diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index febb8ffdad3..d96eb6a6c4d 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -7188,36 +7188,39 @@ namespace ts {
         // Return the lower bound of the key type in a mapped type. Intuitively, the lower
         // bound includes those keys that are known to always be present, for example because
         // because of constraints on type parameters (e.g. 'keyof T' for a constrained T).
-        function getLowerBoundOfKeyType(type: Type): Type {
+        // The isIndexType flag indicates that the type is the index type of an indexed
+        // access that is the target of an assignment.
+        function getLowerBoundOfKeyType(type: Type, isIndexType: boolean): Type {
             if (type.flags & (TypeFlags.Any | TypeFlags.Primitive)) {
                 return type;
             }
             if (type.flags & TypeFlags.Index) {
-                return getIndexType(getApparentType((<IndexType>type).type));
+                const keys = getIndexType(getApparentType((<IndexType>type).type));
+                return isIndexType ? filterType(keys, t => !!(t.flags & TypeFlags.Literal)) : keys;
             }
             if (type.flags & TypeFlags.Conditional) {
-                return getLowerBoundOfConditionalType(<ConditionalType>type);
+                if ((<ConditionalType>type).root.isDistributive) {
+                    const checkType = (<ConditionalType>type).checkType;
+                    const constraint = getLowerBoundOfKeyType(checkType, isIndexType);
+                    if (constraint !== checkType) {
+                        const mapper = makeUnaryTypeMapper((<ConditionalType>type).root.checkType, constraint);
+                        return getConditionalTypeInstantiation(<ConditionalType>type, combineTypeMappers(mapper, (<ConditionalType>type).mapper));
+                    }
+                }
+                return type;
             }
             if (type.flags & TypeFlags.Union) {
-                return getUnionType(sameMap((<UnionType>type).types, getLowerBoundOfKeyType));
+                return getUnionType(sameMap((<UnionType>type).types, t => getLowerBoundOfKeyType(t, isIndexType)));
             }
             if (type.flags & TypeFlags.Intersection) {
-                return getIntersectionType(sameMap((<UnionType>type).types, getLowerBoundOfKeyType));
+                return getIntersectionType(sameMap((<UnionType>type).types, t => getLowerBoundOfKeyType(t, isIndexType)));
+            }
+            if (isIndexType && type.flags & TypeFlags.Instantiable) {
+                return getLowerBoundOfKeyType(getConstraintOfType(type) || neverType, isIndexType);
             }
             return neverType;
         }
 
-        function getLowerBoundOfConditionalType(type: ConditionalType) {
-            if (type.root.isDistributive) {
-                const constraint = getLowerBoundOfKeyType(type.checkType);
-                if (constraint !== type.checkType) {
-                    const mapper = makeUnaryTypeMapper(type.root.checkType, constraint);
-                    return getConditionalTypeInstantiation(type, combineTypeMappers(mapper, type.mapper));
-                }
-            }
-            return type;
-        }
-
         /** Resolve the members of a mapped type { [P in K]: T } */
         function resolveMappedTypeMembers(type: MappedType) {
             const members: SymbolTable = createSymbolTable();
@@ -7246,7 +7249,7 @@ namespace ts {
                 }
             }
             else {
-                forEachType(getLowerBoundOfKeyType(constraintType), addMemberForKeyType);
+                forEachType(getLowerBoundOfKeyType(constraintType, /*isIndexType*/ false), addMemberForKeyType);
             }
             setStructuredTypeMembers(type, members, emptyArray, emptyArray, stringIndexInfo, numberIndexInfo);
 
