Unified: Support Swift subscript calls and declarations - #23111

Swift subscripts were not modelled. xs[i] was extracted as a call to xs itself, and subscript declarations became unsupported_node, so no data flowed through them.

Representation

• xs[i] is now a call to a member named subscript, i.e. xs.subscript(i).
• A subscript declaration becomes a function_declaration named subscript for the getter. If there is a setter, it is extracted as a separate function_declaration named subscript=, placed next to the getter and tagged chained_declaration.
• The setter gets its own copy of the subscript's parameters, plus one extra trailing positional parameter: newValue, or the name given in set(v). It has no return type.
• A subscript parameter with only one name has no argument label, as in Swift.
• A potential clash with a backticked identifier named subscript is ignored for now.

Data flow

• A function named like foo= is a SetterMethodDeclaration. resolveSetterCallTarget resolves x.foo(...) to foo= on the receiver type, for both instance and static members.
• This builds on the existing property getter/setter logic: a call that receives an incoming value or a post-update value also creates a setter call. That covers x[i] = v, x[i] += v, x[i].f = v on value types, and &x[i].
• The new value is passed after the positional arguments. The receiver flows into setter calls, and its updated value flows back out, the same way as for ordinary calls.
• The Swift write-back rule now goes through call callees, so c.s[0] = v also writes back through s's setter.

LegacyMaD

• subscript(...) models already match getter calls, since matching is by member name plus argument labels.
• Setter calls match subscript=(...) models. The labels are the call's labels plus one extra _: for the value.
• A call with a member callee that is a direct assignment target always creates a setter call, even when no setter is found in source. This lets models for library types match.

Review notes

• The consistency query now excludes index arguments shared by the getter and setter calls of x[i] += v.
• Expected outputs in the CFG, BasicTest and type-inference tests changed only because of the new ... .subscript callee node.
• New tests are in ql/test/library-tests/dataflow/subscripts.swift and the extractor corpus (tests/corpus/swift/subscripts/).