completeUnusedClass eventually calls into CGDebugInfo::completeClass which makes use of the TypeCache. What does it mean for a class to have "an external vtable?" Can you give me an example?

Here's some sample code that uses a type in the DWARF but due to the vtable being external (an external vtable is one that's guaranteed to be emitted in some other object file - due to the key function optimization in the Itanium ABI - since all virtual functions must be defined if a type is ODR used, the compiler can make a shared assumption that the vtable will only be emitted with a single consistently chosen virtual function (ideally: the first non-inline one) and emit the vtable only in that object and nowhere else) the type is emitted as a declaration (when using "-fno-standalone-debug"):

struct t1 {
  virtual void f1();
};
t1 v1;

Cool, thanks for the example. For your example:
clang -g -emit-llvm -S foo.cpp -o -:

...
DIGlobalVariable(name: "v1"
...
DICompositeType(tag: DW_TAG_structure_type, name: "t1"
...

clang -g -fno-eliminate-unused-debug-types -emit-llvm -S foo.cpp -o -:

...
DIGlobalVariable(name: "v1"
...
DICompositeType(tag: DW_TAG_structure_type, name: "t1"
...
DIDerivedType(tag: DW_TAG_member, name: "_vptr$t1"
DIDerivedType(tag: DW_TAG_pointer_type
DIDerivedType(tag: DW_TAG_pointer_type, name: "__vtbl_ptr_type"
DIBasicType(name: "int"
DISubprogram(name: "f1"
DISubroutineType
DIDerivedType(tag: DW_TAG_pointer_type
...

So even though f1 was not defined, we still emit debug info for it.

Comparing the output of llvm-dwarfdump on $CC -g -fno-eliminate-unused-debug-types -c <example.cpp> for g++ vs clang++; we're definitely producing more debug info (the vtable info).

That said, the creation of v1 technically means that t1 is no longer an "unused type." If we comment out its creation, then compare $CC -g -fno-eliminate-unused-debug-types -c <example.cpp> for g++ vs clang++, g++ produces no debug info (That seems like a bug in g++ to me), while clang++ with this patch produces the debug info for the CXXRecord, and additional info for the vtable ptr and signature of f1.

I'm not sure what's expected in this case (we should at least emit the DW_TAG_structure_type for t1, but I'm not sure about the DW_TAG_subprogram for f1. I assume that's because we've enabled more-than-full-debuginfo behavior? WDYT?

I think it'd be nice to handle this the same as GCC (though technically we don't offer the same customization that GCC offers - I imagine the GCC behavior of -femit-class-debug-always, which Clang doesn't have a configuration option for (well, not specifically - again, comes back to that "independent configuration" or "overriding behavior" implementation choice) - I'd sort of think of it like that flag is always enabled)

But I guess if we think of it as a series of escalations, then, yeah, it makes sense in the Clang way of thinking of it, as -fno-eliminate-unused-debug-types as overriding the default -fno-emit-class-debug-always... *shrug* don't feel /super/ strongly either way I guess. If the current implementation is such that -fno-eliminate-unused-debug-types overrides all other type homing techniques (vtable based, explicit template specialization decl/def, ctor based, and not-required-to-be-complete (define a type but only use pouinters to that type and never dereference them)) that's fine by me.