sample of debug constraints

output.txt

// swiftc -Xfrontend -debug-constraints Source.swift
// swift-driver version: 1.90.11.1 Apple Swift version 5.10 (swiftlang-5.10.0.13 clang-1500.3.9.4)
// Target: arm64-apple-macosx14.0
---Constraint solving at [Source.swift:10:5 - line:10:22]---
  (overload set choice binding $T1 := () -> Cat)

---Initial constraints for the given expression---
(array_expr type='[Cat]' location=Source.swift:10:16 range=[Source.swift:10:16 - line:10:22] initializer=**NULL**
  (call_expr type='Cat' location=Source.swift:10:17 range=[Source.swift:10:17 - line:10:21] isolationCrossing=none
    (type_expr type='Cat.Type' location=Source.swift:10:17 range=[Source.swift:10:17 - line:10:17] 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:10:8 - line:10:12]
Type Variables:
  $T0 as Cat @ locator@0x1601c3230 [Call@Source.swift:10:17 → function result]
  $T1 as () -> Cat @ locator@0x1601c3300 [Call@Source.swift:10:17 → apply function → constructor member type (for short-form or self.init call)]
  $T2 as () @ locator@0x1601c32d8 [Call@Source.swift:10:17 → apply argument]
  $T3 as [Cat] @ locator@0x1601c3550 [Array@Source.swift:10:16 → contextual type → pattern match → pattern match → named pattern decl]
Resolved overloads:
  selected overload set choice Cat.Type.init: $T1 == () -> Cat for locator@0x1601c3370 [Call@Source.swift:10:17 → 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: 4.030000e-01ms

---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@0x1601c3230 [Call@Source.swift:10:17 → function result]
  $T1 as () -> Cat @ locator@0x1601c3300 [Call@Source.swift:10:17 → apply function → constructor member type (for short-form or self.init call)]
  $T2 as () @ locator@0x1601c32d8 [Call@Source.swift:10:17 → apply argument]
  $T3 as [Cat] @ locator@0x1601c3550 [Array@Source.swift:10:16 → contextual type → pattern match → pattern match → named pattern decl]

Overload choices:
  locator@0x1601c3370 [Call@Source.swift:10:17 → apply function → constructor member] with Source.(file).Cat.init()@Source.swift:2:19 as Cat.Type.init: () -> Cat
Trailing closure matching:
  locator@0x1601c32d8 [Call@Source.swift:10:17 → apply argument]: forward

---Type-checked expression---
(array_expr type='[Cat]' location=Source.swift:10:16 range=[Source.swift:10:16 - line:10:22] initializer=Swift.(file).Array extension.init(arrayLiteral:) [with (substitution_map generic_signature=<Element> (substitution Element -> Cat))]
  (call_expr type='Cat' location=Source.swift:10:17 range=[Source.swift:10:17 - line:10:21] isolationCrossing=none
    (constructor_ref_call_expr type='() -> Cat' location=Source.swift:10:17 range=[Source.swift:10:17 - line:10:17] isolationCrossing=none
      (declref_expr implicit type='(Cat.Type) -> () -> Cat' location=Source.swift:10:17 range=[Source.swift:10:17 - line:10:17] decl=Source.(file).Cat.init()@Source.swift:2:19 function_ref=single)
      (argument_list implicit
        (argument
          (type_expr type='Cat.Type' location=Source.swift:10:17 range=[Source.swift:10:17 - line:10:17] typerepr='Cat'))))
    (argument_list)))

---Constraint solving at [Source.swift:12:1 - line:12:4]---
  (overload set choice binding $T1 := [Cat])
(common result type for $T0 is ())

---Initial constraints for the given expression---
(call_expr type='()' location=Source.swift:12:1 range=[Source.swift:12:1 - line:12:4] isolationCrossing=none
  (overloaded_decl_ref_expr type='$T0' location=Source.swift:12:1 range=[Source.swift:12:1 - line:12:1] name=f number_of_decls=3 function_ref=single decls=[
    Source.(file).f@Source.swift:4:6,
    Source.(file).f@Source.swift:6:6,
    Source.(file).f@Source.swift:8:6])
  (argument_list
    (argument
      (declref_expr type='[Cat]' location=Source.swift:12:3 range=[Source.swift:12:3 - line:12:3] decl=Source.(file).a@Source.swift:10:5 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@0x15781d200 [OverloadedDeclRef@Source.swift:12:1]
  $T1 as [Cat] @ locator@0x15781d408 [DeclRef@Source.swift:12:3]
  $T2 as () @ locator@0x15781d468 [Call@Source.swift:12:1 → function result]
Inactive Constraints:
  disjunction @ locator@0x15781d200 [OverloadedDeclRef@Source.swift:12:1]:
  >             $T0 bound to decl Source.(file).f@Source.swift:4:6 : ([Cat?]) -> ()
  >             $T0 bound to decl Source.(file).f@Source.swift:6:6 : ([Any]) -> ()
  >             $T0 bound to decl Source.(file).f@Source.swift:8:6 : (UnsafePointer<Cat>) -> ()
  ([Cat]) -> $T2 applicable fn $T0 @ locator@0x15781d500 [Call@Source.swift:12:1 → apply function]
Resolved overloads:
  selected overload set choice a: $T1 == [Cat] for locator@0x15781d408 [DeclRef@Source.swift:12:3]

