When I'm working on complicated template code and debugging a mixture of bugs in my templates and ICEs in g++ caused by my buggy templates I sometimes wish I could see a dump of all the templates that the compiler is instantiating, so I can see at which point in a deeply-nested instantiation things don't happen the way I expected them to. With variadic templates in particular it's sometimes hard to visualize what's happening to your code.
Someone else asked for this the other day on gcc-help (http://gcc.gnu.org/ml/gcc-help/2011-06/msg00193.html) so I thought I'd take a look at implementing it. It turns out that or class templates all that's needed is something like: verbatim ("instantiating class template %qT", type) in instantiate_class_template_1 in gcc/cp/pt.c Presumably it wouldn't be much harder to do the same for function templates. Is there something that does this already that I've missed? If I were to add a switch like -ftrace-template-instantiation to control this, is there any chance of getting it accepted? It might be useful if the trace said something like: attempting to instantiate foo<bar>... attempting to instantiate foo<bar>::baz<int> ... successfully instantiated foo<bar>::baz<int> failed to instantiate foo<bar> with nesting to see which instantiations depend on others and to see which pass/fail, but I've already found it useful just to see a list of instantiations, in order to verify whether some refactoring reduces the number of instantiations. Would anyone else find this useful, or should I just keep it in my own source tree for my own use?
