diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 6df6b1f9ce..41921e3c78 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -5911,7 +5911,13 @@ namespace ts {
                 return transformed;
             }
             const baseObjectType = getBaseConstraintOfType(type.objectType);
-            const baseIndexType = getBaseConstraintOfType(type.indexType);
+            const keepTypeParameterForMappedType = baseObjectType && getObjectFlags(baseObjectType) & ObjectFlags.Mapped && type.indexType.flags & TypeFlags.TypeParameter;
+            const baseIndexType = !keepTypeParameterForMappedType && getBaseConstraintOfType(type.indexType);
+            if (baseObjectType && baseIndexType === stringType && !getIndexInfoOfType(baseObjectType, IndexKind.String)) {
+                // getIndexedAccessType returns `any` for X[string] where X doesn't have an index signature.
+                // instead, return undefined so that the indexed access check fails
+                return undefined;
+            }
             return baseObjectType || baseIndexType ? getIndexedAccessType(baseObjectType || type.objectType, baseIndexType || type.indexType) : undefined;
         }
 
@@ -5961,8 +5967,9 @@ namespace ts {
             function computeBaseConstraint(t: Type): Type {
                 if (t.flags & TypeFlags.TypeParameter) {
                     const constraint = getConstraintFromTypeParameter(<TypeParameter>t);
-                    return (<TypeParameter>t).isThisType ? constraint :
-                        constraint ? getBaseConstraint(constraint) : undefined;
+                    return (t as TypeParameter).isThisType || !constraint ?
+                        constraint :
+                        getBaseConstraint(constraint);
                 }
                 if (t.flags & TypeFlags.UnionOrIntersection) {
                     const types = (<UnionOrIntersectionType>t).types;
@@ -5990,9 +5997,6 @@ namespace ts {
                     const baseIndexedAccess = baseObjectType && baseIndexType ? getIndexedAccessType(baseObjectType, baseIndexType) : undefined;
                     return baseIndexedAccess && baseIndexedAccess !== unknownType ? getBaseConstraint(baseIndexedAccess) : undefined;
                 }
-                if (isGenericMappedType(t)) {
-                    return emptyObjectType;
-                }
                 return t;
             }
         }
@@ -9289,7 +9293,7 @@ namespace ts {
                 }
                 else if (target.flags & TypeFlags.IndexedAccess) {
                     // A type S is related to a type T[K] if S is related to A[K], where K is string-like and
-                    // A is the apparent type of S.
+                    // A is the apparent type of T.
                     const constraint = getConstraintOfType(<IndexedAccessType>target);
                     if (constraint) {
                         if (result = isRelatedTo(source, constraint, reportErrors)) {
diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index 8cab1b4115..422df2ead0 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -718,7 +718,7 @@ namespace ts {
     export function sum<T extends Record<K, number>, K extends string>(array: T[], prop: K): number {
         let result = 0;
         for (const v of array) {
-            // Note: we need the following type assertion because of GH #17069
+            // TODO: Remove the following type assertion once the fix for #17069 is merged
             result += v[prop] as number;
         }
         return result;