  (Potential Binding(s): 
      Disjunction(s) = [$T0]
  )
  (attempting disjunction choice $T0 bound to decl Source.(file).f@Source.swift:4:6 : ([Cat?]) -> () @ locator@0x15781d200 [OverloadedDeclRef@Source.swift:12:1]
    (overload set choice binding $T0 := ([Cat?]) -> ())
    (considering: ([Cat]) -> $T2 applicable fn $T0 @ locator@0x15781d500 [Call@Source.swift:12:1 → apply function]
      (simplification result:
        (added constraint: disjunction @ locator@0x15781d600 [Call@Source.swift:12:1 → 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@0x15781d500 [Call@Source.swift:12:1 → apply function])
      )
      (outcome: simplified)
    )
    (Changes:
      (Newly Bound: 
        > $T0 := ([Cat?]) -> ()
      )
      (Added Constraint: 
        > disjunction @ locator@0x15781d600 [Call@Source.swift:12:1 → 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@0x15781d500 [Call@Source.swift:12:1 → apply function]
      )
    )
    (attempting disjunction choice [Cat] bind [Cat?] [deep equality] @ locator@0x15781d600 [Call@Source.swift:12:1 → apply argument → comparing call argument #0 to parameter #0]
          (failed constraint [Cat] bind [Cat?] [deep equality] @ locator@0x15781d600 [Call@Source.swift:12:1 → apply argument → comparing call argument #0 to parameter #0])
    )
    (attempting disjunction choice [Cat] arg conv [Cat?] [array-upcast] @ locator@0x15781d600 [Call@Source.swift:12:1 → apply argument → comparing call argument #0 to parameter #0]
      (increasing 'collection upcast conversion' score by 1 @ locator@0x15781d600 [Call@Source.swift:12:1 → apply argument → comparing call argument #0 to parameter #0])
      (increasing 'value to optional promotion' score by 1 @ locator@0x15781d600 [Call@Source.swift:12:1 → 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).f@Source.swift:6:6 : ([Any]) -> () @ locator@0x15781d200 [OverloadedDeclRef@Source.swift:12:1]
    (overload set choice binding $T0 := ([Any]) -> ())
    (considering: ([Cat]) -> $T2 applicable fn $T0 @ locator@0x15781d500 [Call@Source.swift:12:1 → apply function]
      (simplification result:
        (added constraint: disjunction @ locator@0x15781d600 [Call@Source.swift:12:1 → 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@0x15781d500 [Call@Source.swift:12:1 → apply function])
      )
      (outcome: simplified)
    )
    (Changes:
      (Newly Bound: 
        > $T0 := ([Any]) -> ()
      )
      (Added Constraint: 
        > disjunction @ locator@0x15781d600 [Call@Source.swift:12:1 → 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@0x15781d500 [Call@Source.swift:12:1 → apply function]
      )
    )
    (attempting disjunction choice [Cat] bind [Any] [deep equality] @ locator@0x15781d600 [Call@Source.swift:12:1 → apply argument → comparing call argument #0 to parameter #0]
          (failed constraint [Cat] bind [Any] [deep equality] @ locator@0x15781d600 [Call@Source.swift:12:1 → apply argument → comparing call argument #0 to parameter #0])
    )
    (attempting disjunction choice [Cat] arg conv [Any] [array-upcast] @ locator@0x15781d600 [Call@Source.swift:12:1 → apply argument → comparing call argument #0 to parameter #0]
      (increasing 'collection upcast conversion' score by 1 @ locator@0x15781d600 [Call@Source.swift:12:1 → apply argument → comparing call argument #0 to parameter #0])
      (increasing 'empty-existential conversion' score by 1 @ locator@0x15781d600 [Call@Source.swift:12:1 → 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])
    )
  )
  (attempting disjunction choice $T0 bound to decl Source.(file).f@Source.swift:8:6 : (UnsafePointer<Cat>) -> () @ locator@0x15781d200 [OverloadedDeclRef@Source.swift:12:1]
    (overload set choice binding $T0 := (UnsafePointer<Cat>) -> ())
    (considering: ([Cat]) -> $T2 applicable fn $T0 @ locator@0x15781d500 [Call@Source.swift:12:1 → apply function]
      (simplification result:
    (increasing 'value-to-pointer conversion' score by 1 @ locator@0x15781d5d8 [Call@Source.swift:12:1 → apply argument] -> comparing call argument #0 to parameter #0)
        (removed constraint: ([Cat]) -> $T2 applicable fn $T0 @ locator@0x15781d500 [Call@Source.swift:12:1 → apply function])
      )
      (outcome: simplified)
    )
    (Changes:
      (Newly Bound: 
        > $T0 := (UnsafePointer<Cat>) -> ()
      )
      (Removed Constraint: 
        > ([Cat]) -> $T2 applicable fn $T0 @ locator@0x15781d500 [Call@Source.swift:12:1 → apply function]
      )
    )
    (found solution: [component: value-to-pointer conversion(s), value: 1])
  )

