clang++/g++ disagree on how template arguments in substitutions are resolved

[rocallahan]

Basically the question is: when a template parameter reference like T_ occurs in a substitution, is the reference looked up in the template instance where the substitution is defined, or where it is used?

It appears that llvm-cxxfilt assumes the former, but c++filt assumes the latter. Consider the (hand-written) mangled symbol _Z5helloIXadL_Z6ignoreI9RangitotoEvT_EEEvS2_:

[roc@localhost cpp_demangle]$ c++filt --version
GNU c++filt (GNU Binutils) 2.29.51
Copyright (C) 2018 Free Software Foundation, Inc.
This program is free software; you may redistribute it under the terms of
the GNU General Public License version 3 or (at your option) any later version.
This program has absolutely no warranty.
[roc@localhost cpp_demangle]$ c++filt _Z5helloIXadL_Z6ignoreI9RangitotoEvT_EEEvS2_
void hello<&(void ignore<Rangitoto>(Rangitoto))>(&(void ignore<Rangitoto>(Rangitoto)))
[roc@localhost cpp_demangle]$ llvm-cxxfilt --version
LLVM (http://llvm.org/):
 LLVM version 7.0.0svn
 Optimized build.
 Default target: x86_64-unknown-linux-gnu
 Host CPU: skylake
[roc@localhost cpp_demangle]$ llvm-cxxfilt _Z5helloIXadL_Z6ignoreI9RangitotoEvT_EEEvS2_
void hello<&(void ignore<Rangitoto>(Rangitoto))>(Rangitoto)

In this case both tools agree that the S2_ substitution refers to the T_, but they disagree on what that expands to.

To my reading, the spec isn't clear on this issue. The most relevant text I can find is

Note that substitutable components are the represented symbolic constructs, not their associated mangling character strings.

which suggests the definition instance is preferred, which also seems logical to me.

[rocallahan]

I suspect the same problem could occur with function parameter references, though an example would have to be very complicated.