From 7d104581ef253380dc05a28d5f8b949369c8842f Mon Sep 17 00:00:00 2001
From: Travis Prescott <tjprescott@users.noreply.github.com>
Date: Wed, 18 Oct 2023 09:52:05 -0700
Subject: [PATCH] Fix issue with `@overload` in projections (#2582)

Fix #2533.
---
 .../compiler/booleanTrueFix_2023-10-17-22-35.json | 10 ++++++++++
 .../openapi3/booleanTrueFix_2023-10-17-22-35.json | 10 ++++++++++
 packages/compiler/src/lib/decorators.ts           |  8 ++++----
 packages/openapi3/test/overloads.test.ts          | 15 +++++++++++++++
 4 files changed, 39 insertions(+), 4 deletions(-)
 create mode 100644 common/changes/@typespec/compiler/booleanTrueFix_2023-10-17-22-35.json
 create mode 100644 common/changes/@typespec/openapi3/booleanTrueFix_2023-10-17-22-35.json

diff --git a/common/changes/@typespec/compiler/booleanTrueFix_2023-10-17-22-35.json b/common/changes/@typespec/compiler/booleanTrueFix_2023-10-17-22-35.json
new file mode 100644
index 0000000000..b62fc76e27
--- /dev/null
+++ b/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"
+}
diff --git a/common/changes/@typespec/openapi3/booleanTrueFix_2023-10-17-22-35.json b/common/changes/@typespec/openapi3/booleanTrueFix_2023-10-17-22-35.json
new file mode 100644
index 0000000000..d9a70151de
--- /dev/null
+++ b/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"
+}
\ No newline at end of file
diff --git a/packages/compiler/src/lib/decorators.ts b/packages/compiler/src/lib/decorators.ts
index 65762720b5..88e65dd4a7 100644
--- a/packages/compiler/src/lib/decorators.ts
+++ b/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);
diff --git a/packages/openapi3/test/overloads.test.ts b/packages/openapi3/test/overloads.test.ts
index 102b34d336..5aaceb21c3 100644
--- a/packages/openapi3/test/overloads.test.ts
+++ b/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;
+    `);
+  });
 });