Implicit constraints in non-distributive '[T] extends [U] ? X : Y' types

ahejlsberg · ahejlsberg · commit 420e58c44462 · 2018-03-20T14:08:05.000-07:00
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
@@ -7349,13 +7349,24 @@ namespace ts {
             return result;
         }
 
+        function isUnaryTupleTypeNode(node: TypeNode) {
+            return node.kind === SyntaxKind.TupleType && (<TupleTypeNode>node).elementTypes.length === 1;
+        }
+
+        function getImpliedConstraint(typeVariable: TypeVariable, checkNode: TypeNode, extendsNode: TypeNode): Type {
+            return isUnaryTupleTypeNode(checkNode) && isUnaryTupleTypeNode(extendsNode) ? getImpliedConstraint(typeVariable, (<TupleTypeNode>checkNode).elementTypes[0], (<TupleTypeNode>extendsNode).elementTypes[0]) :
+                getActualTypeVariable(getTypeFromTypeNode(checkNode)) === typeVariable ? getTypeFromTypeNode(extendsNode) :
+                undefined;
+        }
+
         function getConstrainedTypeVariable(typeVariable: TypeVariable, node: Node) {
             let constraints: Type[];
             while (isPartOfTypeNode(node)) {
                 const parent = node.parent;
                 if (parent.kind === SyntaxKind.ConditionalType && node === (<ConditionalTypeNode>parent).trueType) {
-                    if (getActualTypeVariable(getTypeFromTypeNode((<ConditionalTypeNode>parent).checkType)) === typeVariable) {
-                        constraints = append(constraints, getTypeFromTypeNode((<ConditionalTypeNode>parent).extendsType));
+                    const constraint = getImpliedConstraint(typeVariable, (<ConditionalTypeNode>parent).checkType, (<ConditionalTypeNode>parent).extendsType);
+                    if (constraint) {
+                        constraints = append(constraints, constraint);
                     }
                 }
                 node = parent;
