Permit reverse mapped types to be created from partially inferable types

Author: ahejlsberg

src/compiler/checker.ts

@@ -14887,10 +14887,22 @@ namespace ts {
             return type;
         }
 
+        // We consider a type to be partially inferable if it isn't marked non-inferable or if it is
+        // an object literal type with at least one property of an inferable type. For example, an object
+        // literal { a: 123, b: x => true } is marked non-inferable because it contains a context sensitive
+        // arrow function, but is considered partially inferable because property 'a' has an inferable type.
+        function isPartiallyInferableType(type: Type): boolean {
+            return !(getObjectFlags(type) & ObjectFlags.NonInferrableType) ||
+                isObjectLiteralType(type) && some(getPropertiesOfType(type), prop => isPartiallyInferableType(getTypeOfSymbol(prop)));
+        }
+
         function createReverseMappedType(source: Type, target: MappedType, constraint: IndexType) {
-            // If any property contains context sensitive functions that have been skipped, the source type
-            // is incomplete and we can't infer a meaningful input type.
-            if (getObjectFlags(source) & ObjectFlags.NonInferrableType || getPropertiesOfType(source).length === 0 && !getIndexInfoOfType(source, IndexKind.String)) {
+            // We consider a source type reverse mappable if it has a string index signature or if
+            // it has one or more properties and all properties have inferable types.
+            const properties = getPropertiesOfType(source);
+            const isReverseMappable = getIndexInfoOfType(source, IndexKind.String) ||
+                properties.length !== 0 && every(properties, prop => isPartiallyInferableType(getTypeOfSymbol(prop)));
+            if (!isReverseMappable) {
                 return undefined;
             }
             // For arrays and tuples we infer new arrays and tuples where the reverse mapping has been