[PATCH] D51789: [clang] Add the exclude_from_explicit_instantiation attribute

[Louis Dionne via Phabricator via cfe-commits]

ldionne added inline comments.


================
Comment at: clang/include/clang/Basic/DiagnosticSemaKinds.td:4683-4686
+  "Member '%0' marked with 'exclude_from_explicit_instantiation' attribute is "
+  "not defined but an explicit template instantiation declaration exists. "
+  "Reliance on this member being defined by an explicit template instantiation "
+  "will lead to link errors.">;
----------------
rsmith wrote:
> Diagnostics should start with a lowercase letter and not end with a period.
> 
> That said, I'm not sure I see why this diagnostic is correct / useful. If the entity is never used, then there's no link error. And if it is ever used, then you should get an implicit instantiation like normal, and we already have a diagnostic for the case where an entity is implicitly instantiated and no definition is available.
> Diagnostics should start with a lowercase letter and not end with a period.

Done.

> That said, I'm not sure I see why this diagnostic is correct / useful. If the entity is never used, then there's no link error. And if it is ever used, then you should get an implicit instantiation like normal, and we already have a diagnostic for the case where an entity is implicitly instantiated and no definition is available.

This is not what happens right now. If you don't provide a definition but you try to call the function, an extern call will be produced (and that will result in a link error because any potential explicit instantiation won't provide the function). For example:

```
cat <<EOF | ./install/bin/clang++ -cc1 -stdlib=libc++ -xc++ -emit-llvm -o - -
template <class T>
struct Foo {
  __attribute__((exclude_from_explicit_instantiation)) static void static_member_function();
};

extern template struct Foo<int>;

int main() {
  Foo<int>::static_member_function();
}
EOF
```

Results in the following LLVM IR:

```
; Function Attrs: noinline norecurse nounwind optnone
define i32 @main() #0 {
entry:
  call void @_ZN3FooIiE22static_member_functionEv()
  ret i32 0
}

declare void @_ZN3FooIiE22static_member_functionEv() #1
```

I guess we should be getting a warning or an error on the point of implicit instantiation instead, or is this behavior acceptable?



================
Comment at: clang/lib/Sema/SemaTemplateInstantiate.cpp:2581-2582
+        if (Function->hasAttr<ExcludeFromExplicitInstantiationAttr>()) {
+          if (TSK == TSK_ExplicitInstantiationDeclaration &&
+                                                      !Pattern->isDefined()) {
+            Diag(Function->getLocation(),
----------------
rsmith wrote:
> Nit: we prefer to left-align continuation lines (clang-format will do that for you).
Thanks for the heads up. I ran clang-format on all the lines I touched in this file.


Repository:
  rC Clang

https://reviews.llvm.org/D51789