Symbol type parameters can be piracy

[timholy]

As described in #374, types like

ClosedInterval{T} = Interval{:closed,:closed,T}

from IntervalSets blocked piracy-detection because :closed Symbol params caused is_foreign to return false unconditionally.

Fundamentally, this revealed problems with the heuristic used by is_foreign. This was essentially acknowledged as a problem in the comments.

The strategy taken here is to change the heuristic: if the combination SomeType{:foo} appears in the module that defines SomeType, then this combination is treated as a foreign type. If no such combination exists, then the Symbol is treated as a user-defined dispatch tag, and the type is treated as a user-defined type. To avoid the need for "grepping" through source code, there is one additional requirement for it to be treated as foreign: the type/symbol combination must be defined as a type alias (UnionAll) in the owning module.

Fixes #374.

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.61%. Comparing base (cdc679f) to head (5c672c8).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #375      +/-   ##
==========================================
+ Coverage   87.47%   87.61%   +0.13%     
==========================================
  Files          12       12              
  Lines         527      541      +14     
==========================================
+ Hits          461      474      +13     
- Misses         66       67       +1     

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[timholy]

I saw that the one failure was about a label. I don't have suitable privileges to add labels, but I did add an entry to the CHANGELOG.

[lgoettgens]

Sorry for ignoring this for so long. I now had the time to finally review it. Unfortunately, this now has non-trivial conflicts due to #377. Could you try to resolve them?

One semantical question, because I am not sure what you are concretely trying to do here:
Let's assume that a dependency defines

ClosedInterval{T} = Interval{:closed,:closed,T}

and a function foo

As described earlier, defining foo(::ClosedInterval) is type piracy and foo(::Interval{:myownsymbol,:bar}) is not. But what about foo(::Interval{:closed,:closed,T}) where T?