<stdin>:6:22: error: cannot convert value of type 'KeyPath<User, String>' to type '(User) -> String' in coercion
let f3 = \User.email as (User) -> String
         ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~

---Constraint solving at [<stdin>:6:10 - line:6:35]---
---Initial constraints for the given expression---
(coerce_expr type='(User) -> String' location=<stdin>:6:22 range=[<stdin>:6:10 - line:6:35] writtenType='(User) -> String'
  (keypath_expr type='$T3' location=<stdin>:6:10 range=[<stdin>:6:10 - line:6:16]
    (components      
      (unresolved_property decl_name='email' type='_'))
    (parsed_root
      (unresolved_dot_expr type='<null>' field 'email' function_ref=unapplied
        (type_expr type='<null>' typerepr='User')))))
Score: 0 0 0 0 0 0 0 0 0 0 0 0 0
Contextual Type: <null>
Type Variables:
  $T0 [noescape allowed] bindings={(supertypes of) User} @ locator@0x7fe1e6938800 [KeyPath@<stdin>:6:10 -> key path root]
  $T1 [lvalue allowed] [noescape allowed] fully_bound subtype_of_existential involves_type_vars bindings={} @ locator@0x7fe1e6938910 [KeyPath@<stdin>:6:10 -> key path component #&#8203;0 -> key path component result]
  $T2 [noescape allowed] subtype_of_existential involves_type_vars bindings={} @ locator@0x7fe1e69389e0 [KeyPath@<stdin>:6:10 -> key path value]
  $T3 [noescape allowed] involves_type_vars #defaultable_bindings=1 bindings={<<unresolvedtype>>} @ locator@0x7fe1e6938aa0 [KeyPath@<stdin>:6:10 -> key path type]

Active Constraints:

Inactive Constraints:
  User subtype $T0 [[locator@0x7fe1e6938820 [KeyPath@<stdin>:6:10]]];
  @lvalue $T0[.email: value] == $T1 [[locator@0x7fe1e69388f0 [KeyPath@<stdin>:6:10 -> key path component #&#8203;0]]];
  $T1 equal $T2 [[locator@0x7fe1e6938820 [KeyPath@<stdin>:6:10]]];
  $T3 key path from $T0 -> $T2 [[locator@0x7fe1e6938820 [KeyPath@<stdin>:6:10]]];
  disjunction (remembered) [[locator@0x7fe1e6938b70 [Coerce@<stdin>:6:22]]]:$T3 conv (User) -> String [[locator@0x7fe1e6938b70 [Coerce@<stdin>:6:22]]]; or $T3 bridging conv (User) -> String [[locator@0x7fe1e6938b70 [Coerce@<stdin>:6:22]]];
  ($T0 bindings={(supertypes of) User})
  Initial bindings: $T0 := User
  (attempting type variable $T0 := User
    (overload set choice binding $T1 := String)
    (attempting disjunction choice $T3 conv (User) -> String [[locator@0x7fe1e6938b70 [Coerce@<stdin>:6:22]]];
    )
    (attempting disjunction choice $T3 bridging conv (User) -> String [[locator@0x7fe1e6938b70 [Coerce@<stdin>:6:22]]];
      (increasing score due to user conversion)
    )
  )
---Solver statistics---
Total number of scopes explored: 4
Maximum depth reached while exploring solutions: 3
Time: 6.445000e+00ms
---Attempting to salvage and emit diagnostics---