Javac not trying to recover unresolved parameterized types

[Mickael Istria]

Hi all,

As part of our work to allow Eclipse JDT to use Javac as backend, we
noticed that in case of unresolved parameterized types, the compiler
doesn't generate a nice recovered TypeSymbol with interesting data (such as
the name) that can be used later.
For example, my latest issue is that code like
```
class K {
  Regular reg;
  Parameterized<Object> par;
}
```
where neither Regular nor Parameterized exist, make Javac produce a
"recovered" symbol for `Regular` (at least the ErrorTypr stores the
"Regular" name) when asking for the tsym of `reg` VarSymbol, while it
doesn't produce anything useful for `Parameterized` when asking for the
tsym of `par` VarSymbol. In this later case, the symbol points to `noType`
which isn't usable by the IDE to generate a minimal DOM object.

In comparison, ECJ generates a recovered type binding for `Parameterized`,
which can be later used by some refactorings as if the type were existing.
Such refactorings that work with ECJ backend cannot work with the Javac
backend because of the symbol being unusable.

We're currently adding a bunch of workarounds in many places to handle this
case, but we'd obviously rather prefer if Javac can come with such
smartness out of the box.

So this behavior in Javac seems to me like a small issue or limitation in
Javac. What do you think? Is this "problem" already known? Should this be
reported as a bug? Does it make sense to expect Javac to provide such
recovery for parameterized types to? Should we try to fix it?...

Thanks in advance for you guidance
-- 
Mickael Istria
Eclipse IDE <https://www.eclipse.org/eclipseide> developer, for Red Hat
<https://developers.redhat.com/>
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