feat(parser): add TS1174 error for classes extending multiple base classes

Implement TypeScript error TS1174 which is emitted when a class attempts
to extend multiple base classes using comma-separated syntax (e.g.,
'class C extends A, B { }').

In TypeScript, classes can only extend a single class, unlike interfaces
which can extend multiple interfaces. This implementation:

- Adds the diagnostic message with helpful guidance to use interfaces
  for multiple inheritance
- Checks the extends clause after parsing to detect multiple base classes
- Emits an error for each additional base class beyond the first

Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: sapphi-red

crates/oxc_parser/src/diagnostics.rs

@@ -327,6 +327,14 @@ pub fn extends_clause_must_precede_implements(span: Span, implements_span: Span)
         .with_help("Move the 'extends' clause before the 'implements' clause")
 }
 
+// Classes can only extend a single class. ts(1174)
+#[cold]
+pub fn classes_can_only_extend_single_class(span: Span) -> OxcDiagnostic {
+    ts_error("1174", "Classes can only extend a single class.")
+        .with_label(span)
+        .with_help("Remove the extra base class or use interfaces for multiple inheritance")
+}
+
 // 'implements' clause already seen. ts(1175)
 #[cold]
 pub fn implements_clause_already_seen(span: Span, seen_span: Span) -> OxcDiagnostic {

crates/oxc_parser/src/js/class.rs

@@ -90,6 +90,9 @@ impl<'a> ParserImpl<'a> {
             let first_extends = extends.remove(0);
             super_class = Some(first_extends.0);
             super_type_parameters = first_extends.1;
+            for (_, _, span) in extends {
+                self.error(diagnostics::classes_can_only_extend_single_class(span));
+            }
         }
         let body = self.parse_class_body();
 

tasks/coverage/snapshots/parser_typescript.snap

@@ -3,7 +3,7 @@ commit: 669c25c0
 parser_typescript Summary:
 AST Parsed     : 9821/9822 (99.99%)
 Positive Passed: 9812/9822 (99.90%)
-Negative Passed: 1482/2547 (58.19%)
+Negative Passed: 1485/2547 (58.30%)
 Expect Syntax Error: tasks/coverage/typescript/tests/cases/compiler/FunctionDeclaration3.ts
 
 Expect Syntax Error: tasks/coverage/typescript/tests/cases/compiler/FunctionDeclaration4.ts
@@ -120,8 +120,6 @@ Expect Syntax Error: tasks/coverage/typescript/tests/cases/compiler/classExtendi
 
 Expect Syntax Error: tasks/coverage/typescript/tests/cases/compiler/classExtendsInterface_not.ts
 
-Expect Syntax Error: tasks/coverage/typescript/tests/cases/compiler/classExtendsMultipleBaseClasses.ts
-
 Expect Syntax Error: tasks/coverage/typescript/tests/cases/compiler/classExtendsNull.ts
 
 Expect Syntax Error: tasks/coverage/typescript/tests/cases/compiler/classExtendsNull2.ts
@@ -686,8 +684,6 @@ Expect Syntax Error: tasks/coverage/typescript/tests/cases/compiler/multipleExpo
 
 Expect Syntax Error: tasks/coverage/typescript/tests/cases/compiler/multipleExports.ts
 
-Expect Syntax Error: tasks/coverage/typescript/tests/cases/compiler/multipleInheritance.ts
-
 Expect Syntax Error: tasks/coverage/typescript/tests/cases/compiler/multivar.ts
 
 Expect Syntax Error: tasks/coverage/typescript/tests/cases/compiler/namespaceMergedWithImportAliasNoCrash.ts
@@ -1758,8 +1754,6 @@ Expect Syntax Error: tasks/coverage/typescript/tests/cases/conformance/parser/ec
 
 Expect Syntax Error: tasks/coverage/typescript/tests/cases/conformance/parser/ecmascript5/ClassDeclarations/parserClassDeclaration18.ts
 
-Expect Syntax Error: tasks/coverage/typescript/tests/cases/conformance/parser/ecmascript5/ClassDeclarations/parserClassDeclaration6.ts
-
 Expect Syntax Error: tasks/coverage/typescript/tests/cases/conformance/parser/ecmascript5/ClassDeclarations/parserClassDeclaration7.ts
 
 Expect Syntax Error: tasks/coverage/typescript/tests/cases/conformance/parser/ecmascript5/ComputedPropertyNames/parserES5ComputedPropertyName4.ts
@@ -3628,6 +3622,14 @@ Expect to Parse: tasks/coverage/typescript/tests/cases/conformance/parser/ecmasc
  4 │ }
    ╰────
 
