From eb1185103db31a5897f4edfd103dee974343e366 Mon Sep 17 00:00:00 2001
From: Timothee Guerin <tiguerin@microsoft.com>
Date: Mon, 22 Apr 2024 18:50:06 -0700
Subject: [PATCH] Fix: Do not crash when trying to access member of aliased
 expression (#3188)

---
 ...ler-crash-alias-union-2024-3-18-2-39-55.md |  8 +++++
 packages/compiler/src/core/checker.ts         | 32 ++++++++++++++---
 packages/compiler/test/checker/alias.test.ts  | 34 +++++++++++++++++++
 3 files changed, 69 insertions(+), 5 deletions(-)
 create mode 100644 .chronus/changes/fix-compiler-crash-alias-union-2024-3-18-2-39-55.md

diff --git a/.chronus/changes/fix-compiler-crash-alias-union-2024-3-18-2-39-55.md b/.chronus/changes/fix-compiler-crash-alias-union-2024-3-18-2-39-55.md
new file mode 100644
index 0000000000..e727e7e96f
--- /dev/null
+++ b/.chronus/changes/fix-compiler-crash-alias-union-2024-3-18-2-39-55.md
@@ -0,0 +1,8 @@
+---
+# Change versionKind to one of: internal, fix, dependencies, feature, deprecation, breaking
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+Fix: Do not crash when trying to access member of aliased expressions
diff --git a/packages/compiler/src/core/checker.ts b/packages/compiler/src/core/checker.ts
index ab97ef8fb3..e42e5f52b7 100644
--- a/packages/compiler/src/core/checker.ts
+++ b/packages/compiler/src/core/checker.ts
@@ -2402,10 +2402,24 @@ export function createChecker(program: Program): Checker {
 
       // when resolving a type reference based on an alias, unwrap the alias.
       if (base.flags & SymbolFlags.Alias) {
-        base = getAliasedSymbol(base, mapper, options);
-        if (!base) {
+        const aliasedSym = getAliasedSymbol(base, mapper, options);
+        if (!aliasedSym) {
+          reportCheckerDiagnostic(
+            createDiagnostic({
+              code: "invalid-ref",
+              messageId: "node",
+              format: {
+                id: node.id.sv,
+                nodeName: base.declarations[0]
+                  ? SyntaxKind[base.declarations[0].kind]
+                  : "Unknown node",
+              },
+              target: node,
+            })
+          );
           return undefined;
         }
+        base = aliasedSym;
       }
 
       if (node.selector === ".") {
@@ -2561,11 +2575,19 @@ export function createChecker(program: Program): Checker {
     while (current.flags & SymbolFlags.Alias) {
       const node = current.declarations[0];
       const targetNode = node.kind === SyntaxKind.AliasStatement ? node.value : node;
-      const sym = resolveTypeReferenceSymInternal(targetNode as any, mapper, options);
-      if (sym === undefined) {
+      if (
+        targetNode.kind === SyntaxKind.TypeReference ||
+        targetNode.kind === SyntaxKind.MemberExpression ||
+        targetNode.kind === SyntaxKind.Identifier
+      ) {
+        const sym = resolveTypeReferenceSymInternal(targetNode, mapper, options);
+        if (sym === undefined) {
+          return undefined;
+        }
+        current = sym;
+      } else {
         return undefined;
       }
-      current = sym;
     }
     const sym = current;
     const node = aliasSymbol.declarations[0];
diff --git a/packages/compiler/test/checker/alias.test.ts b/packages/compiler/test/checker/alias.test.ts
index 551ea8fe57..3cc2201ed8 100644
--- a/packages/compiler/test/checker/alias.test.ts
+++ b/packages/compiler/test/checker/alias.test.ts
@@ -258,4 +258,38 @@ describe("compiler: aliases", () => {
     const diagnostics = await testHost.diagnose("main.tsp");
     expectDiagnosticEmpty(diagnostics);
   });
+
+  // REGRESSION TEST: https://github.com/microsoft/typespec/issues/3125
+  it("trying to access member of aliased union expression shouldn't crash", async () => {
+    testHost.addTypeSpecFile(
+      "main.tsp",
+      `
+      alias A = {foo: string} | {bar: string};
+
+      alias Aliased = A.prop;
+
+      `
+    );
+    const diagnostics = await testHost.diagnose("main.tsp");
+    expectDiagnostics(diagnostics, {
+      code: "invalid-ref",
+      message: `Cannot resolve 'prop' in node AliasStatement since it has no members. Did you mean to use "::" instead of "."?`,
+    });
+  });
+  it("trying to access member of aliased model expression shouldn't crash", async () => {
+    testHost.addTypeSpecFile(
+      "main.tsp",
+      `
+      alias A = {foo: string};
+
+      alias Aliased = A.prop;
+
+      `
+    );
+    const diagnostics = await testHost.diagnose("main.tsp");
+    expectDiagnostics(diagnostics, {
+      code: "invalid-ref",
+      message: `Cannot resolve 'prop' in node AliasStatement since it has no members. Did you mean to use "::" instead of "."?`,
+    });
+  });
 });