diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 76e11d53811..b77b5a62177 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -432,6 +432,7 @@ namespace ts {
             getIndexInfosOfType,
             getSignaturesOfType,
             getIndexTypeOfType: (type, kind) => getIndexTypeOfType(type, kind === IndexKind.String ? stringType : numberType),
+            getIndexType: type => getIndexType(type),
             getBaseTypes,
             getBaseTypeOfLiteralType,
             getWidenedType,
@@ -15344,10 +15345,6 @@ namespace ts {
                 (type.flags & (TypeFlags.InstantiableNonPrimitive | TypeFlags.Index | TypeFlags.TemplateLiteral | TypeFlags.StringMapping) && !isPatternLiteralType(type) ? ObjectFlags.IsGenericIndexType : 0);
         }
 
-        function isThisTypeParameter(type: Type): boolean {
-            return !!(type.flags & TypeFlags.TypeParameter && (type as TypeParameter).isThisType);
-        }
-
         function getSimplifiedType(type: Type, writing: boolean): Type {
             return type.flags & TypeFlags.IndexedAccess ? getSimplifiedIndexedAccessType(type as IndexedAccessType, writing) :
                 type.flags & TypeFlags.Conditional ? getSimplifiedConditionalType(type as ConditionalType, writing) :
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index bdff510977b..e7e60dc0252 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -4162,6 +4162,7 @@ namespace ts {
         getIndexInfosOfType(type: Type): readonly IndexInfo[];
         getSignaturesOfType(type: Type, kind: SignatureKind): readonly Signature[];
         getIndexTypeOfType(type: Type, kind: IndexKind): Type | undefined;
+        /* @internal */ getIndexType(type: Type): Type;
         getBaseTypes(type: InterfaceType): BaseType[];
         getBaseTypeOfLiteralType(type: Type): Type;
         getWidenedType(type: Type): Type;
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index a24f61a93fa..72ce071940d 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -7414,4 +7414,8 @@ namespace ts {
     export function escapeSnippetText(text: string): string {
         return text.replace(/\$/gm, "\\$");
     }
+
+    export function isThisTypeParameter(type: Type): boolean {
+        return !!(type.flags & TypeFlags.TypeParameter && (type as TypeParameter).isThisType);
+    }
 }
diff --git a/src/services/services.ts b/src/services/services.ts
index c9c54c0234e..c65f5221686 100644
--- a/src/services/services.ts
+++ b/src/services/services.ts
@@ -500,6 +500,9 @@ namespace ts {
         isClass(): this is InterfaceType {
             return !!(getObjectFlags(this) & ObjectFlags.Class);
         }
+        isIndexType(): this is IndexType {
+            return !!(this.flags & TypeFlags.Index);
+        }
         /**
          * This polyfills `referenceType.typeArguments` for API consumers
          */
@@ -545,6 +548,16 @@ namespace ts {
         getReturnType(): Type {
             return this.checker.getReturnTypeOfSignature(this);
         }
+        getTypeParameterAtPosition(pos: number): Type {
+            const type = this.checker.getParameterType(this, pos);
+            if (type.isIndexType() && isThisTypeParameter(type.type)) {
+                const constraint = type.type.getConstraint();
+                if (constraint) {
+                    return this.checker.getIndexType(constraint);
+                }
+            }
+            return type;
+        }
 
         getDocumentationComment(): SymbolDisplayPart[] {
             return this.documentationComment || (this.documentationComment = getDocumentationComment(singleElementArray(this.declaration), this.checker));
diff --git a/src/services/stringCompletions.ts b/src/services/stringCompletions.ts
index 2c1a453df6d..b84b578ed4f 100644
--- a/src/services/stringCompletions.ts
+++ b/src/services/stringCompletions.ts
@@ -264,7 +264,7 @@ namespace ts.Completions.StringCompletions {
         checker.getResolvedSignature(argumentInfo.invocation, candidates, argumentInfo.argumentCount);
         const types = flatMap(candidates, candidate => {
             if (!signatureHasRestParameter(candidate) && argumentInfo.argumentCount > candidate.parameters.length) return;
-            const type = checker.getParameterType(candidate, argumentInfo.argumentIndex);
+            const type = candidate.getTypeParameterAtPosition(argumentInfo.argumentIndex);
             isNewIdentifier = isNewIdentifier || !!(type.flags & TypeFlags.String);
             return getStringLiteralTypes(type, uniques);
         });
diff --git a/src/services/types.ts b/src/services/types.ts
index 0659e6295e6..b9bd9f2177d 100644
--- a/src/services/types.ts
+++ b/src/services/types.ts
@@ -72,6 +72,7 @@ namespace ts {
         isTypeParameter(): this is TypeParameter;
         isClassOrInterface(): this is InterfaceType;
         isClass(): this is InterfaceType;
+        isIndexType(): this is IndexType;
     }
 
