clang-10 regression with __thread and deduction guides

[llvmbot]

Bugzilla Link	45124
Resolution	FIXED
Resolved on	Mar 11, 2020 03:39
Version	10.0
OS	Windows NT
Blocks	#43900
Reporter	LLVM Bugzilla Contributor
CC	@froydnj,@zmodem,@zygoloid,@sylvestre

Extended Description

https://godbolt.org/z/9rQ5Mi

template class a { int d; };
template <template class b> class c : b {} ;

__thread c g1;

struct x { ~x(); };
template class y { y(x = x()); };
template y(z)->y;

__thread c g2;

g1 compiles fine, but when a deduction guide is introduced, g2 gives an error: initializer for thread-local variable must be a constant expression even though the deduction guide's classes have nothing to do with the __thread variable's classes.

[llvmbot]

This began with: llvmorg-10-init-5218-gc667cdc850c "[c++20] P1331R2: Allow transient use of uninitialized objects in constant evaluation." c667cdc

[zmodem]

Richard, any ideas?

David, can you comment on the severity here? The commit landed some time ago so I'm surprised it didn't cause problems before.

[llvmbot]

This breaks Firefox builds in https://bugzilla.mozilla.org/show_bug.cgi?id=1619206. The reason we didn't see it before was that we only recently landed our codebase's first use of deduction guides. (Or perhaps one of the first, but the first to be #included before this use of __thread.)

[zygoloid]

Declaration of a deduction guide for 'y' triggered declaration of 'y's implicit deduction guides, and in particular created a 'template y(x = x()) -> x;' guide. But we forgot to capture the cleanups for the x() temporary when transforming the default argument, so this left us with a pending "a temporary was created and needs to be destroyed" flag set.

Then when we reached the __thread declaration's initializer, we noticed the flag and created a wrapper "you need to run destructors here" AST node.

Now, we consider an initializer to be "constant enough" for __thread if either it's a "real" constant initializer (which this case is not, because it leaves a::d uninitialized) or if it's an invocation of a trivial default constructor (or a bunch of other cases). And our "trivial default constructor" check fails due to the wrapper "you need to run destructors here" node -- that's not trivial enough for us.

Fixed for Clang 11 in 6d894af.

[llvmbot]

Confirmed that this fixes our full-size file as well. Thank you!

[sylvestre]

@​Hans, could you please cherry-pick it for 10?
thanks

[zmodem]

@​Hans, could you please cherry-pick it for 10?
thanks

Yes, pushed to 10.x as 5b5a111.