variable template specialization is treated as only a declaration if its type has internal linkage

[zygoloid]

Bugzilla Link	19060
Version	unspecified
OS	Linux
CC	@akyrtzi

Extended Description

We're failing to instantiate the initializer for a variable template specialization if its type has internal linkage:

<stdin>:1:24: warning: variable 'var<<anonymous namespace>::S>' has internal linkage but is not defined [-Wundefined-internal]
template<typename T> T var; namespace { struct S {} s = var<S>; }
                       ^
<stdin>:1:57: note: used here
template<typename T> T var; namespace { struct S {} s = var<S>; }
                                                        ^

The warning is sort of right: if we emit IR for s, we emit it as external global.

[llvmbot]

clang sometimes fails to emit definition of variable template when used
This may be unrelated, but the attached program emits a similar warning, then fails to link (with undefined reference to _ZL1bILb1EE). If I change the return b<B>;" line to "return b<true>; it works as expected.

[zygoloid]

That's an unrelated issue, and is already fixed in trunk.

[zygoloid]

The problem occurs any time a variable template:

1. has no explicit initializer, and
2. either has a primitive type or has a dependent type

This results in the init on the template definition being null, which confuses us into thinking that a definition is missing.

The root cause is that we cannot distinguish between a declaration of a variable template specialization and a definition that has no initializer.

[AaronBallman]

This issue was fixed in Clang 6.0: https://godbolt.org/z/YsKErW5q5

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (ec04fc15-fa35-46f2-80e1-5d271f2ef708)

Details

| | | | --- | --- | | Bugzilla Link | [19060](https://llvm.org/bz19060) | | Version | unspecified | | OS | Linux | | CC | @akyrtzi |

Extended Description

We're failing to instantiate the initializer for a variable template specialization if its type has internal linkage:

<stdin>:1:24: warning: variable 'var<<anonymous namespace>::S>' has internal linkage but is not defined [-Wundefined-internal]
template<typename T> T var; namespace { struct S {} s = var<S>; }
                       ^
<stdin>:1:57: note: used here
template<typename T> T var; namespace { struct S {} s = var<S>; }
                                                        ^

The warning is sort of right: if we emit IR for s, we emit it as external global.