Issue 10378 - Local imports hide local symbols

[9rnsr]

https://issues.dlang.org/show_bug.cgi?id=10378

Reuse the symbol shadowing detection mechanism for WithStatement.

This PR is based on #4857, because the local import behavior change will hit issue 14858 in Phobos unittest.

[dlang-bot]

Fix	Bugzilla	Description
✓	10378	Prevent local imports from hiding local symbols

[MetaLang]

I have revived this pull request here: #5353

[9rnsr]

Rebased.

[ibuclaw]

Looks reasonable at a glance. I'll have a second look later.

[WalterBright]

I want to spend some more time on this. At first glance, it seems like a lot of code just to add a check.

[WalterBright]

This PR deals with shadowing of local symbols within a function. It does not deal with shadowing when using an import in, say, a class declaration. For example:

int writeln() { return 3; }

struct S {
    import std.stdio;
    void bar() { assert(writeln() == 3); }
}

void main() {
    S s;
    s.bar();
}

fails to compile.

[WalterBright]

I'm working on fixing this, so please, no overlapping efforts!

[9rnsr]

At first glance, it seems like a lot of code just to add a check.

The concept of this PR is, a symbol shadowing introduced by local imports, are essentially equivalent with the shadowing done by WithStatement. I think that using same mechanism for the two would increase language consistency.

[9rnsr]

This PR deals with shadowing of local symbols within a function.
It does not deal with shadowing when using an import in, say, a class declaration. For example:

In the example code, the import declaration is not local, it's in struct member scope (it affects all other member functions, if declared). So it's beyond the target of this PR.

[WalterBright]

I understand. But this case is one of the things people complained about. I'm working on a fix that will address all cases of doing imports inside scopes.

[WalterBright]

Anyhow, the reboot based on #deadalnix 's proposed algorithm is now in #5445

[quickfur]

Now that #5445 is merged, can this PR be closed?

[ibuclaw]

The tests may still be useful.

[quickfur]

Should we merge the tests then?

[9rnsr]

The tests are useless anymore.