[core] Disallow overriding a required property with an optional property

packages/compiler/src/core/checker.ts

@@ -4618,6 +4618,8 @@ export function createChecker(program: Program): Checker {
       const parentType = getTypeName(overriddenProp.type);
       const newPropType = getTypeName(newProp.type);
 
+      let invalid = false;
+
       if (!isAssignable) {
         reportCheckerDiagnostic(
           createDiagnostic({
@@ -4626,8 +4628,22 @@ export function createChecker(program: Program): Checker {
             target: diagnosticTarget ?? newProp,
           })
         );
-        return;
+        invalid = true;
       }
+
+      if (!overriddenProp.optional && newProp.optional) {
+        reportCheckerDiagnostic(
+          createDiagnostic({
+            code: "override-property-mismatch",
+            messageId: "disallowedOptionalOverride",
+            format: { propName: overriddenProp.name },
+            target: diagnosticTarget ?? newProp,
+          })
+        );
+        invalid = true;
+      }
+
+      if (invalid) return;
     }
 
     properties.set(newProp.name, newProp);

packages/compiler/src/core/messages.ts

@@ -357,6 +357,7 @@ const diagnostics = {
     severity: "error",
     messages: {
       default: paramMessage`Model has an inherited property named ${"propName"} of type ${"propType"} which cannot override type ${"parentType"}`,
+      disallowedOptionalOverride: paramMessage`Model has a required inherited property named ${"propName"} which cannot be overridden as optional`,
     },
   },
   "extend-scalar": {

packages/compiler/test/checker/model.test.ts

@@ -445,6 +445,73 @@ describe("compiler: models", () => {
       ]);
     });
 
+    it("disallows subtype overriding required parent property with optional property", async () => {
+      testHost.addTypeSpecFile(
+        "main.tsp",
+        `
+        model A { x: int32; }
+        model B extends A { x?: int32; }
+        `
+      );
+
+      const diagnostics = await testHost.diagnose("main.tsp");
+      expectDiagnostics(diagnostics, [
+        {
+          code: "override-property-mismatch",
+          severity: "error",
+          message:
+            "Model has a required inherited property named x which cannot be overridden as optional",
+        },
+      ]);
+    });
+
+    it("disallows subtype overriding required parent property with optional through multiple levels of inheritance", async () => {
+      testHost.addTypeSpecFile(
+        "main.tsp",
+        `
+        model A { x: int32; }
+        model B extends A { }
+        model C extends B { x?: int16; }
+        `
+      );
+
+      const diagnostics = await testHost.diagnose("main.tsp");
+      expectDiagnostics(diagnostics, [
+        {
+          code: "override-property-mismatch",
+          severity: "error",
+          message:
+            "Model has a required inherited property named x which cannot be overridden as optional",
+        },
+      ]);
+    });
+
+    it("shows both errors when an override is optional and not assignable", async () => {
+      testHost.addTypeSpecFile(
+        "main.tsp",
+        `
+        model A { x: int32; }
+        model B extends A { x?: string; }
+        `
+      );
+
+      const diagnostics = await testHost.diagnose("main.tsp");
+      expectDiagnostics(diagnostics, [
+        {
+          code: "override-property-mismatch",
+          severity: "error",
+          message:
+            "Model has an inherited property named x of type string which cannot override type int32",
+        },
+        {
+          code: "override-property-mismatch",
+          severity: "error",
+          message:
+            "Model has a required inherited property named x which cannot be overridden as optional",
+        },
+      ]);
+    });
+
     it("allow multiple overrides", async () => {
       testHost.addTypeSpecFile(
         "main.tsp",