[CSApply] Allow marker existential to superclass conversions

If existential is a protocol composition type where all of the
protocols are `@_marker`, narrowly allow coercion to its superclass
bound (if it matches). Both types have the same representation
which makes it okay.

This is only a problem in Swift 5 mode without strict concurrency
checks. In this mode `@preconcurrency` stripping happens
outside of the solver which means that no conversion restrictions
are recorded for members that got `& Sendable` stripped from
their types.

For example:

```swift
struct S {
  @preconcurrency static let member: KeyPath<String, Int> & Sendable
}

func test() {
  _ = S.member
}
```

Since `member` is `@preconcurrency` its type would get concurrency
annotations stripped, which includes `& Sendable` which means that
the solver uses `KeyPath<String, Int>` type for the reference and
not the original `KeyPath<String, Int> & Sendable`, this is a problem
for `ExprRewritter::adjustTypeForDeclReference` because conversion
between existential and its superclass bound requires a constraint
restriction which won't be available in this case.

Resolves: rdar://132700409
(cherry picked from commit 38aa71d)

Author: xedin

lib/Sema/CSApply.cpp

@@ -7763,6 +7763,20 @@ Expr *ExprRewriter::coerceToType(Expr *expr, Type toType,
     llvm_unreachable("BuiltinTupleType should not show up here");
   }
 
+  // Allow existential-to-supertype conversion if all protocol
+  // bounds are marker protocols. Normally this requires a
+  // conversion restriction but there are situations related
+  // to `@preconcurrency` where the `& Sendable` would be stripped
+  // transparently to the solver.
+  if (auto *existential = fromType->getAs<ExistentialType>()) {
+    if (auto *PCT = existential->getConstraintType()
+                        ->getAs<ProtocolCompositionType>()) {
+      if (PCT->withoutMarkerProtocols()->isEqual(toType)) {
+        return coerceSuperclass(expr, toType);
+      }
+    }
+  }
+
   // Unresolved types come up in diagnostics for lvalue and inout types.
   if (fromType->hasUnresolvedType() || toType->hasUnresolvedType())
     return cs.cacheType(new (ctx) UnresolvedTypeConversionExpr(expr, toType));

test/Concurrency/predates_concurrency.swift

@@ -302,3 +302,33 @@ do {
     }
   }
 }
+
+// rdar://132700409 - coercion PartialKeyPath & Sendable -> PartialKeyPath crashes in CSApply
+do {
+  struct Test {
+    enum KeyPath {
+      static var member: PartialKeyPath<Test> {
+        fatalError()
+      }
+    }
+  }
+
+  struct KeyPathComparator<Compared> {
+    @preconcurrency public let keyPath: any PartialKeyPath<Compared> & Sendable
+
+    func testDirect() {
+      switch keyPath { // Ok
+      case Test.KeyPath.member: break // Ok
+      default: break
+      }
+    }
+
+    func testErasure() {
+      let kp: PartialKeyPath<Compared> = keyPath
+      switch kp { // Ok
+      case Test.KeyPath.member: break // Ok
+      default: break
+      }
+    }
+  }
+}