Add validation for keys in PickProperties

.chronus/changes/feature-pickProperties-key-validation-2025-1-21-11-5-49.md

@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/compiler"
+---
+
+Keys Validation for PickProperties

packages/compiler/src/core/messages.ts

@@ -471,6 +471,7 @@ const diagnostics = {
     severity: "error",
     messages: {
       default: paramMessage`Object value may only specify known properties, and '${"propertyName"}' does not exist in type '${"type"}'.`,
+      typeMissingProperty: paramMessage`Property '${"propertyName"}' does not exist in type '${"type"}'.`,
     },
   },
   "extends-interface": {

packages/compiler/src/lib/decorators.ts

@@ -78,6 +78,7 @@ import {
   DiagnosticTarget,
   Enum,
   EnumValue,
+  IndeterminateEntity,
   Interface,
   Model,
   ModelProperty,
@@ -937,10 +938,43 @@ export const $withPickedProperties: WithPickedPropertiesDecorator = (
     }
   }
 
+  // Validate that all picked properties exist
+  for (const name of pickedNames) {
+    validatePropertyName(context, target, name);
+  }
+
   // Remove all properties not picked
   filterModelPropertiesInPlace(target, (prop) => pickedNames.has(prop.name));
 };
 
+function validatePropertyName(context: DecoratorContext, target: Model, name: string) {
+  const source = target.templateMapper?.args[0] as Model;
+  const entity = target.templateMapper?.args[1];
+  if (source && entity && !target.properties.has(name)) {
+    reportDiagnostic(context.program, {
+      code: "unexpected-property",
+      format: {
+        propertyName: name,
+        type: getTypeName(source),
+      },
+      messageId: "typeMissingProperty",
+      target: getExactDiagnosticTarget(entity, name),
+    });
+  }
+}
+
+function getExactDiagnosticTarget(
+  entity: Type | Value | IndeterminateEntity,
+  name: string,
+): DiagnosticTarget {
+  if ("kind" in entity && entity.kind === "Union") {
+    return (
+      entity.node.options.find((option) => "value" in option && option.value === name) ?? entity
+    );
+  }
+  return entity;
+}
+
 // -- @withoutDefaultValues decorator ----------------------
 
 export const $withoutDefaultValues: WithoutDefaultValuesDecorator = (

packages/compiler/test/decorators/decorators.test.ts

@@ -825,6 +825,50 @@ describe("compiler: built-in decorators", () => {
       const properties = TestModel.kind === "Model" ? Array.from(TestModel.properties.keys()) : [];
       deepStrictEqual(properties, ["pickMe", "pickMeToo"]);
     });
+
+    it("picks model inherited property", async () => {
+      const { TestModel } = await runner.compile(
+        `
+        model ParentModel {
+          pickMe: string;
+        }
+
+        model OriginalModel extends ParentModel {
+          pickMeToo: string;
+          notMe: string;
+        }
+
+        @test
+        model TestModel is PickProperties<OriginalModel, "pickMe"> {
+        }`,
+      );
+
+      const properties = TestModel.kind === "Model" ? Array.from(TestModel.properties.keys()) : [];
+      deepStrictEqual(properties, ["pickMe"]);
+    });
+
+    it("emits diagnostics if any given key is not a property of a model", async () => {
+      const diagnostics = await runner.diagnose(
+        `
+        model OriginalModel {
+          pickMe: string;
+          pickMeToo: string;
+          notMe: string;
+        }
+
+        model TestModel is PickProperties<OriginalModel, "pickMe" | "notMee"> {
+        }`,
+      );
+
+      expectDiagnostics(diagnostics, [
+        {
+          code: "unexpected-property",
+          message: "Property 'notMee' does not exist in type 'OriginalModel'.",
+          pos: 190,
+          end: 198,
+        },
+      ]);
+    });
   });
 
   describe("@overload", () => {