erichkeane added inline comments.
================
Comment at: clang/lib/AST/ItaniumMangle.cpp:5576
+ "program to refer to the anonymous union, and there is therefore no need
"
+ "to mangle its name. '");
+}
----------------
rjmccall wrote:
> erichkeane wrote:
> > rjmccall wrote:
> > > Unfortunately, I think class value mangling has a counter-example to
> > > this: you can have an anonymous struct or union which doesn't declare any
> > > named members but will of course nonetheless show up in the list of
> > > members in the enclosing class. Code can even give it a non-zero
> > > initializer using list-initialization; that value can never be read, but
> > > it's there, and presumably it's part of uniquely determining a different
> > > template argument value and therefore needs to be mangled.
> > >
> > > I don't know what we should do about this in the ABI, but we shouldn't
> > > crash in the compiler.
> > I'm not really understanding what you mean? Can you provide an example?
> > All of the cases I could come up with like this ended up getting caught by
> > the 'zero -init' case.
> >
> > That said, I considered this 'unreachable' to be a distinct improvement
> > over our current behavior which is to crash at effectively line 5558.
> >
> > Would you prefer some sort of 'fatal-error' emit here?
> I think the test case would be something like:
>
> ```
> struct A {
> union { unsigned: 10; };
> int x;
> constexpr A(int x) : x(x) {}
> };
>
> template <A a> void foo() {}
> template void foo<A(5)>();
> ```
>
> But also consider:
>
> ```
> struct B {
> struct Nested {
> union { unsigned: 10; };
> int x;
> } nested;
> constexpr B(int x) : nested{5, x} {}
> };
>
> template <B b> void bar() {}
> template void bar<B(10)>();
> ```
With my current patch, that first example mangles to:
`@_Z3fooIXtl1ALNS0_Ut_EELi5EEEEvv` (that is, it doesn't touch this crash).
Unfortunately, that doesn't 'demangle' with llvm-cxxfilt.
For the 2nd example, i had to modify it to be:
```
struct B {
struct Nested {
union {
unsigned : 10;
};
int x;
} nested;
constexpr B(int x) : nested{{}, x} {}
};
template <B b> void bar() {}
template void bar<B(10)>();
```
As you wrote it, we get the error:
`temp.cpp:20:31: error: initializer for aggregate with no elements requires
explicit braces `
I tried putting a '5' in those brackets, and get:
`temp.cpp:20:32: error: excess elements in union initializer`
HOWEVER, I think it makes sense to have a Diag here rather than an
llvm-unreachable, so I've done that instead.
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D122820/new/
https://reviews.llvm.org/D122820
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