diff --git a/tests/baselines/reference/additionOperatorWithConstrainedTypeParameter.js b/tests/baselines/reference/additionOperatorWithConstrainedTypeParameter.js
new file mode 100644
index 0000000000..1366e24218
--- /dev/null
+++ b/tests/baselines/reference/additionOperatorWithConstrainedTypeParameter.js
@@ -0,0 +1,27 @@
+//// [additionOperatorWithConstrainedTypeParameter.ts]
+// test for #17069
+function sum<T extends Record<K, number>, K extends string>(n: number, v: T, k: K) {
+    n = n + v[k];
+    n += v[k]; // += should work the same way
+}
+function realSum<T extends Record<K, number>, K extends string>(n: number, vs: T[], k: K) {
+    for (const v of vs) {
+        n = n + v[k];
+        n += v[k];
+    }
+}
+
+
+//// [additionOperatorWithConstrainedTypeParameter.js]
+// test for #17069
+function sum(n, v, k) {
+    n = n + v[k];
+    n += v[k]; // += should work the same way
+}
+function realSum(n, vs, k) {
+    for (var _i = 0, vs_1 = vs; _i < vs_1.length; _i++) {
+        var v = vs_1[_i];
+        n = n + v[k];
+        n += v[k];
+    }
+}
diff --git a/tests/baselines/reference/additionOperatorWithConstrainedTypeParameter.symbols b/tests/baselines/reference/additionOperatorWithConstrainedTypeParameter.symbols
new file mode 100644
index 0000000000..e7055c1e38
--- /dev/null
+++ b/tests/baselines/reference/additionOperatorWithConstrainedTypeParameter.symbols
@@ -0,0 +1,54 @@
+=== tests/cases/conformance/expressions/binaryOperators/additionOperator/additionOperatorWithConstrainedTypeParameter.ts ===
+// test for #17069
+function sum<T extends Record<K, number>, K extends string>(n: number, v: T, k: K) {
+>sum : Symbol(sum, Decl(additionOperatorWithConstrainedTypeParameter.ts, 0, 0))
+>T : Symbol(T, Decl(additionOperatorWithConstrainedTypeParameter.ts, 1, 13))
+>Record : Symbol(Record, Decl(lib.d.ts, --, --))
+>K : Symbol(K, Decl(additionOperatorWithConstrainedTypeParameter.ts, 1, 41))
+>K : Symbol(K, Decl(additionOperatorWithConstrainedTypeParameter.ts, 1, 41))
+>n : Symbol(n, Decl(additionOperatorWithConstrainedTypeParameter.ts, 1, 60))
+>v : Symbol(v, Decl(additionOperatorWithConstrainedTypeParameter.ts, 1, 70))
+>T : Symbol(T, Decl(additionOperatorWithConstrainedTypeParameter.ts, 1, 13))
+>k : Symbol(k, Decl(additionOperatorWithConstrainedTypeParameter.ts, 1, 76))
+>K : Symbol(K, Decl(additionOperatorWithConstrainedTypeParameter.ts, 1, 41))
+
+    n = n + v[k];
+>n : Symbol(n, Decl(additionOperatorWithConstrainedTypeParameter.ts, 1, 60))
+>n : Symbol(n, Decl(additionOperatorWithConstrainedTypeParameter.ts, 1, 60))
+>v : Symbol(v, Decl(additionOperatorWithConstrainedTypeParameter.ts, 1, 70))
+>k : Symbol(k, Decl(additionOperatorWithConstrainedTypeParameter.ts, 1, 76))
+
+    n += v[k]; // += should work the same way
+>n : Symbol(n, Decl(additionOperatorWithConstrainedTypeParameter.ts, 1, 60))
+>v : Symbol(v, Decl(additionOperatorWithConstrainedTypeParameter.ts, 1, 70))
+>k : Symbol(k, Decl(additionOperatorWithConstrainedTypeParameter.ts, 1, 76))
+}
+function realSum<T extends Record<K, number>, K extends string>(n: number, vs: T[], k: K) {
+>realSum : Symbol(realSum, Decl(additionOperatorWithConstrainedTypeParameter.ts, 4, 1))
+>T : Symbol(T, Decl(additionOperatorWithConstrainedTypeParameter.ts, 5, 17))
+>Record : Symbol(Record, Decl(lib.d.ts, --, --))
+>K : Symbol(K, Decl(additionOperatorWithConstrainedTypeParameter.ts, 5, 45))
+>K : Symbol(K, Decl(additionOperatorWithConstrainedTypeParameter.ts, 5, 45))
+>n : Symbol(n, Decl(additionOperatorWithConstrainedTypeParameter.ts, 5, 64))
