[CSSimplify] Broaden delay of the key path assignment

If key path is connected to a disjunction directly or indirectly
(i.e. via ApplicableFunction constraint) do not attempt to bind
it until disjunction is taken, otherwise there is a risk to miss
a valid keypath-to-function conversion.

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -12409,7 +12409,8 @@ ConstraintSystem::simplifyKeyPathConstraint(
     return SolutionKind::Solved;
   } else if (!anyComponentsUnresolved ||
              (definitelyKeyPathType && capability == ReadOnly)) {
-    // If key path is connected to a function application it cannot be
+    // If key path is connected to a disjunction (i.e. through coercion
+    // or used as an argument to a function/subscipt call) it cannot be
     // bound until the choice is selected because it's undetermined
     // until then whether key path is implicitly converted to a function
     // type or not.
@@ -12436,7 +12437,8 @@ ConstraintSystem::simplifyKeyPathConstraint(
                        typeVar, OptionalWrappingDirection::Unwrap,
                        [&](Constraint *match, TypeVariableType *) {
                          return match->getKind() ==
-                                ConstraintKind::ApplicableFunction;
+                                    ConstraintKind::ApplicableFunction ||
+                                match->getKind() == ConstraintKind::Disjunction;
                        })) {
       formUnsolved();
     } else {

test/expr/unary/keypath/keypath.swift

@@ -1213,3 +1213,14 @@ func keypath_to_func_conversion_as_arg_to_overloaded_func() {
     test(arr, \Data.value) // Ok
   }
 }
+
+// https://github.com/apple/swift/issues/55436
+func test_keypath_coercion_to_function() {
+  struct User {
+    let email: String
+  }
+
+  let users = [User]()
+  let fn = \User.email as (User) -> String // Ok
+  _ = users.map(fn) // Ok
+}