[Bug c++/90508] New: GCC does not produce full template backtraces when instantiating but not calling virtual functions

[m101010a at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90508

            Bug ID: 90508
           Summary: GCC does not produce full template backtraces when
                    instantiating but not calling virtual functions
           Product: gcc
           Version: 8.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: m101010a at gmail dot com
  Target Milestone: ---

$ cat x.cpp
template <class b> struct a {
    virtual b g() { return 0; }
};
template <class T>
void foo() {
    a<T>();
}
void bar() {
    foo<void>();
}
$ g++ -fsyntax-only x.cpp
x.cpp: In instantiation of ‘b a<b>::g() [with b = void]’:
x.cpp:2:15:   required from here
x.cpp:2:28: error: return-statement with a value, in function returning ‘void’
[-fpermissive]
     virtual b g() { return 0; }
                            ^
$ cat y.cpp
template <class b> struct a {
    virtual b g() { return 0; }
};
template <class T>
void foo() {
    a<T>().g();
}
void bar() {
    foo<void>();
}
$ g++ -fsyntax-only y.cpp
y.cpp: In instantiation of ‘b a<b>::g() [with b = void]’:
y.cpp:6:5:   required from ‘void foo() [with T = void]’
y.cpp:9:15:   required from here
y.cpp:2:28: error: return-statement with a value, in function returning ‘void’
[-fpermissive]
     virtual b g() { return 0; }
                            ^
$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/8.3.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure --prefix=/usr --libdir=/usr/lib
--libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-shared
--enable-threads=posix --enable-libmpx --with-system-zlib --with-isl
--enable-__cxa_atexit --disable-libunwind-exceptions --enable-clocale=gnu
--disable-libstdcxx-pch --disable-libssp --enable-gnu-unique-object
--enable-linker-build-id --enable-lto --enable-plugin
--enable-install-libiberty --with-linker-hash-style=gnu
--enable-gnu-indirect-function --enable-multilib --disable-werror
--enable-checking=release --enable-default-pie --enable-default-ssp
--enable-cet=auto
Thread model: posix
gcc version 8.3.0 (GCC) 
$ 

Both x.cpp and y.cpp fail to instantiate a<void>::g(), but y.cpp's failure
gives the full template stack whereas x.cpp's failure just gives the location
of g.  This also happens in GCC 9 and trunk.  However, in 7 and below it would
point at the end of the function at the bottom of the template stack (e.g.
x.cpp:10:1 instead of x.cpp:2:15).