On Mon, Jan 04, 2021 at 04:01:25PM -0500, Jason Merrill via Gcc-patches wrote: > On 1/4/21 3:48 PM, Jakub Jelinek wrote: > > On Mon, Jan 04, 2021 at 03:44:46PM -0500, Jason Merrill wrote: > > > This change is OK, but part of the problem is that we're trying to do > > > overload resolution for an S copy/move constructor, which we shouldn't be > > > because bit_cast is a prvalue, so in C++17 and up we should use it to > > > directly initialize the target without any implied constructor call. > > > > > > It seems we're mishandling this because the code in > > > build_special_member_call specifically looks for TARGET_EXPR or > > > CONSTRUCTOR, > > > and BIT_CAST_EXPR is neither of those. > > > > > > Wrapping a BIT_CAST_EXPR of aggregate type in a TARGET_EXPR would address > > > this, and any other places that expect a class prvalue to come in the form > > > of a TARGET_EXPR. > > > > I can try that tomorrow. Won't that cause copying through extra temporary > > in some cases though, or is that guaranteed to be optimized? > > It won't cause any extra copying when it's used to initialize another object > (like the return value of std::bit_cast). Class prvalues are always > expressed with a TARGET_EXPR in the front end; the TARGET_EXPR melts away > when used as an initializer, it only creates a temporary when it's used in > another way.
Ok, this version wraps it into a TARGET_EXPR then, it alone fixes the bug, but I've kept the constexpr.c change too. Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk? 2021-01-05 Jakub Jelinek <ja...@redhat.com> PR c++/98469 * constexpr.c (cxx_eval_constant_expression) <case BIT_CAST_EXPR>: Punt if lval is true. * semantics.c (cp_build_bit_cast): Call get_target_expr_sfinae on the result if it has a class type. * g++.dg/cpp2a/bit-cast8.C: New test. * g++.dg/cpp2a/bit-cast9.C: New test. --- gcc/cp/constexpr.c.jj 2021-01-04 10:25:48.750121531 +0100 +++ gcc/cp/constexpr.c 2021-01-05 11:41:38.315032636 +0100 @@ -6900,6 +6900,15 @@ cxx_eval_constant_expression (const cons return t; case BIT_CAST_EXPR: + if (lval) + { + if (!ctx->quiet) + error_at (EXPR_LOCATION (t), + "address of a call to %qs is not a constant expression", + "__builtin_bit_cast"); + *non_constant_p = true; + return t; + } r = cxx_eval_bit_cast (ctx, t, non_constant_p, overflow_p); break; --- gcc/cp/semantics.c.jj 2021-01-04 10:25:48.489124486 +0100 +++ gcc/cp/semantics.c 2021-01-05 11:27:49.327372582 +0100 @@ -10761,6 +10761,10 @@ cp_build_bit_cast (location_t loc, tree tree ret = build_min (BIT_CAST_EXPR, type, arg); SET_EXPR_LOCATION (ret, loc); + + if (!processing_template_decl && CLASS_TYPE_P (type)) + ret = get_target_expr_sfinae (ret, complain); + return ret; } --- gcc/testsuite/g++.dg/cpp2a/bit-cast8.C.jj 2021-01-05 11:41:38.315032636 +0100 +++ gcc/testsuite/g++.dg/cpp2a/bit-cast8.C 2021-01-05 11:41:38.315032636 +0100 @@ -0,0 +1,11 @@ +// PR c++/98469 +// { dg-do compile { target c++20 } } +// { dg-options "-Wall" } + +struct S { int s; }; + +S +foo () +{ + return __builtin_bit_cast (S, 0); +} --- gcc/testsuite/g++.dg/cpp2a/bit-cast9.C.jj 2021-01-05 11:41:38.315032636 +0100 +++ gcc/testsuite/g++.dg/cpp2a/bit-cast9.C 2021-01-05 11:41:38.315032636 +0100 @@ -0,0 +1,15 @@ +// PR c++/98469 +// { dg-do compile { target c++20 } } +// { dg-options "-Wall" } + +template<typename T, typename F> +constexpr T +bit_cast (const F &f) noexcept +{ + return __builtin_bit_cast (T, f); +} +struct S { int s; }; +constexpr int foo (const S &x) { return x.s; } +constexpr int bar () { return foo (bit_cast<S> (0)); } +constexpr int x = bar (); +static_assert (!x); Jakub