https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116710
Bug ID: 116710
Summary: Deduction succeeds despite mismatch of reference kind
(lvalue versus rvalue)
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Keywords: rejects-valid
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: hstong at ca dot ibm.com
Target Milestone: ---
GCC allows template argument deduction to succeed even when deducing an lvalue
reference type (i.e., not a forwarding reference) from an rvalue reference
type.
As demonstrated by the case below, this can cause SFINAE to fail and trigger
errors outside the immediate context.
Note: The forwarding reference behaviour that allows a forwarding reference to
be deduced as an lvalue reference does not apply in the deduction context
involved below anyway.
MSVC has a similar bug.
Online compiler link: https://godbolt.org/z/8o48Pr6rW
### SOURCE (<stdin>):
template <typename T> struct A : T {};
template <typename T>
void f(void (*)(T &), typename A<T>::type * = 0);
void f(...);
void g(int &&);
void q() { f(g); }
### COMPILER INVOCATION:
g++ -fsyntax-only -std=c++11 -xc++ -
### EXPECTED OUTPUT:
(clean compile)
### ACTUAL OUTPUT:
<stdin>: In instantiation of 'struct A<int>':
<stdin>:4:6: required by substitution of 'template<class T> void f(void
(*)(T&), typename A<T>::type*) [with T = int]'
<stdin>:10:13: required from here
<stdin>:1:30: error: base type 'int' fails to be a struct or class type
### COMPILER VERSION INFO (g++ -v):
Using built-in specs.
COLLECT_GCC=/opt/wandbox/gcc-head/bin/g++
COLLECT_LTO_WRAPPER=/opt/wandbox/gcc-head/libexec/gcc/x86_64-pc-linux-gnu/15.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../source/configure --prefix=/opt/wandbox/gcc-head
--enable-languages=c,c++ --disable-multilib --without-ppl --without-cloog-ppl
--enable-checking=release --disable-nls --enable-lto
LDFLAGS=-Wl,-rpath,/opt/wandbox/gcc-head/lib,-rpath,/opt/wandbox/gcc-head/lib64,-rpath,/opt/wandbox/gcc-head/lib32
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 15.0.0 20240913 (experimental) (GCC)