+>vs : Symbol(vs, Decl(additionOperatorWithConstrainedTypeParameter.ts, 5, 74))
+>T : Symbol(T, Decl(additionOperatorWithConstrainedTypeParameter.ts, 5, 17))
+>k : Symbol(k, Decl(additionOperatorWithConstrainedTypeParameter.ts, 5, 83))
+>K : Symbol(K, Decl(additionOperatorWithConstrainedTypeParameter.ts, 5, 45))
+
+    for (const v of vs) {
+>v : Symbol(v, Decl(additionOperatorWithConstrainedTypeParameter.ts, 6, 14))
+>vs : Symbol(vs, Decl(additionOperatorWithConstrainedTypeParameter.ts, 5, 74))
+
+        n = n + v[k];
+>n : Symbol(n, Decl(additionOperatorWithConstrainedTypeParameter.ts, 5, 64))
+>n : Symbol(n, Decl(additionOperatorWithConstrainedTypeParameter.ts, 5, 64))
+>v : Symbol(v, Decl(additionOperatorWithConstrainedTypeParameter.ts, 6, 14))
+>k : Symbol(k, Decl(additionOperatorWithConstrainedTypeParameter.ts, 5, 83))
+
+        n += v[k];
+>n : Symbol(n, Decl(additionOperatorWithConstrainedTypeParameter.ts, 5, 64))
+>v : Symbol(v, Decl(additionOperatorWithConstrainedTypeParameter.ts, 6, 14))
+>k : Symbol(k, Decl(additionOperatorWithConstrainedTypeParameter.ts, 5, 83))
+    }
+}
+
diff --git a/tests/baselines/reference/additionOperatorWithConstrainedTypeParameter.types b/tests/baselines/reference/additionOperatorWithConstrainedTypeParameter.types
new file mode 100644
index 0000000000..d52c77a94f
--- /dev/null
+++ b/tests/baselines/reference/additionOperatorWithConstrainedTypeParameter.types
@@ -0,0 +1,64 @@
+=== tests/cases/conformance/expressions/binaryOperators/additionOperator/additionOperatorWithConstrainedTypeParameter.ts ===
+// test for #17069
+function sum<T extends Record<K, number>, K extends string>(n: number, v: T, k: K) {
+>sum : <T extends Record<K, number>, K extends string>(n: number, v: T, k: K) => void
+>T : T
+>Record : Record<K, T>
+>K : K
+>K : K
+>n : number
+>v : T
+>T : T
+>k : K
+>K : K
+
+    n = n + v[k];
+>n = n + v[k] : number
+>n : number
+>n + v[k] : number
+>n : number
+>v[k] : T[K]
+>v : T
+>k : K
+
+    n += v[k]; // += should work the same way
+>n += v[k] : number
+>n : number
+>v[k] : T[K]
+>v : T
+>k : K
+}
+function realSum<T extends Record<K, number>, K extends string>(n: number, vs: T[], k: K) {
+>realSum : <T extends Record<K, number>, K extends string>(n: number, vs: T[], k: K) => void
+>T : T
+>Record : Record<K, T>
+>K : K
+>K : K
+>n : number
+>vs : T[]
+>T : T
+>k : K
+>K : K
+
+    for (const v of vs) {
+>v : T
+>vs : T[]
+
+        n = n + v[k];
+>n = n + v[k] : number
+>n : number
+>n + v[k] : number
+>n : number
+>v[k] : T[K]
+>v : T
+>k : K
+
+        n += v[k];
+>n += v[k] : number
+>n : number
+>v[k] : T[K]
+>v : T
+>k : K
+    }
+}
+
diff --git a/tests/baselines/reference/nonPrimitiveConstraintOfIndexAccessType.errors.txt b/tests/baselines/reference/nonPrimitiveConstraintOfIndexAccessType.errors.txt
new file mode 100644
index 0000000000..5678115713
--- /dev/null
+++ b/tests/baselines/reference/nonPrimitiveConstraintOfIndexAccessType.errors.txt
@@ -0,0 +1,67 @@
+tests/cases/conformance/types/nonPrimitive/nonPrimitiveConstraintOfIndexAccessType.ts(3,5): error TS2322: Type 'string' is not assignable to type 'T[P]'.
+tests/cases/conformance/types/nonPrimitive/nonPrimitiveConstraintOfIndexAccessType.ts(6,5): error TS2322: Type 'string' is not assignable to type 'T[P]'.
+tests/cases/conformance/types/nonPrimitive/nonPrimitiveConstraintOfIndexAccessType.ts(9,5): error TS2322: Type 'string' is not assignable to type 'T[P]'.
+tests/cases/conformance/types/nonPrimitive/nonPrimitiveConstraintOfIndexAccessType.ts(12,5): error TS2322: Type 'string' is not assignable to type 'T[P]'.