     export interface TypeReference {
@@ -82,6 +83,7 @@ namespace ts {
         getDeclaration(): SignatureDeclaration;
         getTypeParameters(): TypeParameter[] | undefined;
         getParameters(): Symbol[];
+        getTypeParameterAtPosition(pos: number): Type;
         getReturnType(): Type;
         getDocumentationComment(typeChecker: TypeChecker | undefined): SymbolDisplayPart[];
         getJsDocTags(): JSDocTagInfo[];
diff --git a/tests/baselines/reference/api/tsserverlibrary.d.ts b/tests/baselines/reference/api/tsserverlibrary.d.ts
index 752f6b892bb..c332bd508eb 100644
--- a/tests/baselines/reference/api/tsserverlibrary.d.ts
+++ b/tests/baselines/reference/api/tsserverlibrary.d.ts
@@ -5560,6 +5560,7 @@ declare namespace ts {
         isTypeParameter(): this is TypeParameter;
         isClassOrInterface(): this is InterfaceType;
         isClass(): this is InterfaceType;
+        isIndexType(): this is IndexType;
     }
     interface TypeReference {
         typeArguments?: readonly Type[];
@@ -5568,6 +5569,7 @@ declare namespace ts {
         getDeclaration(): SignatureDeclaration;
         getTypeParameters(): TypeParameter[] | undefined;
         getParameters(): Symbol[];
+        getTypeParameterAtPosition(pos: number): Type;
         getReturnType(): Type;
         getDocumentationComment(typeChecker: TypeChecker | undefined): SymbolDisplayPart[];
         getJsDocTags(): JSDocTagInfo[];
diff --git a/tests/baselines/reference/api/typescript.d.ts b/tests/baselines/reference/api/typescript.d.ts
index a9d7e9c449b..f17b673f250 100644
--- a/tests/baselines/reference/api/typescript.d.ts
+++ b/tests/baselines/reference/api/typescript.d.ts
@@ -5560,6 +5560,7 @@ declare namespace ts {
         isTypeParameter(): this is TypeParameter;
         isClassOrInterface(): this is InterfaceType;
         isClass(): this is InterfaceType;
+        isIndexType(): this is IndexType;
     }
     interface TypeReference {
         typeArguments?: readonly Type[];
@@ -5568,6 +5569,7 @@ declare namespace ts {
         getDeclaration(): SignatureDeclaration;
         getTypeParameters(): TypeParameter[] | undefined;
         getParameters(): Symbol[];
+        getTypeParameterAtPosition(pos: number): Type;
         getReturnType(): Type;
         getDocumentationComment(typeChecker: TypeChecker | undefined): SymbolDisplayPart[];
         getJsDocTags(): JSDocTagInfo[];
diff --git a/tests/cases/fourslash/completionListAtThisType.ts b/tests/cases/fourslash/completionListAtThisType.ts
new file mode 100644
index 00000000000..3842b6c3cfe
--- /dev/null
+++ b/tests/cases/fourslash/completionListAtThisType.ts
@@ -0,0 +1,19 @@
+/// <reference path='fourslash.ts'/>
+
+////class Test {
+////    foo() {}
+////
+////    bar() {
+////        this.baz(this, "/*1*/");
+////
+////        const t = new Test()
+////        this.baz(t, "/*2*/");
+////    }
+////
+////    baz<T>(a: T, k: keyof T) {}
+////}
+
+verify.completions({
+    marker: ["1", "2"],
+    exact: ["foo", "bar", "baz"]
+});