Fix: Do not crash when trying to access member of aliased expression

.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

packages/compiler/src/core/checker.ts

@@ -2400,10 +2400,24 @@ 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 === ".") {
@@ -2559,11 +2573,19 @@ 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];

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 "."?`,
+    });
+  });
 });