+tests/cases/conformance/types/nonPrimitive/nonPrimitiveConstraintOfIndexAccessType.ts(15,5): error TS2322: Type 'string' is not assignable to type 'T[P]'.
+tests/cases/conformance/types/nonPrimitive/nonPrimitiveConstraintOfIndexAccessType.ts(18,5): error TS2322: Type 'string' is not assignable to type 'T[P]'.
+tests/cases/conformance/types/nonPrimitive/nonPrimitiveConstraintOfIndexAccessType.ts(21,5): error TS2322: Type 'string' is not assignable to type 'T[P]'.
+tests/cases/conformance/types/nonPrimitive/nonPrimitiveConstraintOfIndexAccessType.ts(24,5): error TS2322: Type 'string' is not assignable to type 'T[P]'.
+tests/cases/conformance/types/nonPrimitive/nonPrimitiveConstraintOfIndexAccessType.ts(27,5): error TS2322: Type 'string' is not assignable to type 'T[P]'.
+tests/cases/conformance/types/nonPrimitive/nonPrimitiveConstraintOfIndexAccessType.ts(30,5): error TS2322: Type 'string' is not assignable to type 'T[P]'.
+  Type 'string' is not assignable to type 'number'.
+
+
+==== tests/cases/conformance/types/nonPrimitive/nonPrimitiveConstraintOfIndexAccessType.ts (10 errors) ====
+    // test for #15371
+    function f<T extends object, P extends keyof T>(s: string, tp: T[P]): void {
+        tp = s;
+        ~~
+!!! error TS2322: Type 'string' is not assignable to type 'T[P]'.
+    }
+    function g<T extends null, P extends keyof T>(s: string, tp: T[P]): void {
+        tp = s;
+        ~~
+!!! error TS2322: Type 'string' is not assignable to type 'T[P]'.
+    }
+    function h<T extends undefined, P extends keyof T>(s: string, tp: T[P]): void {
+        tp = s;
+        ~~
+!!! error TS2322: Type 'string' is not assignable to type 'T[P]'.
+    }
+    function i<T extends void, P extends keyof T>(s: string, tp: T[P]): void {
+        tp = s;
+        ~~
+!!! error TS2322: Type 'string' is not assignable to type 'T[P]'.
+    }
+    function j<T extends never, P extends keyof T>(s: string, tp: T[P]): void {
+        tp = s;
+        ~~
+!!! error TS2322: Type 'string' is not assignable to type 'T[P]'.
+    }
+    function k<T extends number, P extends keyof T>(s: string, tp: T[P]): void {
+        tp = s;
+        ~~
+!!! error TS2322: Type 'string' is not assignable to type 'T[P]'.
+    }
+    function o<T extends string, P extends keyof T>(s: string, tp: T[P]): void {
+        tp = s;
+        ~~
+!!! error TS2322: Type 'string' is not assignable to type 'T[P]'.
+    }
+    function l<T extends {}, P extends keyof T>(s: string, tp: T[P]): void {
+        tp = s;
+        ~~
+!!! error TS2322: Type 'string' is not assignable to type 'T[P]'.
+    }
+    function m<T extends { a: number }, P extends keyof T>(s: string, tp: T[P]): void {
+        tp = s;
+        ~~
+!!! error TS2322: Type 'string' is not assignable to type 'T[P]'.
+    }
+    function n<T extends { [s: string]: number }, P extends keyof T>(s: string, tp: T[P]): void {
+        tp = s;
+        ~~
+!!! error TS2322: Type 'string' is not assignable to type 'T[P]'.
+!!! error TS2322:   Type 'string' is not assignable to type 'number'.