@@ -7523,7 +7526,7 @@ namespace ts {
             // a union - once negated types exist and are applied to the conditional false branch, this "constraint"
             // likely doesn't need to exist.
             if (type.root.isDistributive && type.restrictiveInstantiation !== type) {
-                const simplified = getSimplifiedType(type.checkType);
+                const simplified = getSimplifiedType(type.checkType, /*writing*/ false);
                 const constraint = simplified === type.checkType ? getConstraintOfType(simplified) : simplified;
                 if (constraint && constraint !== type.checkType) {
                     const mapper = makeUnaryTypeMapper(type.root.checkType, constraint);
@@ -7630,7 +7633,7 @@ namespace ts {
                         return t.immediateBaseConstraint = noConstraintType;
                     }
                     constraintDepth++;
-                    let result = computeBaseConstraint(getSimplifiedType(t));
+                    let result = computeBaseConstraint(getSimplifiedType(t, /*writing*/ false));
                     constraintDepth--;
                     if (!popTypeResolution()) {
                         if (t.flags & TypeFlags.TypeParameter) {
@@ -10003,53 +10006,68 @@ namespace ts {
             return maybeTypeOfKind(type, TypeFlags.InstantiableNonPrimitive | TypeFlags.Index);
         }
 
-        function getSimplifiedType(type: Type): Type {
-            return type.flags & TypeFlags.IndexedAccess ? getSimplifiedIndexedAccessType(<IndexedAccessType>type) : type;
+        function getSimplifiedType(type: Type, writing: boolean): Type {
+            return type.flags & TypeFlags.IndexedAccess ? getSimplifiedIndexedAccessType(<IndexedAccessType>type, writing) : type;
         }
 
-        function distributeIndexOverObjectType(objectType: Type, indexType: Type) {
-            // (T | U)[K] -> T[K] | U[K]
-            if (objectType.flags & TypeFlags.Union) {
-                return mapType(objectType, t => getSimplifiedType(getIndexedAccessType(t, indexType)));
-            }
+        function distributeIndexOverObjectType(objectType: Type, indexType: Type, writing: boolean) {
+            // (T | U)[K] -> T[K] | U[K] (reading)
+            // (T | U)[K] -> T[K] & U[K] (writing)
             // (T & U)[K] -> T[K] & U[K]
-            if (objectType.flags & TypeFlags.Intersection) {
-                return getIntersectionType(map((objectType as IntersectionType).types, t => getSimplifiedType(getIndexedAccessType(t, indexType))));
+            if (objectType.flags & TypeFlags.UnionOrIntersection) {
+                const types = map((objectType as UnionOrIntersectionType).types, t => getSimplifiedType(getIndexedAccessType(t, indexType), writing));
+                return objectType.flags & TypeFlags.Intersection || writing ? getIntersectionType(types) : getUnionType(types);
+            }
+        }
+
+        function distributeObjectOverIndexType(objectType: Type, indexType: Type, writing: boolean) {
+            // T[A | B] -> T[A] | T[B] (reading)
+            // T[A | B] -> T[A] & T[B] (writing)
+            if (indexType.flags & TypeFlags.Union) {
+                const types = map((indexType as UnionType).types, t => getSimplifiedType(getIndexedAccessType(objectType, t), writing));
+                return writing ? getIntersectionType(types) : getUnionType(types);
             }
         }
 
