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

[Richard Smith - zygoloid via Phabricator via cfe-commits]

rsmith added inline comments.


================
Comment at: clang/test/CodeGenCXX/mangle-exprs.cpp:218
   template void a<int>(decltype(noexcept(int())));
-  // CHECK: void @_ZN5test51aIiEEvDTnxcvT__EE(
+  // CHECK: void @_ZN5test51aIiEEvb
 }
----------------
As with the other case, the expression here is instantiation-dependent so should be mangled.


================
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:
> 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.
Looks like we need the single-step-desugaring loop below the big comment you quoted to stop when it hits a `decltype` type. That's presumably where we step from the instantiation-dependent-but-not-type-dependent `decltype` node to its desugared type.


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Comment at: clang/test/SemaTemplate/dependent-expr.cpp:132
   }
-  template void f<int>(); // expected-note {{instantiation of}}
+  template void f<int>();
 
----------------
sammccall wrote:
> Took me a while to understand this, I think this is an improvement as we now diagnose when we see the template rather than when we see the instantiation.
Right. We treat statement-expressions in a template as always being value- and instantiation-dependent, in part to match GCC and in part to avoid needing to define value- and instantiation-dependence for all statements and block-scope declarations, so decltype(statement expression) would have been a dependent type before.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D87349/new/

https://reviews.llvm.org/D87349