From 221bc8b1ab5d39a64282037c0c167339af7f3709 Mon Sep 17 00:00:00 2001
From: Timothee Guerin <tiguerin@microsoft.com>
Date: Tue, 20 Feb 2024 12:37:29 -0800
Subject: [PATCH] Report error when having a circular template constraint
 (#2932)

fix [#2788](https://github.com/microsoft/typespec/issues/2788)
---
 ...-template-constraint-2024-1-16-22-25-53.md |  8 +++++
 packages/compiler/src/core/checker.ts         | 17 +++++++++++
 packages/compiler/src/core/messages.ts        |  6 ++++
 .../compiler/test/checker/templates.test.ts   | 30 +++++++++++++++++++
 4 files changed, 61 insertions(+)
 create mode 100644 .chronus/changes/fix-circular-template-constraint-2024-1-16-22-25-53.md

diff --git a/.chronus/changes/fix-circular-template-constraint-2024-1-16-22-25-53.md b/.chronus/changes/fix-circular-template-constraint-2024-1-16-22-25-53.md
new file mode 100644
index 0000000000..97e9b5cc05
--- /dev/null
+++ b/.chronus/changes/fix-circular-template-constraint-2024-1-16-22-25-53.md
@@ -0,0 +1,8 @@
+---
+# Change versionKind to one of: internal, fix, dependencies, feature, deprecation, breaking
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+Report error when having a circular template constraint e.g. `model Example<T extends T>`
diff --git a/packages/compiler/src/core/checker.ts b/packages/compiler/src/core/checker.ts
index 42d8da6db2..f9c6688a7a 100644
--- a/packages/compiler/src/core/checker.ts
+++ b/packages/compiler/src/core/checker.ts
@@ -711,6 +711,7 @@ export function createChecker(program: Program): Checker {
       | AliasStatementNode
       | InterfaceStatementNode
       | OperationStatementNode
+      | TemplateParameterDeclarationNode
       | UnionStatementNode
   ): number {
     const symbol =
@@ -750,6 +751,19 @@ export function createChecker(program: Program): Checker {
     const grandParentNode = parentNode.parent;
     const links = getSymbolLinks(node.symbol);
 
+    if (pendingResolutions.has(getNodeSymId(node), ResolutionKind.Constraint)) {
+      if (mapper === undefined) {
+        reportCheckerDiagnostic(
+          createDiagnostic({
+            code: "circular-constraint",
+            format: { typeName: node.id.sv },
+            target: node.constraint!,
+          })
+        );
+      }
+      return errorType;
+    }
+
     let type: TemplateParameter | undefined = links.declaredType as TemplateParameter;
     if (type === undefined) {
       if (grandParentNode) {
@@ -770,7 +784,9 @@ export function createChecker(program: Program): Checker {
       });
 
       if (node.constraint) {
+        pendingResolutions.start(getNodeSymId(node), ResolutionKind.Constraint);
         type.constraint = getTypeOrValueTypeForNode(node.constraint);
+        pendingResolutions.finish(getNodeSymId(node), ResolutionKind.Constraint);
       }
       if (node.default) {
         type.default = checkTemplateParameterDefault(
@@ -6330,6 +6346,7 @@ const _assertReflectionNameToKind: Record<string, Type["kind"]> = ReflectionName
 enum ResolutionKind {
   Type,
   BaseType,
+  Constraint,
 }
 
 class PendingResolutions {
diff --git a/packages/compiler/src/core/messages.ts b/packages/compiler/src/core/messages.ts
index d7b09b4752..b6edb9f520 100644
--- a/packages/compiler/src/core/messages.ts
+++ b/packages/compiler/src/core/messages.ts
@@ -854,6 +854,12 @@ const diagnostics = {
       default: paramMessage`Type '${"typeName"}' recursively references itself as a base type.`,
     },
   },
+  "circular-constraint": {
+    severity: "error",
+    messages: {
+      default: paramMessage`Type parameter '${"typeName"}' has a circular constraint.`,
+    },
+  },
   "circular-op-signature": {
     severity: "error",
     messages: {
diff --git a/packages/compiler/test/checker/templates.test.ts b/packages/compiler/test/checker/templates.test.ts
index 84d974d414..4538a72c53 100644
--- a/packages/compiler/test/checker/templates.test.ts
+++ b/packages/compiler/test/checker/templates.test.ts
@@ -428,6 +428,36 @@ describe("compiler: templates", () => {
       `);
     });
 
+    it("emits diagnostic when constraint reference itself", async () => {
+      testHost.addTypeSpecFile("main.tsp", `model Test<A extends A> {}`);
+
+      const diagnostics = await testHost.diagnose("main.tsp");
+      expectDiagnostics(diagnostics, {
+        code: "circular-constraint",
+        message: "Type parameter 'A' has a circular constraint.",
+      });
+    });
+
+    it("emits diagnostic when constraint reference other parameter in circular constraint", async () => {
+      testHost.addTypeSpecFile("main.tsp", `model Test<A extends B, B extends A> {}`);
+
+      const diagnostics = await testHost.diagnose("main.tsp");
+      expectDiagnostics(diagnostics, {
+        code: "circular-constraint",
+        message: "Type parameter 'A' has a circular constraint.",
+      });
+    });
+
+    it("emits diagnostic when constraint reference itself inside an expression", async () => {
+      testHost.addTypeSpecFile("main.tsp", `model Test<A extends {name: A}> {}`);
+
+      const diagnostics = await testHost.diagnose("main.tsp");
+      expectDiagnostics(diagnostics, {
+        code: "circular-constraint",
+        message: "Type parameter 'A' has a circular constraint.",
+      });
+    });
+
     it("emit diagnostics if template default is not assignable to constraint", async () => {
       const { source, pos, end } = extractSquiggles(`
         model A<T extends string = ~~~123~~~> { a: T }