         // Transform an indexed access to a simpler form, if possible. Return the simpler form, or return
-        // the type itself if no transformation is possible.
-        function getSimplifiedIndexedAccessType(type: IndexedAccessType): Type {
-            if (type.simplified) {
-                return type.simplified === circularConstraintType ? type : type.simplified;
+        // the type itself if no transformation is possible. The writing flag indicates that the type is
+        // the target of an assignment.
+        function getSimplifiedIndexedAccessType(type: IndexedAccessType, writing: boolean): Type {
+            const cache = writing ? "simplifiedForWriting" : "simplifiedForReading";
+            if (type[cache]) {
+                return type[cache] === circularConstraintType ? type : type[cache]!;
             }
-            type.simplified = circularConstraintType;
+            type[cache] = circularConstraintType;
             // We recursively simplify the object type as it may in turn be an indexed access type. For example, with
             // '{ [P in T]: { [Q in U]: number } }[T][U]' we want to first simplify the inner indexed access type.
-            const objectType = getSimplifiedType(type.objectType);
-            const indexType = getSimplifiedType(type.indexType);
-            // T[A | B] -> T[A] | T[B]
-            if (indexType.flags & TypeFlags.Union) {
-                return type.simplified = mapType(indexType, t => getSimplifiedType(getIndexedAccessType(objectType, t)));
+            const objectType = getSimplifiedType(type.objectType, writing);
+            const indexType = getSimplifiedType(type.indexType, writing);
+            // T[A | B] -> T[A] | T[B] (reading)
+            // T[A | B] -> T[A] & T[B] (writing)
+            const distributedOverIndex = distributeObjectOverIndexType(objectType, indexType, writing);
+            if (distributedOverIndex) {
+                return type[cache] = distributedOverIndex;
             }
             // Only do the inner distributions if the index can no longer be instantiated to cause index distribution again
             if (!(indexType.flags & TypeFlags.Instantiable)) {
-                const simplified = distributeIndexOverObjectType(objectType, indexType);
-                if (simplified) {
-                    return type.simplified = simplified;
+                // (T | U)[K] -> T[K] | U[K] (reading)
+                // (T | U)[K] -> T[K] & U[K] (writing)
+                // (T & U)[K] -> T[K] & U[K]
+                const distributedOverObject = distributeIndexOverObjectType(objectType, indexType, writing);
+                if (distributedOverObject) {
+                    return type[cache] = distributedOverObject;
                 }
             }
-            // So ultimately:
+            // So ultimately (reading):
             // ((A & B) | C)[K1 | K2] -> ((A & B) | C)[K1] | ((A & B) | C)[K2] -> (A & B)[K1] | C[K1] | (A & B)[K2] | C[K2] -> (A[K1] & B[K1]) | C[K1] | (A[K2] & B[K2]) | C[K2]
 
             // If the object type is a mapped type { [P in K]: E }, where K is generic, instantiate E using a mapper
             // that substitutes the index type for P. For example, for an index access { [P in K]: Box<T[P]> }[X], we
             // construct the type Box<T[X]>.
             if (isGenericMappedType(objectType)) {
-                return type.simplified = mapType(substituteIndexedMappedType(objectType, type.indexType), getSimplifiedType);
+                return type[cache] = mapType(substituteIndexedMappedType(objectType, type.indexType), t => getSimplifiedType(t, writing));
             }
-            return type.simplified = type;
+            return type[cache] = type;
         }
 
         function substituteIndexedMappedType(objectType: MappedType, index: Type) {
@@ -12197,10 +12215,10 @@ namespace ts {
                     target = (<SubstitutionType>target).typeVariable;
                 }
                 if (source.flags & TypeFlags.IndexedAccess) {
-                    source = getSimplifiedType(source);
+                    source = getSimplifiedType(source, /*writing*/ false);
                 }
                 if (target.flags & TypeFlags.IndexedAccess) {
-                    target = getSimplifiedType(target);
+                    target = getSimplifiedType(target, /*writing*/ true);
                 }
 
