Allow spreading a model that has props added in previous version

.chronus/changes/allow-spreading-model-previous-version-2024-6-19-18-50-40.md

@@ -0,0 +1,8 @@
+---
+# Change versionKind to one of: internal, fix, dependencies, feature, deprecation, breaking
+changeKind: fix
+packages:
+  - "@typespec/versioning"
+---
+
+Allow spreading a model that has props added in previous version

packages/versioning/src/validate.ts

@@ -6,12 +6,15 @@ import {
   isTemplateInstance,
   isType,
   navigateProgram,
+  type ModelProperty,
   type Namespace,
   type Program,
   type Type,
   type TypeNameOptions,
 } from "@typespec/compiler";
 import {
+  $added,
+  $removed,
   findVersionedNamespace,
   getMadeOptionalOn,
   getMadeRequiredOn,
@@ -772,6 +775,30 @@ function validateAvailabilityForRef(
   }
 }
 
+function canIgnoreDependentVersioning(type: Type, versioning: "added" | "removed") {
+  if (type.kind === "ModelProperty") {
+    return canIgnoreVersioningOnProperty(type, versioning);
+  }
+  return false;
+}
+
+function canIgnoreVersioningOnProperty(
+  prop: ModelProperty,
+  versioning: "added" | "removed"
+): boolean {
+  if (prop.sourceProperty === undefined) {
+    return false;
+  }
+
+  const decoratorFn = versioning === "added" ? $added : $removed;
+  // Check if the decorator was defined on this property or a source property. If source property ignore.
+  const selfDecorators = prop.decorators.filter((x) => x.decorator === decoratorFn);
+  const sourceDecorators = prop.sourceProperty.decorators.filter(
+    (x) => x.decorator === decoratorFn
+  );
+  return !selfDecorators.some((x) => !sourceDecorators.some((y) => x.node === y.node));
+}
+
 function validateAvailabilityForContains(
   program: Program,
   sourceAvail: Map<string, Availability> | undefined,
@@ -791,7 +818,8 @@ function validateAvailabilityForContains(
     if (sourceVal === targetVal) continue;
     if (
       [Availability.Added].includes(targetVal) &&
-      [Availability.Removed, Availability.Unavailable].includes(sourceVal)
+      [Availability.Removed, Availability.Unavailable].includes(sourceVal) &&
+      !canIgnoreDependentVersioning(target, "added")
     ) {
       const sourceAddedOn = findAvailabilityOnOrBeforeVersion(key, Availability.Added, sourceAvail);
       reportDiagnostic(program, {
@@ -808,7 +836,8 @@ function validateAvailabilityForContains(
     }
     if (
       [Availability.Removed].includes(sourceVal) &&
-      [Availability.Added, Availability.Available].includes(targetVal)
+      [Availability.Added, Availability.Available].includes(targetVal) &&
+      !canIgnoreDependentVersioning(target, "removed")
     ) {
       const targetRemovedOn = findAvailabilityAfterVersion(key, Availability.Removed, targetAvail);
       reportDiagnostic(program, {

packages/versioning/test/incompatible-versioning.test.ts

@@ -412,6 +412,34 @@ describe("versioning: validate incompatible references", () => {
       expectDiagnosticEmpty(diagnostics);
     });
 
+    it("succeed when spreading a model that might have add properties added in previous versions", async () => {
+      const diagnostics = await runner.diagnose(`
+        model Base {
+          @added(Versions.v1) name: string;
+        }
+
+        @added(Versions.v2)
+        model Child {
+          ...Base;
+        }
+      `);
+      expectDiagnosticEmpty(diagnostics);
+    });
+
+    it("succeed when spreading a model that might have add properties removed after the model", async () => {
+      const diagnostics = await runner.diagnose(`
+        model Base {
+          @removed(Versions.v3) name: string;
+        }
+
+        @removed(Versions.v2)
+        model Child {
+          ...Base;
+        }
+      `);
+      expectDiagnosticEmpty(diagnostics);
+    });
+
     it("emit diagnostic when model property was added before model itself", async () => {
       const diagnostics = await runner.diagnose(`
         @added(Versions.v3)