sample of debug constraints

output.txt

// swiftc -Xfrontend -debug-constraints Source.swift
// swift-driver version: 1.115 Apple Swift version 6.0 (swiftlang-6.0.0.9.10 clang-1600.0.26.2)
// Target: arm64-apple-macosx14.0
---Constraint solving at [Source.swift:7:5 - line:7:22]---
  (overload set choice binding $T1 := () -> Cat)

---Initial constraints for the given expression---
(array_expr type="[Cat]" location=Source.swift:7:16 range=[Source.swift:7:16 - line:7:22] initializer="**NULL**"
  (call_expr type="Cat" location=Source.swift:7:17 range=[Source.swift:7:17 - line:7:21] isolation_crossing="none"
    (type_expr type="Cat.Type" location=Source.swift:7:17 range=[Source.swift:7:17 - line:7:17] typerepr="Cat")
    (argument_list)))

Score: <default 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0>
Contextual Type: [Cat] at [Source.swift:7:8 - line:7:12]
Type Variables:
  $T0 as Cat @ locator@0x122131630 [[email protected]:7:17 → function result]
  $T1 as () -> Cat @ locator@0x122131700 [[email protected]:7:17 → apply function → constructor member type (for short-form or self.init call)]
  $T2 as () @ locator@0x1221316d8 [[email protected]:7:17 → apply argument]
  $T3 as [Cat] @ locator@0x122131950 [[email protected]:7:16 → contextual type → pattern match → pattern match → named pattern decl]
Resolved overloads:
  selected overload set choice Cat.Type.init: $T1 == () -> Cat for locator@0x122131770 [[email protected]:7:17 → apply function → constructor member]

  (found solution: <default 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0>)

---Solver statistics---
Total number of scopes explored: 1
Maximum depth reached while exploring solutions: 1
Time: 1.383000e+00ms

---Solution---
Fixed score: <default 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0>
Type variables:
  $T0 as Cat @ locator@0x122131630 [[email protected]:7:17 → function result]
  $T1 as () -> Cat @ locator@0x122131700 [[email protected]:7:17 → apply function → constructor member type (for short-form or self.init call)]
  $T2 as () @ locator@0x1221316d8 [[email protected]:7:17 → apply argument]
  $T3 as [Cat] @ locator@0x122131950 [[email protected]:7:16 → contextual type → pattern match → pattern match → named pattern decl]

Overload choices:
  locator@0x122131770 [[email protected]:7:17 → apply function → constructor member] with Source.(file).Cat.init()@Source.swift:2:19 as Cat.Type.init: () -> Cat
Trailing closure matching:
  locator@0x1221316d8 [[email protected]:7:17 → apply argument]: forward

---Type-checked expression---
(array_expr type="[Cat]" location=Source.swift:7:16 range=[Source.swift:7:16 - line:7:22] initializer="Swift.(file).Array extension.init(arrayLiteral:) [with (substitution_map generic_signature=<Element where Element : Copyable, Element : Escapable> Element -> Cat)]"
  (call_expr type="Cat" location=Source.swift:7:17 range=[Source.swift:7:17 - line:7:21] isolation_crossing="none"
    (constructor_ref_call_expr type="() -> Cat" location=Source.swift:7:17 range=[Source.swift:7:17 - line:7:17] isolation_crossing="none"
      (declref_expr implicit type="(Cat.Type) -> () -> Cat" location=Source.swift:7:17 range=[Source.swift:7:17 - line:7:17] decl="Source.(file).Cat.init()@Source.swift:2:19" function_ref=single)
      (argument_list implicit
        (argument
          (type_expr type="Cat.Type" location=Source.swift:7:17 range=[Source.swift:7:17 - line:7:17] typerepr="Cat"))))
    (argument_list)))

---Constraint solving at [Source.swift:8:1 - line:8:4]---
  (overload set choice binding $T1 := [Cat])
(common result type for $T0 is ())

---Initial constraints for the given expression---
(call_expr type="()" location=Source.swift:8:1 range=[Source.swift:8:1 - line:8:4] isolation_crossing="none"
  (overloaded_decl_ref_expr type="$T0" location=Source.swift:8:1 range=[Source.swift:8:1 - line:8:1] name="f" number_of_decls=2 function_ref=single
    (candidate_decl "Source.(file)[email protected]:4:6")
    (candidate_decl "Source.(file)[email protected]:5:6"))
  (argument_list
    (argument
      (declref_expr type="[Cat]" location=Source.swift:8:3 range=[Source.swift:8:3 - line:8:3] decl="Source.(file)[email protected]:7:5" function_ref=unapplied))))

