Fix issue with @overload in projections (#2582)

Fix #2533.

common/changes/@typespec/compiler/booleanTrueFix_2023-10-17-22-35.json

@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@typespec/compiler",
+      "comment": "Fix issue where using `@overload` could result in incorrect `unassignable` type errors.",
+      "type": "none"
+    }
+  ],
+  "packageName": "@typespec/compiler"
+}

common/changes/@typespec/openapi3/booleanTrueFix_2023-10-17-22-35.json

@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@typespec/openapi3",
+      "comment": "",
+      "type": "none"
+    }
+  ],
+  "packageName": "@typespec/openapi3"
+}

packages/compiler/src/lib/decorators.ts

@@ -1162,15 +1162,15 @@ const overloadsOperationKey = createStateSymbol("overloadsOperation");
 export function $overload(context: DecoratorContext, target: Operation, overloadBase: Operation) {
   // Ensure that the overloaded method arguments are a subtype of the original operation.
   const [paramValid, paramDiagnostics] = context.program.checker.isTypeAssignableTo(
-    target.parameters,
-    overloadBase.parameters,
+    target.parameters.projectionBase ?? target.parameters,
+    overloadBase.parameters.projectionBase ?? overloadBase.parameters,
     target
   );
   if (!paramValid) context.program.reportDiagnostics(paramDiagnostics);
 
   const [returnTypeValid, returnTypeDiagnostics] = context.program.checker.isTypeAssignableTo(
-    target.returnType,
-    overloadBase.returnType,
+    target.returnType.projectionBase ?? target.returnType,
+    overloadBase.returnType.projectionBase ?? overloadBase.returnType,
     target
   );
   if (!returnTypeValid) context.program.reportDiagnostics(returnTypeDiagnostics);

packages/openapi3/test/overloads.test.ts

@@ -109,4 +109,19 @@ describe("openapi3: overloads", () => {
       }
     `);
   });
+
+  it("can overload a boolean property with true or false", async () => {
+    const _ = await openApiFor(`
+      @test
+      op someThing(param: boolean): string | int32;
+
+      @test
+      @overload(someThing)
+      op someStringThing(param: true): string;
+      
+      @test
+      @overload(someThing)
+      op someNumberThing(param: false): int32;
+    `);
+  });
 });