Properly compute lower bound of key type in a mapped type

Author: ahejlsberg

src/compiler/checker.ts

@@ -7084,6 +7084,39 @@ namespace ts {
             setStructuredTypeMembers(type, members, emptyArray, emptyArray, stringIndexInfo, undefined);
         }
 
+        // 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 {
+            if (type.flags & (TypeFlags.Any | TypeFlags.Primitive)) {
+                return type;
+            }
+            if (type.flags & TypeFlags.Index) {
+                return getIndexType(getApparentType((<IndexType>type).type));
+            }
+            if (type.flags & TypeFlags.Conditional) {
+                return getLowerBoundOfConditionalType(<ConditionalType>type);
+            }
+            if (type.flags & TypeFlags.Union) {
+                return getUnionType(sameMap((<UnionType>type).types, getLowerBoundOfKeyType));
+            }
+            if (type.flags & TypeFlags.Intersection) {
+                return getIntersectionType(sameMap((<UnionType>type).types, getLowerBoundOfKeyType));
+            }
+            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();
@@ -7112,10 +7145,7 @@ namespace ts {
                 }
             }
             else {
-                // If the key type is a 'keyof X', obtain 'keyof C' where C is the base constraint of X.
-                // Then iterate over the constituents of the key type.
-                const iterationType = constraintType.flags & TypeFlags.Index ? getIndexType(getApparentType((<IndexType>constraintType).type)) : constraintType;
-                forEachType(iterationType, addMemberForKeyType);
+                forEachType(getLowerBoundOfKeyType(constraintType), addMemberForKeyType);
             }
             setStructuredTypeMembers(type, members, emptyArray, emptyArray, stringIndexInfo, numberIndexInfo);