Score: <default 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0>
Type Variables:
  $T0 [allows bindings to: lvalue, noescape] [attributes: delayed] [with possible bindings: <empty>] @ locator@0x121019a00 [[email protected]:8:1]
  $T1 as [Cat] @ locator@0x121019b98 [[email protected]:8:3]
  $T2 as () @ locator@0x121019bf8 [[email protected]:8:1 → function result]
Inactive Constraints:
  disjunction @ locator@0x121019a00 [[email protected]:8:1]:
  >             $T0 bound to decl Source.(file)[email protected]:4:6 : ([Cat?]) -> ()
  >             $T0 bound to decl Source.(file)[email protected]:5:6 : ([Any]) -> ()
  ([Cat]) -> $T2 applicable fn $T0 @ locator@0x121019c90 [[email protected]:8:1 → apply function]
Resolved overloads:
  selected overload set choice a: $T1 == [Cat] for locator@0x121019b98 [[email protected]:8:3]

  (Potential Binding(s):
      Disjunction(s) = [$T0]
  )
  (attempting disjunction choice $T0 bound to decl Source.(file)[email protected]:4:6 : ([Cat?]) -> () @ locator@0x121019a00 [[email protected]:8:1]
    (overload set choice binding $T0 := ([Cat?]) -> ())
    (considering: ([Cat]) -> $T2 applicable fn $T0 @ locator@0x121019c90 [[email protected]:8:1 → apply function]
      (simplification result:
        (added constraint: disjunction @ locator@0x121019d90 [[email protected]:8:1 → apply argument → comparing call argument #0 to parameter #0]:
          >  [favored]  [Cat] bind [Cat?] [deep equality]
          >             [Cat] arg conv [Cat?] [array-upcast])
        (removed constraint: ([Cat]) -> $T2 applicable fn $T0 @ locator@0x121019c90 [[email protected]:8:1 → apply function])
      )
      (outcome: simplified)
    )
    (Changes:
      (Newly Bound:
        > $T0 := ([Cat?]) -> ()
      )
      (Added Constraint:
        > disjunction @ locator@0x121019d90 [[email protected]:8:1 → apply argument → comparing call argument #0 to parameter #0]:
            >  [favored]  [Cat] bind [Cat?] [deep equality]
            >             [Cat] arg conv [Cat?] [array-upcast]
      )
      (Removed Constraint:
        > ([Cat]) -> $T2 applicable fn $T0 @ locator@0x121019c90 [[email protected]:8:1 → apply function]
      )
    )
    (attempting disjunction choice [Cat] bind [Cat?] [deep equality] @ locator@0x121019d90 [[email protected]:8:1 → apply argument → comparing call argument #0 to parameter #0]
          (failed constraint [Cat] bind [Cat?] [deep equality] @ locator@0x121019d90 [[email protected]:8:1 → apply argument → comparing call argument #0 to parameter #0])
    )
    (attempting disjunction choice [Cat] arg conv [Cat?] [array-upcast] @ locator@0x121019d90 [[email protected]:8:1 → apply argument → comparing call argument #0 to parameter #0]
      (increasing 'collection upcast conversion' score by 1 @ locator@0x121019d90 [[email protected]:8:1 → apply argument → comparing call argument #0 to parameter #0])
      (increasing 'value to optional promotion' score by 1 @ locator@0x121019d90 [[email protected]:8:1 → apply argument → comparing call argument #0 to parameter #0] -> generic argument #0)
      (Changes:
      )
      (found solution: [component: collection upcast conversion(s), value: 1] [component: value to optional promotion(s), value: 1])
    )
  )
  (attempting disjunction choice $T0 bound to decl Source.(file)[email protected]:5:6 : ([Any]) -> () @ locator@0x121019a00 [[email protected]:8:1]
    (overload set choice binding $T0 := ([Any]) -> ())
    (considering: ([Cat]) -> $T2 applicable fn $T0 @ locator@0x121019c90 [[email protected]:8:1 → apply function]
      (simplification result:
        (added constraint: disjunction @ locator@0x121019d90 [[email protected]:8:1 → apply argument → comparing call argument #0 to parameter #0]:
          >  [favored]  [Cat] bind [Any] [deep equality]
          >             [Cat] arg conv [Any] [array-upcast])
        (removed constraint: ([Cat]) -> $T2 applicable fn $T0 @ locator@0x121019c90 [[email protected]:8:1 → apply function])
      )
      (outcome: simplified)
    )
    (Changes:
      (Newly Bound:
        > $T0 := ([Any]) -> ()
      )
      (Added Constraint:
        > disjunction @ locator@0x121019d90 [[email protected]:8:1 → apply argument → comparing call argument #0 to parameter #0]:
            >  [favored]  [Cat] bind [Any] [deep equality]
            >             [Cat] arg conv [Any] [array-upcast]
      )
      (Removed Constraint:
        > ([Cat]) -> $T2 applicable fn $T0 @ locator@0x121019c90 [[email protected]:8:1 → apply function]
      )
    )
    (attempting disjunction choice [Cat] bind [Any] [deep equality] @ locator@0x121019d90 [[email protected]:8:1 → apply argument → comparing call argument #0 to parameter #0]
          (failed constraint [Cat] bind [Any] [deep equality] @ locator@0x121019d90 [[email protected]:8:1 → apply argument → comparing call argument #0 to parameter #0])
    )
    (attempting disjunction choice [Cat] arg conv [Any] [array-upcast] @ locator@0x121019d90 [[email protected]:8:1 → apply argument → comparing call argument #0 to parameter #0]
      (increasing 'collection upcast conversion' score by 1 @ locator@0x121019d90 [[email protected]:8:1 → apply argument → comparing call argument #0 to parameter #0])
      (increasing 'empty-existential conversion' score by 1 @ locator@0x121019d90 [[email protected]:8:1 → apply argument → comparing call argument #0 to parameter #0] -> generic argument #0)
      (Changes:
      )
      (found solution: [component: collection upcast conversion(s), value: 1] [component: empty-existential conversion(s), value: 1])
    )
  )

