fix(37519): forbid trailing comma in a index signature (#37535)

src/compiler/checker.ts

@@ -36658,6 +36658,7 @@ namespace ts {
                     return grammarErrorOnNode(node, Diagnostics.An_index_signature_must_have_exactly_one_parameter);
                 }
             }
+            checkGrammarForDisallowedTrailingComma(node.parameters, Diagnostics.An_index_signature_cannot_have_a_trailing_comma);
             if (parameter.dotDotDotToken) {
                 return grammarErrorOnNode(parameter.dotDotDotToken, Diagnostics.An_index_signature_cannot_have_a_rest_parameter);
             }

src/compiler/diagnosticMessages.json

@@ -83,6 +83,10 @@
         "category": "Error",
         "code": 1024
     },
+    "An index signature cannot have a trailing comma.": {
+        "category": "Error",
+        "code": 1025
+    },
     "Accessibility modifier already seen.": {
         "category": "Error",
         "code": 1028

tests/baselines/reference/indexSignatureWithTrailingComma.errors.txt

@@ -0,0 +1,24 @@
+tests/cases/compiler/indexSignatureWithTrailingComma.ts(2,17): error TS1025: An index signature cannot have a trailing comma.
+tests/cases/compiler/indexSignatureWithTrailingComma.ts(6,17): error TS1025: An index signature cannot have a trailing comma.
+tests/cases/compiler/indexSignatureWithTrailingComma.ts(10,17): error TS1025: An index signature cannot have a trailing comma.
+
+
+==== tests/cases/compiler/indexSignatureWithTrailingComma.ts (3 errors) ====
+    type A = {
+        [key: string,]: string;
+                    ~
+!!! error TS1025: An index signature cannot have a trailing comma.
+    };
+    
+    interface B {
+        [key: string,]: string;
+                    ~
+!!! error TS1025: An index signature cannot have a trailing comma.
+    }
+    
+    class C {
+        [key: string,]: null;
+                    ~
+!!! error TS1025: An index signature cannot have a trailing comma.
+    }
+    

tests/baselines/reference/indexSignatureWithTrailingComma.js

@@ -0,0 +1,20 @@
+//// [indexSignatureWithTrailingComma.ts]
+type A = {
+    [key: string,]: string;
+};
+
+interface B {
+    [key: string,]: string;
+}
+
+class C {
+    [key: string,]: null;
+}
+
+
+//// [indexSignatureWithTrailingComma.js]
+var C = /** @class */ (function () {
+    function C() {
+    }
+    return C;
+}());

tests/baselines/reference/indexSignatureWithTrailingComma.symbols

@@ -0,0 +1,23 @@
+=== tests/cases/compiler/indexSignatureWithTrailingComma.ts ===
+type A = {
+>A : Symbol(A, Decl(indexSignatureWithTrailingComma.ts, 0, 0))
+
+    [key: string,]: string;
+>key : Symbol(key, Decl(indexSignatureWithTrailingComma.ts, 1, 5))
+
+};
+
+interface B {
+>B : Symbol(B, Decl(indexSignatureWithTrailingComma.ts, 2, 2))
+
+    [key: string,]: string;
+>key : Symbol(key, Decl(indexSignatureWithTrailingComma.ts, 5, 5))
+}
+
+class C {
+>C : Symbol(C, Decl(indexSignatureWithTrailingComma.ts, 6, 1))
+
+    [key: string,]: null;
+>key : Symbol(key, Decl(indexSignatureWithTrailingComma.ts, 9, 5))
+}
+

tests/baselines/reference/indexSignatureWithTrailingComma.types

@@ -0,0 +1,22 @@
+=== tests/cases/compiler/indexSignatureWithTrailingComma.ts ===
+type A = {
+>A : A
+
+    [key: string,]: string;
+>key : string
+
+};
+
+interface B {
+    [key: string,]: string;
+>key : string
+}
+
+class C {
+>C : C
+
+    [key: string,]: null;
+>key : string
+>null : null
+}
+

tests/cases/compiler/indexSignatureWithTrailingComma.ts

@@ -0,0 +1,11 @@
+type A = {
+    [key: string,]: string;
+};
+
+interface B {
+    [key: string,]: string;
+}
+
+class C {
+    [key: string,]: null;
+}