[Bug python/28647] New: Python pretty-printer doesn't work with std::variant containing local type

["bugmenot at mailinator dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=28647

            Bug ID: 28647
           Summary: Python pretty-printer doesn't work with std::variant
                    containing local type
           Product: gdb
           Version: 10.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: python
          Assignee: unassigned at sourceware dot org
          Reporter: bugmenot at mailinator dot com
  Target Milestone: ---

When I compile this program:

```
#include <variant>
#include <vector>

struct G {
    int x;
};

int main() {
    struct A {
        int x;
    };
    A a;
    std::variant<A> b {a};
    std::vector<A> c {a};
    std::variant<G> d {G {}};

    __builtin_trap();
}
```

Then run it in gdb, note that `print b` prints

```
std::variant<> [no contained value]
```

but `print d` prints

```
std::variant<G> [index 0] = {{x = 0}}
```

In the first case the print is wrong.

It's caused by `Type.template_argument` function: If this is called in the
Python shell

```
>>> gdb.lookup_symbol("b")[0].type.template_argument(0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: syntax error
```

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