linker fails with multiple definitions after inline thread_local var within class

[Arthur Norman <acn1 at cam dot ac dot uk>]

#! /bin/bash
# When I run this on Ubuntu-x86_64, Macintosh(clang) or a Raspberry pi
# the code links and when it runs it just prints 999 - which is the behaviour
# that I expect. On both Cygwin and using x86_64-w64-mingw32-g++  (and i686)
# I get a linker diagnostic of the form
# ./tltest.sh
# /usr/lib/gcc/x86_64-pc-cygwin/8.3.0/../../../../x86_64-pc-cygwin/bin/ld:
#   /tmp/cczVTcZ1.o:t2.cpp:(.text+0x86): multiple definition of
#   `TLS init function for Data::valref';
#   /tmp/ccyLQlRb.o:t1.cpp:(.text+0x4a): first defined here
# collect2: error: ld returned 1 exit status
#
# I believe that given the specification of C++17 "inline" variables this
# is incorrect, but there are better experts who may be able to explain
# otherwise.
#
# When people raise issues here I often see other asking "Why are you doing
# that?". This is a cut-down version of my real code where rather than
# storing a reference to the thread_local Record that I am interested in
# in a simple array (tvec) at a fixed offset (1) I store and retrieve a
# referece to it using TlsAlloc(), TlsGetValue and TlsSetValue - those being
# the Microsoft API for thread-local access, and for my purposes my
# measurements suggest that using them gives me useful performance gains
# over the emutls code that g++ creates on the relevant platforms.
# And I am then (trying to) build a header-only library (for which t.h is
# the surrogate here) which can be included from several other compilation
# units but by virtue of "inline variables" its private (including thread-
# local) date can be defined within that header file  so that the files that
# #include the header-only library do not need to contain anything beyond
# uses of it. To illustrate this I have two source files which each include
# t.h but the bad behaviour does not need any other code in the first one!
# The second just contains a tiny main program that inspects data from the
# library data.

# Have I misunderstood C++ and so am I doing something wrong or is this
# a g++/Windows bug?

cat <<XXX > t.h
#include <cstdint>

inline void *tvec[4];

class Record
{
public:
    int val = 999;
};

class Data_Ref
{   static inline thread_local Record val;
public:
    static Record* get()  // Get reference to Record via tvec[]
    {   return (Record*)tvec[1];
    }
    Data_Ref()            // Stash it in tvec[] for later use
    {   tvec[1] = (void *)&val;
    }
};

class Data
{   static inline thread_local Data_Ref valref;
public:
    static Record &get()
    {   return *valref.get(); // note that get() is static in Data_Ref
    }
};

XXX
cat <<XXX > t1.cpp
#include <iostream>
#include "t.h"  // First copy

XXX
cat <<XXX > t2.cpp
#include <iostream>
#include "t.h"  // Second copy

int main()
{   std::cout << Data::get().val << std::endl;
    return 0;
}
XXX

${1:-g++} -std=c++17 -I. t1.cpp t2.cpp -o t
./t

# end of test script

Attachment: tltest.sh
Description: Binary data

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