From e6fd02cc6d874c523466250a1cb724e0c7af9d75 Mon Sep 17 00:00:00 2001 From: Marek Polacek Date: Tue, 3 Nov 2020 15:10:31 -0500 Subject: [PATCH] c++: Fix decltype(auto) deduction with rvalue ref [PR78209] Here's a small deficiency in decltype(auto). [dcl.type.auto.deduct]/5: If the placeholder-type-specifier is of the form decltype(auto), [...] the type deduced for T is determined [...] as though E had been the operand of the decltype. So: int &&i = 0; decltype(auto) j = i; // should behave like int &&j = i; error We deduce j's type in do_auto_deduction via finish_decltype_type which takes an 'id' argument. Currently we compute 'id' as false, because stripped_init is *i (a REFERENCE_REF_P). But it seems to me we should rather set 'id' to true here, by looking through the REFERENCE_REF_P, so that finish_decltype_type DTRT. gcc/cp/ChangeLog: PR c++/78209 * pt.c (do_auto_deduction): If init is REFERENCE_REF_P, use its first operand. gcc/testsuite/ChangeLog: PR c++/78209 * g++.dg/cpp1y/decltype-auto1.C: New test. --- gcc/cp/pt.c | 2 ++ gcc/testsuite/g++.dg/cpp1y/decltype-auto1.C | 8 ++++++++ 2 files changed, 10 insertions(+) create mode 100644 gcc/testsuite/g++.dg/cpp1y/decltype-auto1.C diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c index f401c75b9e5..c033a286407 100644 --- a/gcc/cp/pt.c +++ b/gcc/cp/pt.c @@ -29278,6 +29278,8 @@ do_auto_deduction (tree type, tree init, tree auto_node, else if (AUTO_IS_DECLTYPE (auto_node)) { tree stripped_init = tree_strip_any_location_wrapper (init); + if (REFERENCE_REF_P (stripped_init)) + stripped_init = TREE_OPERAND (stripped_init, 0); bool id = (DECL_P (stripped_init) || ((TREE_CODE (init) == COMPONENT_REF || TREE_CODE (init) == SCOPE_REF) diff --git a/gcc/testsuite/g++.dg/cpp1y/decltype-auto1.C b/gcc/testsuite/g++.dg/cpp1y/decltype-auto1.C new file mode 100644 index 00000000000..13baf8eba06 --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp1y/decltype-auto1.C @@ -0,0 +1,8 @@ +// PR c++/78209 +// { dg-do compile { target c++14 } } + +int main() +{ + int &&i = 0; + decltype(auto) j = i; // { dg-error "cannot bind rvalue reference" } +} -- 2.30.2