[C++ diagnostic Patch] Partially fix c++/58363 and more

[Paolo Carlini <paolo dot carlini at oracle dot com>]

Hi all, Gaby,

I was having a look to c++/58363 and besides the main issue, that is we 
probably want to help the user and tell him/her something about 
destructors, etc, I noticed that we aren't able to pretty print the 
pseudo destructor expression at issue:

cannot convert ‘f.#‘var_decl’ not supported by dump_type#<type 
error>::~’ (type ‘void’) to type ‘int’

Weird. Thus I went to cp-tree.def and found the very clear comment:

/* A pseudo-destructor, of the form "OBJECT.~DESTRUCTOR" or
"OBJECT.SCOPE::~DESTRUCTOR. The first operand is the OBJECT. The
second operand (if non-NULL) is the SCOPE. The third operand is
the TYPE node corresponding to the DESTRUCTOR.

which in fact is inconsistent with the code in error.c:dump_expr. As 
regards cxx-pretty-print.c, the code in postfix_expression seems largely 
Ok (that confirmed my analysis), only I don't think the case of NULL 
second operand is handled correctly. What do you think about the below? 
Certainly passes the testsuite and the pretty printing for 58363 is Ok.

Thanks!
Paolo.

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