                 // Try to see if we're relating something like `Foo` -> `Bar | null | undefined`.
@@ -12780,7 +12798,7 @@ namespace ts {
                     }
                     // A type S is assignable to keyof T if S is assignable to keyof C, where C is the
                     // simplified form of T or, if T doesn't simplify, the constraint of T.
-                    const simplified = getSimplifiedType((<IndexType>target).type);
+                    const simplified = getSimplifiedType((<IndexType>target).type, /*writing*/ false);
                     const constraint = simplified !== (<IndexType>target).type ? simplified : getConstraintOfType((<IndexType>target).type);
                     if (constraint) {
                         // We require Ternary.True here such that circular constraints don't cause
@@ -12795,12 +12813,26 @@ namespace ts {
                 else if (target.flags & TypeFlags.IndexedAccess) {
                     // A type S is related to a type T[K], where T and K aren't both type variables, if S is related to C,
                     // where C is the base constraint of T[K]
-                    if (relation !== identityRelation &&
-                        !(isGenericObjectType((<IndexedAccessType>target).objectType) && isGenericIndexType((<IndexedAccessType>target).indexType))) {
-                        const constraint = getBaseConstraintOfType(target);
-                        if (constraint && constraint !== target) {
-                            if (result = isRelatedTo(source, constraint, reportErrors)) {
-                                return result;
+                    if (relation !== identityRelation) {
+                        const objectType = (<IndexedAccessType>target).objectType
+                        const indexType = (<IndexedAccessType>target).indexType;
+                        if (indexType.flags & TypeFlags.StructuredOrInstantiable) {
+                            const keyType = getLowerBoundOfKeyType(indexType, /*isIndexType*/ true);
+                            if (keyType !== indexType && !(keyType.flags & TypeFlags.Never)) {
+                                const targetType = keyType.flags & TypeFlags.Union ?
+                                    getIntersectionType(map((<UnionType>keyType).types, t => getIndexedAccessType(objectType, t))) :
+                                    getIndexedAccessType(objectType, keyType);
+                                if (result = isRelatedTo(source, targetType, reportErrors)) {
+                                    return result;
+                                }
+                            }
+                        }
+                        else {
+                            const constraint = getConstraintOfType(objectType);
+                            if (constraint) {
+                                if (result = isRelatedTo(source, getIndexedAccessType(constraint, indexType), reportErrors)) {
+                                    return result;
+                                }
                             }
                         }
                     }
@@ -14660,16 +14692,16 @@ namespace ts {
                     }
                     else {
                         // Infer to the simplified version of an indexed access, if possible, to (hopefully) expose more bare type parameters to the inference engine
-                        const simplified = getSimplifiedType(target);
+                        const simplified = getSimplifiedType(target, /*writing*/ false);
                         if (simplified !== target) {
                             inferFromTypesOnce(source, simplified);
                         }
                         else if (target.flags & TypeFlags.IndexedAccess) {
-                            const indexType = getSimplifiedType((target as IndexedAccessType).indexType);
+                            const indexType = getSimplifiedType((target as IndexedAccessType).indexType, /*writing*/ false);
                             // Generally simplifications of instantiable indexes are avoided to keep relationship checking correct, however if our target is an access, we can consider
                             // that key of that access to be "instantiated", since we're looking to find the infernce goal in any way we can.
                             if (indexType.flags & TypeFlags.Instantiable) {
-                                const simplified = distributeIndexOverObjectType(getSimplifiedType((target as IndexedAccessType).objectType), indexType);
+                                const simplified = distributeIndexOverObjectType(getSimplifiedType((target as IndexedAccessType).objectType, /*writing*/ false), indexType, /*writing*/ false);
                                 if (simplified && simplified !== target) {
                                     inferFromTypesOnce(source, simplified);
                                 }
@@ -15697,24 +15729,19 @@ namespace ts {
             if (!(type.flags & TypeFlags.Union)) {
                 return mapper(type);
             }
-            const types = (<UnionType>type).types;
-            let mappedType: Type | undefined;
             let mappedTypes: Type[] | undefined;
-            for (const current of types) {
-                const t = mapper(current);
-                if (t) {
-                    if (!mappedType) {
-                        mappedType = t;
-                    }
-                    else if (!mappedTypes) {
-                        mappedTypes = [mappedType, t];
+            for (const t of (<UnionType>type).types) {
+                const mapped = mapper(t);
+                if (mapped) {
+                    if (!mappedTypes) {
+                        mappedTypes = [mapped];
                     }
                     else {
-                        mappedTypes.push(t);
+                        mappedTypes.push(mapped);
                     }
                 }
             }
-            return mappedTypes ? getUnionType(mappedTypes, noReductions ? UnionReduction.None : UnionReduction.Literal) : mappedType;
+            return mappedTypes && getUnionType(mappedTypes, noReductions ? UnionReduction.None : UnionReduction.Literal);
         }
 
         function extractTypesOfKind(type: Type, kind: TypeFlags) {
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index bcc3218a017..7c9b7cee1e5 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -4279,7 +4279,8 @@ namespace ts {
         objectType: Type;
         indexType: Type;
         constraint?: Type;
-        simplified?: Type;
+        simplifiedForReading?: Type;
+        simplifiedForWriting?: Type;
     }
 
     export type TypeVariable = TypeParameter | IndexedAccessType;