Template instantiated with ErrorType will get the arg changed to unkn…

…own (#2874)

fix #2722

---------

Co-authored-by: Mark Cowlishaw <markcowl@microsoft.com>

.changeset/green-starfishes-cry.md

@@ -0,0 +1,5 @@
+---
+"@typespec/compiler": patch
+---
+
+Template instantiated with ErrorType will get the arg changed to unknown

packages/compiler/src/core/checker.ts

@@ -1090,11 +1090,15 @@ export function createChecker(program: Program): Checker {
           commit(param, effectiveType);
           continue;
         }
+      } else if (isErrorType(type)) {
+        // If we got an error type we don't want to keep passing it through so we reduce to unknown
+        // Similar to the above where if the type is not assignable to the constraint we reduce to the constraint
+        commit(param, unknownType);
+        continue;
       }
 
       commit(param, type);
     }
-
     return finalMap;
   }
 

packages/compiler/test/checker/templates.test.ts

@@ -1,7 +1,8 @@
-import { deepStrictEqual, fail, strictEqual } from "assert";
+import { deepStrictEqual, fail, ok, strictEqual } from "assert";
 import { beforeEach, describe, it } from "vitest";
 import { getSourceLocation } from "../../src/core/diagnostics.js";
 import { Diagnostic, Model, StringLiteral, Type } from "../../src/core/types.js";
+import { isUnknownType } from "../../src/index.js";
 import {
   BasicTestRunner,
   TestHost,
@@ -244,6 +245,30 @@ describe("compiler: templates", () => {
       });
     });
 
+    it("an error type should revert to unknown", async () => {
+      testHost.addTypeSpecFile(
+        "main.tsp",
+        `
+          model Test<T> {
+            @test prop: T;
+          }
+          
+          model Bar {
+            a: Test<notExists>;
+          }
+        `
+      );
+      const [{ prop }, diagnostics] = await testHost.compileAndDiagnose("main.tsp");
+      // Only one error
+      expectDiagnostics(diagnostics, {
+        code: "unknown-identifier",
+        message: "Unknown identifier notExists",
+      });
+
+      strictEqual(prop.kind, "ModelProperty");
+      ok(isUnknownType(prop.type), "Prop type should be unknown");
+    });
+
     it("operation should still be able to be used(no extra diagnostic)", async () => {
       const { pos, source } = extractCursor(`
     op Action<T extends {}>(): T;