+    }
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/nonPrimitiveConstraintOfIndexAccessType.js b/tests/baselines/reference/nonPrimitiveConstraintOfIndexAccessType.js
new file mode 100644
index 0000000000..7a24345a94
--- /dev/null
+++ b/tests/baselines/reference/nonPrimitiveConstraintOfIndexAccessType.js
@@ -0,0 +1,67 @@
+//// [nonPrimitiveConstraintOfIndexAccessType.ts]
+// test for #15371
+function f<T extends object, P extends keyof T>(s: string, tp: T[P]): void {
+    tp = s;
+}
+function g<T extends null, P extends keyof T>(s: string, tp: T[P]): void {
+    tp = s;
+}
+function h<T extends undefined, P extends keyof T>(s: string, tp: T[P]): void {
+    tp = s;
+}
+function i<T extends void, P extends keyof T>(s: string, tp: T[P]): void {
+    tp = s;
+}
+function j<T extends never, P extends keyof T>(s: string, tp: T[P]): void {
+    tp = s;
+}
+function k<T extends number, P extends keyof T>(s: string, tp: T[P]): void {
+    tp = s;
+}
+function o<T extends string, P extends keyof T>(s: string, tp: T[P]): void {
+    tp = s;
+}
+function l<T extends {}, P extends keyof T>(s: string, tp: T[P]): void {
+    tp = s;
+}
+function m<T extends { a: number }, P extends keyof T>(s: string, tp: T[P]): void {
+    tp = s;
+}
+function n<T extends { [s: string]: number }, P extends keyof T>(s: string, tp: T[P]): void {
+    tp = s;
+}
+
+
+//// [nonPrimitiveConstraintOfIndexAccessType.js]
+"use strict";
+// test for #15371
+function f(s, tp) {
+    tp = s;
+}
+function g(s, tp) {
+    tp = s;
+}
+function h(s, tp) {
+    tp = s;
+}
+function i(s, tp) {
+    tp = s;
+}
+function j(s, tp) {
+    tp = s;
+}
+function k(s, tp) {
+    tp = s;
+}
+function o(s, tp) {
+    tp = s;
+}
+function l(s, tp) {
+    tp = s;
+}
+function m(s, tp) {
+    tp = s;
+}
+function n(s, tp) {
+    tp = s;
+}
diff --git a/tests/cases/conformance/expressions/binaryOperators/additionOperator/additionOperatorWithConstrainedTypeParameter.ts b/tests/cases/conformance/expressions/binaryOperators/additionOperator/additionOperatorWithConstrainedTypeParameter.ts
new file mode 100644
index 0000000000..2303bb3394
--- /dev/null
+++ b/tests/cases/conformance/expressions/binaryOperators/additionOperator/additionOperatorWithConstrainedTypeParameter.ts
@@ -0,0 +1,11 @@
+// test for #17069
+function sum<T extends Record<K, number>, K extends string>(n: number, v: T, k: K) {
+    n = n + v[k];
+    n += v[k]; // += should work the same way
+}
+function realSum<T extends Record<K, number>, K extends string>(n: number, vs: T[], k: K) {
+    for (const v of vs) {
+        n = n + v[k];
+        n += v[k];
+    }
+}
diff --git a/tests/cases/conformance/types/nonPrimitive/nonPrimitiveConstraintOfIndexAccessType.ts b/tests/cases/conformance/types/nonPrimitive/nonPrimitiveConstraintOfIndexAccessType.ts
new file mode 100644
index 0000000000..1e852f12f9
--- /dev/null
+++ b/tests/cases/conformance/types/nonPrimitive/nonPrimitiveConstraintOfIndexAccessType.ts
@@ -0,0 +1,32 @@
+// @strict: true
+// test for #15371
+function f<T extends object, P extends keyof T>(s: string, tp: T[P]): void {
+    tp = s;
+}
+function g<T extends null, P extends keyof T>(s: string, tp: T[P]): void {
+    tp = s;
+}
+function h<T extends undefined, P extends keyof T>(s: string, tp: T[P]): void {
+    tp = s;
+}
+function i<T extends void, P extends keyof T>(s: string, tp: T[P]): void {
+    tp = s;
+}
+function j<T extends never, P extends keyof T>(s: string, tp: T[P]): void {
+    tp = s;
+}
+function k<T extends number, P extends keyof T>(s: string, tp: T[P]): void {
+    tp = s;
+}
+function o<T extends string, P extends keyof T>(s: string, tp: T[P]): void {
+    tp = s;
+}
+function l<T extends {}, P extends keyof T>(s: string, tp: T[P]): void {
+    tp = s;
+}
+function m<T extends { a: number }, P extends keyof T>(s: string, tp: T[P]): void {
+    tp = s;
+}
+function n<T extends { [s: string]: number }, P extends keyof T>(s: string, tp: T[P]): void {
+    tp = s;
+}