Or constructors combined with static objects; you get the static initialization order fiasco.
Or constness with template functions; you get two, four, or eight copies of each generic function in your source code according to which things are const.
Or (compile-time) overloading with (run-time) overriding; you get C++’s weird “hiding” rule about the other overrides you didn't override.
Or separate compilation with implicitly instantiated templates; you get geological build times as the compiler instantiates the same templates in every .C file and then throws away all but one of the identical instantiations at link time. (To be fair, this is far from the only reason C++ compiles slowly.)
Overriding combined with type conversions through implicitly invoked constructors (and implicit referencing and implicit casting to const) gives you annoying bugs that are unnecessarily hard to figure out.
Cleanup from exceptions via RAII combined with C’s unspecified argument evaluation order led to a situation where resource leaks during certain kinds of operations couldn't be avoided reliably, a bug in the language definition that wasn't noticed for several years, though I think it's fixed now.
The grammar is undecidable because of the number of different things that have been added, which sounds like a hyperbolic joke but is actually literally true, and a significant obstacle to implementing something like gofmt for C++.
The combination of template parameters using <> for parameters, the traditional longest-leftmost tokenization rule, and the >> operator for bitshifting made foo<bar<baz>> an unexpected syntactic pitfall, one that is now fixed.
There wasn't an exception-safe version of the STL for a number of years, which isn't really an interaction between templates and exceptions —a non-template container had to deal with the same exception-safety problems— but it did mean that fit quite a while you could use the STL or exceptions but not both.
This is far from the extent of the problem. The C++ FAQ consists largely of affirmations of the form, “Doing [reasonable thing 1] works, and [reasonable thing 2] works too, but if you do them both, you will die horribly for your immorality.” It's exaggerating about the death part but the surprising problems are quite real.
I don't hate C++, and I think it's the best existing language for some problem spaces, but it's very much the poster boy for unexpected problems arising from interactions between features.
As far as the keyboard goes, https://GitHub.com/kragen/xcompose and http://canonical.org/~kragen/setting-up-keyboard are your friends.
It’s good that they’re being careful and conservative; but I feel that they’re going out of their way to avoid all existing paths, whereas it would actually be safer to use some existing system(s) as a starting point, as both the advantages and disadvantages -- in practice, not just theoretically! -- are known.
I definitely agree that interactions between features need to be carefully thought through. One likely wart with the current proposal is that user-defined generic types still won’t look or behave quite like the built-in ones, as those have special syntax and special operators. Will it be considered good practice to expose raw slices, maps etc in APIs? Or will people start wrapping them (just as in Java, an array would most often be wrapped in an ArrayList for convenience)? This might have been easier to resolve if generics had been thinkable much earlier in the language design.
I agree that, with this design, it won't be as comfortable to use generic containers from a library as to use the built-in containers. That's been a persistent problem with C++ templates and Java generics, too, though (I think reasonable initializers for vector finally landed in C++17?) so maybe the lesson they took was that the base language should have slices and maps because there was no known reasonable design that allowed them to be seamlessly defined in a library while supporting static typing and object nesting? Maybe there is a solution if you design the language around generic containers from the beginning—have you tried D?
