Created attachment 1760 [details]
Patch fixing this issue

This is like a special case of Issue 20046, caused by a wrong instantiation of emplaceRef. I spent days trying to find a minimal test case, but no success.

The error can easily be reproduced with a unittest build of https://github.com/thaven/OneFile

(This issue is currently blocking a first release of that package)

The unittest build will produce output like:

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$ dub test -f
Generating test runner configuration 'onefile-test-library' for 'library' (library).
Performing "unittest" build using /usr/bin/dmd for x86_64.
onefile ~master: building configuration "onefile-test-library"...
/usr/include/dmd/phobos/std/conv.d(4419,19): Error: function core.stdc.string.memcpy(return void* s1, scope const(void*) s2, ulong n) is not callable using argument types (shared(Root)*, immutable(Root)*, ulong)
/usr/include/dmd/phobos/std/conv.d(4419,19):        cannot pass argument &chunk of type shared(Root)* to parameter return void* s1
/usr/include/dmd/phobos/std/conv.d(4339,27): Error: template instance std.conv.emplaceInitializer!(shared(Root)) error instantiating
/usr/include/dmd/phobos/std/experimental/allocator/package.d(1178,29):        instantiated from here: emplaceRef!(shared(Root), shared(Root))
source/onefile/stm.d(1218,39):        instantiated from here: make!(shared(Root), shared(AlignedMallocator))
source/onefile/container/slist.d(223,40):        instantiated from here: tmMake!(shared(Root))
source/onefile/container/slist.d(675,18):        instantiated from here: SList!uint
/usr/include/dmd/phobos/std/experimental/allocator/package.d(1190,53): Error: forward reference to inferred return type of function call function () @trusted
{
alloc.deallocate(m);
}
()
<...snip...>

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The attached patch to Phobos fixes the problem. (Also to be found at https://github.com/dlang/phobos/compare/master...thaven:fix/make-emplace-shared)

Any help finding the exact trigger for the bug, and thus a minimal test case, is appreciated.