---Solver statistics---
Total number of scopes explored: 7
Maximum depth reached while exploring solutions: 3
Time: 1.053000e+00ms
Comparing 2 viable solutions

--- Solution #0 ---
Fixed score: [component: collection upcast conversion(s), value: 1] [component: value to optional promotion(s), value: 1]
Type variables:
  $T0 as ([Cat?]) -> () @ locator@0x121019a00 [[email protected]:8:1]
  $T1 as [Cat] @ locator@0x121019b98 [[email protected]:8:3]
  $T2 as () @ locator@0x121019bf8 [[email protected]:8:1 → function result]

Overload choices:
  locator@0x121019a00 [[email protected]:8:1] with Source.(file)[email protected]:4:6 as f: ([Cat?]) -> ()
  locator@0x121019b98 [[email protected]:8:3] with Source.(file)[email protected]:7:5 as a: [Cat]
Constraint restrictions:
  Array<Cat> to Array<Optional<Cat>> is [array-upcast]
  Cat to Optional<Cat> is [value-to-optional]
Trailing closure matching:
  locator@0x121019d68 [[email protected]:8:1 → apply argument]: forward

--- Solution #1 ---
Fixed score: [component: collection upcast conversion(s), value: 1] [component: empty-existential conversion(s), value: 1]
Type variables:
  $T0 as ([Any]) -> () @ locator@0x121019a00 [[email protected]:8:1]
  $T1 as [Cat] @ locator@0x121019b98 [[email protected]:8:3]
  $T2 as () @ locator@0x121019bf8 [[email protected]:8:1 → function result]

Overload choices:
  locator@0x121019a00 [[email protected]:8:1] with Source.(file)[email protected]:5:6 as f: ([Any]) -> ()
  locator@0x121019b98 [[email protected]:8:3] with Source.(file)[email protected]:7:5 as a: [Cat]
Constraint restrictions:
  Cat to Any is [existential]
  Array<Cat> to Array<Any> is [array-upcast]
Trailing closure matching:
  locator@0x121019d68 [[email protected]:8:1 → apply argument]: forward
comparing solutions 1 and 0
comparing solutions 1 and 0

---Solution---
Fixed score: [component: collection upcast conversion(s), value: 1] [component: empty-existential conversion(s), value: 1]
Type variables:
  $T0 as ([Any]) -> () @ locator@0x121019a00 [[email protected]:8:1]
  $T1 as [Cat] @ locator@0x121019b98 [[email protected]:8:3]
  $T2 as () @ locator@0x121019bf8 [[email protected]:8:1 → function result]

Overload choices:
  locator@0x121019a00 [[email protected]:8:1] with Source.(file)[email protected]:5:6 as f: ([Any]) -> ()
  locator@0x121019b98 [[email protected]:8:3] with Source.(file)[email protected]:7:5 as a: [Cat]
Constraint restrictions:
  Cat to Any is [existential]
  Array<Cat> to Array<Any> is [array-upcast]
Trailing closure matching:
  locator@0x121019d68 [[email protected]:8:1 → apply argument]: forward

---Type-checked expression---
(call_expr type="()" location=Source.swift:8:1 range=[Source.swift:8:1 - line:8:4] isolation_crossing="none"
  (declref_expr type="([Any]) -> ()" location=Source.swift:8:1 range=[Source.swift:8:1 - line:8:1] decl="Source.(file)[email protected]:5:6" function_ref=single)
  (argument_list
    (argument
      (collection_upcast_expr implicit type="[Any]" location=Source.swift:8:3 range=[Source.swift:8:3 - line:8:3]
        (declref_expr type="[Cat]" location=Source.swift:8:3 range=[Source.swift:8:3 - line:8:3] decl="Source.(file)[email protected]:7:5" function_ref=unapplied)
        (value_conversion=erasure_expr implicit type="Any" location=Source.swift:8:3 range=[Source.swift:8:3 - line:8:3]
          (builtin_conformance type="Cat" protocol="Copyable")
          (builtin_conformance type="Cat" protocol="Escapable")
          (opaque_value_expr implicit type="Cat" location=Source.swift:8:3 range=[Source.swift:8:3 - line:8:3] "0x12101c050"))))))

Source.swift

class Animal {}
class Cat: Animal {}

func f(_ a: [Cat?]) { }
func f(_ a: [Any]) { }

let a: [Cat] = [Cat()]
f(a)