[Type checker] Validate subscript declaration before using it.

test/multifile/Inputs/sr11458.swift

@@ -0,0 +1,54 @@
+protocol Observed: AnyObject {
+}
+
+struct Other<Value: Equatable> {
+  var value: Value
+
+  func hello() -> String {
+    return "Hello from \(value)"
+  }
+}
+
+@propertyWrapper
+struct Observable<Value: Equatable> {
+  private var stored: Value
+
+
+  init(wrappedValue: Value) {
+    self.stored = wrappedValue
+  }
+
+  var wrappedValue: Value {
+    get { fatalError("called wrappedValue getter") }
+    set { fatalError("called wrappedValue setter") }
+  }
+
+  var projectedValue: Other<Value> {
+    get { fatalError("called projectedValue getter") }
+    set { fatalError("called projectedValue setter") }
+  }
+
+  static subscript<EnclosingSelf: Observed, FinalValue>(
+      _enclosingInstance observed: EnclosingSelf,
+      wrapped wrappedKeyPath: ReferenceWritableKeyPath<EnclosingSelf, FinalValue>,
+      storage storageKeyPath: ReferenceWritableKeyPath<EnclosingSelf, Self>
+    ) -> Value {
+    get {
+      fatalError("blah")
+    }
+    set {
+    }
+  }
+
+  static subscript<EnclosingSelf: Observed>(
+      _enclosingInstance observed: EnclosingSelf,
+      projected wrappedKeyPath: ReferenceWritableKeyPath<EnclosingSelf, Other<Value>>,
+      storage storageKeyPath: ReferenceWritableKeyPath<EnclosingSelf, Self>
+    ) -> Other<Value> {
+    get {
+      fatalError("blah")
+    }
+    set {
+    }
+  }
+}

test/multifile/property-wrappers-sr11458.swift

@@ -0,0 +1,10 @@
+// RUN: %target-swift-frontend -typecheck -primary-file %s %S/Inputs/sr11458.swift
+
+// SR-11458: crash involving subscript in other file
+class MyOtherType<T: Equatable>: Observed {
+  @Observable var x: T
+
+  init(x: T) {
+    self.x = x
+  }
+}