702,10 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t, if (TYPE_REF_P (type)) { if (!ctx->quiet) - error_at (loc, - "dereferencing a null pointer"); + error_at (loc, "dereferencing a null pointer"); *non_constant_p = true; return t; } - else if (TYPE_PTR_P (TREE_TYPE (op))) - { - tree from = TREE_TYPE (op); - - if (!can_convert (type, from, tf_none)) - { - if (!ctx->quiet) - error_at (loc, - "conversion of %qT null pointer to %qT " - "is not a constant expression", - from, type); - *non_constant_p = true; - return t; - } - } } else { diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-cast2.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-cast2.C new file mode 100644 index 00000000000..b79e8a90131 --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-cast2.C @@ -0,0 +1,16 @@ +// DR 1312 - Simulated reinterpret_cast in constant expressions. +// PR c++/99176 +// { dg-do compile { target c++11 } } + +static int i; +constexpr void *vp0 = nullptr; +constexpr void *vpi = &i; +constexpr int *p1 = (int *) vp0; // { dg-error "cast from .void\\*. is not allowed" } +constexpr int *p2 = (int *) vpi; // { dg-error "cast from .void\\*. is not allowed" } +constexpr int *p3 = static_cast<int *>(vp0); // { dg-error "cast from .void\\*. is not allowed" } +constexpr int *p4 = static_cast<int *>(vpi); // { dg-error "cast from .void\\*. is not allowed" } +constexpr void *p5 = vp0; +constexpr void *p6 = vpi; + +constexpr int *pi = &i; +constexpr bool b = ((int *)(void *) pi == pi); // { dg-error "cast from .void\\*. is not allowed" } diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-cast3.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-cast3.C new file mode 100644 index 00000000000..a330a99f7de --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-cast3.C @@ -0,0 +1,14 @@ +// PR c++/99176 +// { dg-do compile { target c++11 } } + +constexpr const int *p = nullptr; +constexpr int *q1 = const_cast<int*>(p); +constexpr int *q2 = (int *)(const int *) nullptr; + +struct B { }; +struct D : B { }; +constexpr B *q3 = static_cast<B*>(nullptr); +constexpr D *pd = nullptr; +constexpr B *pb = nullptr; +constexpr B *q4 = static_cast<B*>(pd); +constexpr D *q5 = static_cast<D*>(pb); diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-nullptr-2.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-nullptr-2.C index afb4b37be5a..92f3bbdc0a6 100644 --- a/gcc/testsuite/g++.dg/cpp0x/constexpr-nullptr-2.C +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-nullptr-2.C @@ -163,7 +163,7 @@ constexpr const void *pv2 = pv0; constexpr void *pv3 = pv2; // { dg-error "invalid conversion|not a constant expression" } constexpr const void *pv4 = pv2; -constexpr X *px4 = pv0; // { dg-error "invalid conversion|not a constant expression" } +constexpr X *px4 = pv0; // { dg-error "cast from|invalid conversion|not a constant expression" } } @@ -226,7 +226,7 @@ constexpr A *pa3 = pd0; // { dg-error "ambiguous base" } constexpr A *pa4 = static_cast<A*>(pd0); // { dg-error "ambiguous base" } constexpr B *pb1 = pa0; // { dg-error "invalid conversion|not a constant expression" } -constexpr B *pb2 = static_cast<B*>(pa0); // { dg-error "not a constant expression" } +constexpr B *pb2 = static_cast<B*>(pa0); constexpr C *pc1 = pa0; // { dg-error "invalid conversion|not a constant expression" } constexpr D *pd1 = pa0; // { dg-error "ambiguous base|invalid conversion" } base-commit: 9a4eb720b343324f7f8fd2dceed5d0347e5a0153 -- 2.29.2
