Introduce shouldReportUnimplemented.

Sources/Dependencies/DependencyKey.swift

@@ -250,3 +250,32 @@ extension TestDependencyKey {
   /// which will take precedence over this implementation.
   public static var previewValue: Value { Self.testValue }
 }
+
+extension TestDependencyKey {
+  /// Determines if it is appropriate to report an issue in an accessed `testValue`.
+  ///
+  /// When implementing the `testValue` requirement of ``TestDependencyKey`` you may want to report
+  /// an issue so that the user of the dependency is forced to override it in tests. However, one
+  /// cannot unconditionally report an issue because the getter of `testValue` is invoked when
+  /// setting.
+  ///
+  /// Check this value in order to determine if it is appropriate to report an issue or not:
+  ///
+  /// ```swift
+  /// private enum DefaultDatabaseKey: DependencyKey {
+  ///   static var testValue: any DatabaseWriter {
+  ///     if shouldReportUnimplemented {
+  ///       reportIssue("A blank, in-memory database is being used.")
+  ///     }
+  ///     return InMemoryDatabase()
+  ///   }
+  /// }
+  /// ```
+  public static var shouldReportUnimplemented: Bool {
+    #if DEBUG
+      return !DependencyValues.isSetting
+    #else
+      return false
+    #endif
+  }
+}

Tests/DependenciesTests/DependencyKeyTests.swift

@@ -188,6 +188,28 @@ final class DependencyKeyTests: XCTestCase {
       }
     #endif
   }
+
+#if DEBUG && !os(Linux) && !os(WASI) && !os(Windows)
+  func testShouldReportUnimplemented() {
+    XCTExpectFailure {
+      @Dependency(ReportIssueTestValueClient.self) var client
+      _ = client
+    } issueMatcher: { issue in
+      issue.compactDescription == """
+        failed - Override this dependency.
+        """
+    }
+  }
+  #endif
+
+  func testShouldReportUnimplemented_OverrideDependency() {
+    withDependencies {
+      $0[ReportIssueTestValueClient.self] = ReportIssueTestValueClient()
+    } operation: {
+      @Dependency(ReportIssueTestValueClient.self) var client
+      _ = client
+    }
+  }
 }
 
 private enum LiveKey: DependencyKey {
@@ -211,3 +233,12 @@ extension DependencyValues {
     set { self[NumberClient.self] = newValue }
   }
 }
+
+struct ReportIssueTestValueClient: TestDependencyKey {
+  static var testValue: ReportIssueTestValueClient {
+    if shouldReportUnimplemented {
+      reportIssue("Override this dependency.")
+    }
+    return ReportIssueTestValueClient()
+  }
+}