[PATCH] D87349: [clang] adapt c++17 behavior for dependent decltype-specifiers

Wed Sep 16 07:04:56 PDT 2020

sammccall added inline comments.

================
Comment at: clang/test/CodeGenCXX/mangle.cpp:805

-  // CHECK-LABEL: define weak_odr void @_ZN6test341fIiEEvDTstDTplcvT__EcvS1__EEE
+  // CHECK-LABEL: define weak_odr void @_ZN6test341fIiEEvm
   template void f<int>(decltype(sizeof(1)));
----------------
sammccall wrote:
> GCC mangles this as `_ZN6test341fIiEEvDTstDTplcvT__EcvS1__EEE`, so we're breaking compat here :-\
> 
> And in fact we're just incorrect. https://itanium-cxx-abi.github.io/cxx-abi/abi.html#mangling:
> 
> > If the operand expression of decltype is not instantiation-dependent then the resulting type is encoded directly.
> 
> Here, instantiation-dependent *is* explicitly defined as "type-dependent or value-dependent or ...". And therefore these cases must not be "encoded directly", including the motivating case (decltype(N) where N is a typed constant whose initializer is value-dependent).
> 
> We could consider not two but *three* types of decltypetypes:
>  - decltype(type-dependent), which is sugar for a canonical DependentDeclTypeType
>  - decltype(instantiation-but-not-type-dependent), which is still sugar for a concrete canonical type (per C++17) but mangles using the expr (per cxxabi)
>  - decltype(non-dependent), which is sugar for a concrete canonical type and mangles directly
> 
> This only works if it's OK for mangling to depend on non-canonical type details - I don't know whether this is the case. @rsmith - any hints here?
Hmm, my naive reading of the mangle code matches what I described.

The big comment in ItaniumMangle talks about related issues: https://github.com/llvm/llvm-project/blob/24238f09edb98b0f460aa41139874ae5d4e5cd8d/clang/lib/AST/ItaniumMangle.cpp#L2541-L2572

I don't really understand what's going on here, sorry.

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D87349/new/

https://reviews.llvm.org/D87349