---Solver statistics---
Total number of scopes explored: 8
Maximum depth reached while exploring solutions: 3
Time: 9.840000e-01ms
Comparing 3 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@0x15781d200 [OverloadedDeclRef@Source.swift:12:1]
  $T1 as [Cat] @ locator@0x15781d408 [DeclRef@Source.swift:12:3]
  $T2 as () @ locator@0x15781d468 [Call@Source.swift:12:1 → function result]

Overload choices:
  locator@0x15781d200 [OverloadedDeclRef@Source.swift:12:1] with Source.(file).f@Source.swift:4:6 as f: ([Cat?]) -> ()
  locator@0x15781d408 [DeclRef@Source.swift:12:3] with Source.(file).a@Source.swift:10:5 as a: [Cat]
Constraint restrictions:
  Cat to Optional<Cat> is [value-to-optional]
  Array<Cat> to Array<Optional<Cat>> is [array-upcast]
Trailing closure matching:
  locator@0x15781d5d8 [Call@Source.swift:12:1 → 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@0x15781d200 [OverloadedDeclRef@Source.swift:12:1]
  $T1 as [Cat] @ locator@0x15781d408 [DeclRef@Source.swift:12:3]
  $T2 as () @ locator@0x15781d468 [Call@Source.swift:12:1 → function result]

Overload choices:
  locator@0x15781d200 [OverloadedDeclRef@Source.swift:12:1] with Source.(file).f@Source.swift:6:6 as f: ([Any]) -> ()
  locator@0x15781d408 [DeclRef@Source.swift:12:3] with Source.(file).a@Source.swift:10:5 as a: [Cat]
Constraint restrictions:
  Array<Cat> to Array<Any> is [array-upcast]
  Cat to Any is [existential]
Trailing closure matching:
  locator@0x15781d5d8 [Call@Source.swift:12:1 → apply argument]: forward

--- Solution #2 ---
Fixed score: [component: value-to-pointer conversion(s), value: 1]
Type variables:
  $T0 as (UnsafePointer<Cat>) -> () @ locator@0x15781d200 [OverloadedDeclRef@Source.swift:12:1]
  $T1 as [Cat] @ locator@0x15781d408 [DeclRef@Source.swift:12:3]
  $T2 as () @ locator@0x15781d468 [Call@Source.swift:12:1 → function result]

Overload choices:
  locator@0x15781d200 [OverloadedDeclRef@Source.swift:12:1] with Source.(file).f@Source.swift:8:6 as f: (UnsafePointer<Cat>) -> ()
  locator@0x15781d408 [DeclRef@Source.swift:12:3] with Source.(file).a@Source.swift:10:5 as a: [Cat]
Constraint restrictions:
  Array<Cat> to UnsafePointer<Cat> is [array-to-pointer]
Trailing closure matching:
  locator@0x15781d5d8 [Call@Source.swift:12:1 → apply argument]: forward
comparing solutions 1 and 0
comparing solutions 2 and 1
comparing solutions 2 and 0
comparing solutions 2 and 1

---Solution---
Fixed score: [component: value-to-pointer conversion(s), value: 1]
Type variables:
  $T0 as (UnsafePointer<Cat>) -> () @ locator@0x15781d200 [OverloadedDeclRef@Source.swift:12:1]
  $T1 as [Cat] @ locator@0x15781d408 [DeclRef@Source.swift:12:3]
  $T2 as () @ locator@0x15781d468 [Call@Source.swift:12:1 → function result]

Overload choices:
  locator@0x15781d200 [OverloadedDeclRef@Source.swift:12:1] with Source.(file).f@Source.swift:8:6 as f: (UnsafePointer<Cat>) -> ()
  locator@0x15781d408 [DeclRef@Source.swift:12:3] with Source.(file).a@Source.swift:10:5 as a: [Cat]
Constraint restrictions:
  Array<Cat> to UnsafePointer<Cat> is [array-to-pointer]
Trailing closure matching:
  locator@0x15781d5d8 [Call@Source.swift:12:1 → apply argument]: forward

---Type-checked expression---
(call_expr type='()' location=Source.swift:12:1 range=[Source.swift:12:1 - line:12:4] isolationCrossing=none
  (declref_expr type='(UnsafePointer<Cat>) -> ()' location=Source.swift:12:1 range=[Source.swift:12:1 - line:12:1] decl=Source.(file).f@Source.swift:8:6 function_ref=single)
  (argument_list
    (argument
      (array_to_pointer implicit type='UnsafePointer<Cat>' location=Source.swift:12:3 range=[Source.swift:12:3 - line:12:3]
        (declref_expr type='[Cat]' location=Source.swift:12:3 range=[Source.swift:12:3 - line:12:3] decl=Source.(file).a@Source.swift:10:5 function_ref=unapplied)))))

Source.swift

class Animal {}
class Cat: Animal {}

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

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

func f(_ a: UnsafePointer<Cat>) {}

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

f(a)