+  × TS(1174): Classes can only extend a single class.
+   ╭─[typescript/tests/cases/compiler/classExtendsMultipleBaseClasses.ts:3:19]
+ 2 │ class B { }
+ 3 │ class C extends A,B { }
+   ·                   ─
+   ╰────
+  help: Remove the extra base class or use interfaces for multiple inheritance
+
   × Expected `(` but found `}`
    ╭─[typescript/tests/cases/compiler/classFieldsBrokenConstructorEmitNoCrash1.ts:4:1]
  3 │   constructor
@@ -3636,6 +3638,14 @@ Expect to Parse: tasks/coverage/typescript/tests/cases/conformance/parser/ecmasc
    · ╰── `(` expected
    ╰────
 
+  × TS(1174): Classes can only extend a single class.
+   ╭─[typescript/tests/cases/compiler/classHeritageWithTrailingSeparator.ts:2:20]
+ 1 │     class C { foo: number }
+ 2 │ ╭─▶ class D extends C, {
+ 3 │ ╰─▶ }
+   ╰────
+  help: Remove the extra base class or use interfaces for multiple inheritance
+
   × Expected `{` but found `EOF`
    ╭─[typescript/tests/cases/compiler/classHeritageWithTrailingSeparator.ts:3:2]
  2 │ class D extends C, {
@@ -8415,6 +8425,24 @@ Expect to Parse: tasks/coverage/typescript/tests/cases/conformance/parser/ecmasc
  5 │     public private p4;
    ╰────
 
+  × TS(1174): Classes can only extend a single class.
+    ╭─[typescript/tests/cases/compiler/multipleInheritance.ts:9:21]
+  8 │ 
+  9 │ class C extends B1, B2 { // duplicate member
+    ·                     ──
+ 10 │ }
+    ╰────
+  help: Remove the extra base class or use interfaces for multiple inheritance
+
+  × TS(1174): Classes can only extend a single class.
+    ╭─[typescript/tests/cases/compiler/multipleInheritance.ts:18:21]
+ 17 │ 
+ 18 │ class E extends D1, D2 { // nope, duplicate member
+    ·                     ──
+ 19 │ }
+    ╰────
+  help: Remove the extra base class or use interfaces for multiple inheritance
+
   × Identifier `x` has already been declared
    ╭─[typescript/tests/cases/compiler/nameCollisions.ts:2:9]
  1 │ namespace T {
@@ -21142,6 +21170,14 @@ Expect to Parse: tasks/coverage/typescript/tests/cases/conformance/parser/ecmasc
    ╰────
   help: Merge the two 'implements' clauses into one by a ','
 
+  × TS(1174): Classes can only extend a single class.
+   ╭─[typescript/tests/cases/conformance/parser/ecmascript5/ClassDeclarations/parserClassDeclaration6.ts:1:20]
+ 1 │ class C extends A, B {
+   ·                    ─
+ 2 │ }
+   ╰────
+  help: Remove the extra base class or use interfaces for multiple inheritance
+
   × Constructor implementation is missing.
    ╭─[typescript/tests/cases/conformance/parser/ecmascript5/ClassDeclarations/parserClassDeclaration8.ts:2:3]
  1 │ class C {
@@ -21439,6 +21475,13 @@ Expect to Parse: tasks/coverage/typescript/tests/cases/conformance/parser/ecmasc
  2 │ }
    ╰────
 
+  × TS(1174): Classes can only extend a single class.
+   ╭─[typescript/tests/cases/conformance/parser/ecmascript5/ErrorRecovery/ExtendsOrImplementsClauses/parserErrorRecovery_ExtendsOrImplementsClause2.ts:1:20]
+ 1 │ ╭─▶ class C extends A, {
+ 2 │ ╰─▶ }
+   ╰────
+  help: Remove the extra base class or use interfaces for multiple inheritance
+
   × Expected `{` but found `EOF`
    ╭─[typescript/tests/cases/conformance/parser/ecmascript5/ErrorRecovery/ExtendsOrImplementsClauses/parserErrorRecovery_ExtendsOrImplementsClause2.ts:2:2]
  1 │ class C extends A, {
@@ -21460,6 +21503,14 @@ Expect to Parse: tasks/coverage/typescript/tests/cases/conformance/parser/ecmasc
  2 │ }
    ╰────
 
+  × TS(1174): Classes can only extend a single class.
+   ╭─[typescript/tests/cases/conformance/parser/ecmascript5/ErrorRecovery/ExtendsOrImplementsClauses/parserErrorRecovery_ExtendsOrImplementsClause5.ts:1:20]
+ 1 │ class C extends A, implements B, {
+   ·                    ──────────
+ 2 │ }
+   ╰────
+  help: Remove the extra base class or use interfaces for multiple inheritance
+
   × Expected `{` but found `Identifier`
    ╭─[typescript/tests/cases/conformance/parser/ecmascript5/ErrorRecovery/ExtendsOrImplementsClauses/parserErrorRecovery_ExtendsOrImplementsClause5.ts:1:31]
  1 │ class C extends A, implements B, {