[RFC] Revisit PR 16253 ("Attempt to use a type name...")

  Last year a patch was submitted/approved/commited to eliminate
symbol_matches_domain which was causing this problem.  It was later reverted
because it introduced a (severe) performance regression.

Recap:

(gdb) list
1	enum e {A,B,C} e;
2	int main (void) { return 0; }
3
(gdb) p e
Attempt to use a type name as an expression

The parser attempts to find a symbol named "e" of VAR_DOMAIN.
This gets passed down through lookup_symbol and (eventually) into
block_lookup_symbol_primary, which iterates over the block's dictionary
of symbols:

  for (sym = dict_iter_name_first (block->dict, name, &dict_iter);
       sym != NULL;
       sym = dict_iter_name_next (name, &dict_iter))
    {
      if (symbol_matches_domain (SYMBOL_LANGUAGE (sym),
                                 SYMBOL_DOMAIN (sym), domain))
        return sym;
    }

The problem here is that we have a symbol named "e" in both STRUCT_DOMAIN
and VAR_DOMAIN, and for languages like C++, Java, and Ada, where a tag name
may be used as an implicit typedef of the type, symbol_matches_domain ignores
the difference between VAR_DOMAIN and STRUCT_DOMAIN.  As it happens, the
STRUCT_DOMAIN symbol is found first, considered a match, and that symbol is
returned to the parser, eliciting the (now dreaded) error message.

Since this bug exists specifically because we have both STRUCT and VAR_DOMAIN
symbols in a given block/CU, this patch rather simply/naively changes
block_lookup_symbol_primary so that it continues to search for an exact
domain match on the symbol if symbol_matches_domain returns a symbol
which does not exactly match the requested domain.

This "fixes" the immediate problem, but admittedly might uncover other,
related bugs.  [Paranoia?] However, it causes no regressions (functional
or performance) in the test suite.

I have also resurrected the tests from the previous submission.  However
since we can still be given a matching symbol with a different domain than
requested, we cannot say that a symbol "was not found."  The error messages
today will still be the (dreaded) "Attempt to use a type name..."  I've
updated the tests to reflect this.

ChangeLog

	PR 16253
	* block.c (block_lookup_symbol_primary): If a symbol is found
	which does not exactly match the requested domain, keep searching
	for an exact match.  Otherwise, return the previously found "best"
	symbol.

testsuite/ChangeLog

	PR 16253
	* gdb.cp/var-tag.cc: New file.
	* gdb.cp/var-tag.exp: New file.
---
 gdb/ChangeLog                    |  8 ++++
 gdb/block.c                      | 16 +++++--
 gdb/testsuite/ChangeLog          |  6 +++
 gdb/testsuite/gdb.cp/var-tag.cc  | 44 +++++++++++++++++++
 gdb/testsuite/gdb.cp/var-tag.exp | 94 ++++++++++++++++++++++++++++++++++++++++
 5 files changed, 165 insertions(+), 3 deletions(-)
 create mode 100644 gdb/testsuite/gdb.cp/var-tag.cc
 create mode 100644 gdb/testsuite/gdb.cp/var-tag.exp

[RFC] Revisit PR 16253 ("Attempt to use a type name...")

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