fixAddMissingMember: Support interface and don't crash on type parameter

src/services/codefixes/helpers.ts

@@ -117,6 +117,7 @@ namespace ts.codefix {
         inJs: boolean,
         makeStatic: boolean,
         preferences: UserPreferences,
+        body: boolean,
     ): MethodDeclaration {
         const checker = context.program.getTypeChecker();
         const types = map(args,
@@ -142,7 +143,7 @@ namespace ts.codefix {
                 createTypeParameterDeclaration(CharacterCodes.T + typeArguments!.length - 1 <= CharacterCodes.Z ? String.fromCharCode(CharacterCodes.T + i) : `T${i}`)),
             /*parameters*/ createDummyParameters(args.length, names, types, /*minArgumentCount*/ undefined, inJs),
             /*type*/ inJs ? undefined : createKeywordTypeNode(SyntaxKind.AnyKeyword),
-            createStubbedMethodBody(preferences));
+            body ? createStubbedMethodBody(preferences) : undefined);
     }
 
     function createDummyParameters(argCount: number, names: (string | undefined)[] | undefined, types: (TypeNode | undefined)[] | undefined, minArgumentCount: number | undefined, inJs: boolean): ParameterDeclaration[] {

src/services/textChanges.ts

@@ -409,14 +409,14 @@ namespace ts.textChanges {
             });
         }
 
-        public insertNodeAtClassStart(sourceFile: SourceFile, cls: ClassLikeDeclaration, newElement: ClassElement): void {
+        public insertNodeAtClassStart(sourceFile: SourceFile, cls: ClassLikeDeclaration | InterfaceDeclaration, newElement: ClassElement): void {
             const clsStart = cls.getStart(sourceFile);
             const indentation = formatting.SmartIndenter.findFirstNonWhitespaceColumn(getLineStartPositionForPosition(clsStart, sourceFile), clsStart, sourceFile, this.formatContext.options)
                 + this.formatContext.options.indentSize!;
             this.insertNodeAt(sourceFile, cls.members.pos, newElement, { indentation, ...this.getInsertNodeAtClassStartPrefixSuffix(sourceFile, cls) });
         }
 
-        private getInsertNodeAtClassStartPrefixSuffix(sourceFile: SourceFile, cls: ClassLikeDeclaration): { prefix: string, suffix: string } {
+        private getInsertNodeAtClassStartPrefixSuffix(sourceFile: SourceFile, cls: ClassLikeDeclaration | InterfaceDeclaration): { prefix: string, suffix: string } {
             if (cls.members.length === 0) {
                 if (addToSeen(this.classesWithNodesInsertedAtStart, getNodeId(cls), cls)) {
                     // For `class C {\n}`, don't add the trailing "\n"
@@ -703,7 +703,7 @@ namespace ts.textChanges {
         return skipTrivia(sourceFile.text, getAdjustedStartPosition(sourceFile, node, {}, Position.FullStart), /*stopAfterLineBreak*/ false, /*stopAtComments*/ true);
     }
 
-    function getClassBraceEnds(cls: ClassLikeDeclaration, sourceFile: SourceFile): [number, number] {
+    function getClassBraceEnds(cls: ClassLikeDeclaration | InterfaceDeclaration, sourceFile: SourceFile): [number, number] {
         return [findChildOfKind(cls, SyntaxKind.OpenBraceToken, sourceFile)!.end, findChildOfKind(cls, SyntaxKind.CloseBraceToken, sourceFile)!.end];
     }
 

src/services/utilities.ts

@@ -1232,7 +1232,7 @@ namespace ts {
     }
 
     export function skipConstraint(type: Type): Type {
-        return type.isTypeParameter() ? type.getConstraint()! : type; // TODO: GH#18217
+        return type.isTypeParameter() ? type.getConstraint() || type : type;
     }
 
     export function getNameFromPropertyName(name: PropertyName): string | undefined {

tests/cases/fourslash/codeFixAddMissingMember_typeParameter.ts

@@ -0,0 +1,32 @@
+////interface I {}
+////
+////function f<T>(t: T): number {
+////    return t.foo;
+////}
+////
+////function g<T extends I>(t: T): number {
+////    return t.bar;
+////}
+
+// No code fix for "foo"
+
+verify.codeFixAvailable([
+    { description: "Declare property 'bar'" }, { description: "Add index signature for property 'bar'" },
+])
+
+verify.codeFix({
+    description: "Declare property 'bar'",
+    index: 0,
+    newFileContent:
+`interface I {
+    bar: any;
+}
+
+function f<T>(t: T): number {
+    return t.foo;
+}
+
+function g<T extends I>(t: T): number {
+    return t.bar;
+}`,
+});

tests/cases/fourslash/codeFixUndeclaredPropertyAccesses.ts

@@ -14,6 +14,11 @@
 //// let t: T<number>;
 //// t.x;
 
-verify.codeFixAvailable([{
-    description: "Add missing enum member 'c'"
-}]);
+verify.codeFixAvailable([
+    "Declare property 'y'",
+    "Add index signature for property 'y'",
+    "Declare method 'foo'",
+    "Declare property 'foo'",
+    "Add index signature for property 'foo'",
+    "Add missing enum member 'c'",
+].map(description => ({ description })));