Disallow 'readonly' modifier in ambient module import attributes types

tsc/internal/checker/grammarchecks.go

@@ -2207,6 +2207,13 @@ func (c *Checker) checkGrammarImportAttributesType(attributes *ast.TypeLiteralNo
 			return c.grammarErrorOnNode(member, diagnostics.An_import_attributes_type_may_only_contain_property_signatures)
 		}
 		propertySignature := member.AsPropertySignatureDeclaration()
+		if modifiers := propertySignature.Modifiers(); modifiers != nil {
+			for _, modifier := range modifiers.Nodes {
+				if modifier.Kind == ast.KindReadonlyKeyword {
+					return c.grammarErrorOnNode(modifier, diagnostics.An_import_attributes_property_cannot_have_a_readonly_modifier)
+				}
+			}
+		}
 		if propertySignature.Type == nil {
 			return c.grammarErrorOnNode(member, diagnostics.An_import_attributes_property_must_have_a_type_annotation)
 		}

tsc/internal/diagnostics/diagnosticMessages.json

@@ -1876,6 +1876,10 @@
         "category": "Error",
         "code": 1557
     },
+    "An import attributes property cannot have a 'readonly' modifier.": {
+        "category": "Error",
+        "code": 1558
+    },
 
     "The types of '{0}' are incompatible between these types.": {
         "category": "Error",

tsc/testdata/baselines/reference/compiler/importAttributeTypeReadonly.errors.txt

@@ -0,0 +1,34 @@
+/otherModifierError.d.ts(1,31): error TS1070: 'public' modifier cannot appear on a type member.
+/readonlyError.d.ts(1,31): error TS1558: An import attributes property cannot have a 'readonly' modifier.
+/readonlyError.d.ts(6,32): error TS1558: An import attributes property cannot have a 'readonly' modifier.
+
+
+==== /readonlyError.d.ts (2 errors) ====
+    declare module "*.css" with { readonly type: "css" } {
+                                  ~~~~~~~~
+!!! error TS1558: An import attributes property cannot have a 'readonly' modifier.
+        const stylesheet: CSSStyleSheet;
+        export default stylesheet;
+    }
+
+    declare module "*.json" with { readonly type: "json", readonly kind: "data" } {
+                                   ~~~~~~~~
+!!! error TS1558: An import attributes property cannot have a 'readonly' modifier.
+        const data: any;
+        export default data;
+    }
+
+==== /otherModifierError.d.ts (1 errors) ====
+    declare module "*.svg" with { public type: "svg" } {
+                                  ~~~~~~
+!!! error TS1070: 'public' modifier cannot appear on a type member.
+        const content: string;
+        export default content;
+    }
+
+==== /valid.d.ts (0 errors) ====
+    declare module "*.txt" with { type: "text" } {
+        const text: string;
+        export default text;
+    }
+

tsc/testdata/tests/cases/compiler/importAttributeTypeReadonly.ts

@@ -0,0 +1,27 @@
+// @strict: true
+// @module: preserve
+// @noEmit: true
+// @noTypesAndSymbols: true
+
+// @filename: /readonlyError.d.ts
+declare module "*.css" with { readonly type: "css" } {
+    const stylesheet: CSSStyleSheet;
+    export default stylesheet;
+}
+
+declare module "*.json" with { readonly type: "json", readonly kind: "data" } {
+    const data: any;
+    export default data;
+}
+
+// @filename: /otherModifierError.d.ts
+declare module "*.svg" with { public type: "svg" } {
+    const content: string;
+    export default content;
+}
+
+// @filename: /valid.d.ts
+declare module "*.txt" with { type: "text" } {
+    const text: string;
+    export default text;
+}