Slava Pestov 52479cadd5 AST: Fix source break with new shadowing rules ...

My recent refactoring of top-level lookup replaced the old
shadowing done as part of top-level lookup with two separate
rules:

- If all paths from the current source file to a module 'B'
  go through a module 'A', then declarations in 'A' shadow
  declarations in 'B'.

- If a declaration in module 'A' was found via a scoped
  import and a declaration with the same name in module 'B'
  was found via an unscoped import, prefer the declaration
  from module 'A'.

However this caused a source break when you have a scenario
like the following:

- A source file imports 'A', 'B', and 'B.Foo'.

- 'A' re-exports 'B'.

- Both 'A' and 'B' define a type named 'Foo'.

The problem is that the scoped import 'B.Foo' can actually
find both 'A.Foo' and 'B.Foo', since 'B' re-exports 'A'.

Furthermore, since the source file explicitly imports 'A',
'B' does not shadow 'A' in the import graph.

As a result neither shadowing rule would eliminate the
ambiguity.

The new rule combines the scoped import check and the
shadowing check by considering all access paths to 'A'
that are not shadowed by 'B'. Using this rule, 'A.Foo'
is only seen via the access path 'A', whereas 'B.Foo'
is seen under both 'B' and 'B.Foo'. Since 'B.Foo' is seen
via a scoped import and 'A.Foo' is only seen via an
unscoped import, we can conclude that 'B.Foo' shadows
'A.Foo'.

Fixes <rdar://problem/55205050>.

2019-09-12 21:30:43 -04:00

5.5 KiB

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