Add clarifying comment about checkExpressionCached

src/compiler/checker.ts

@@ -5949,6 +5949,12 @@ module ts {
         }
 
         function checkSpreadElementExpression(node: SpreadElementExpression, contextualMapper?: TypeMapper): Type {
+            // It is usually not safe to call checkExpressionCached if we can be contextually typing.
+            // You can tell that we are contextually typing because of the contextualMapper parameter.
+            // While it is true that a spread element can have a contextual type, it does not do anything
+            // with this type. It is neither affected by it, nor does it propagate it to its operand.
+            // So the fact that contextualMapper is passed is not important, because the operand of a spread
+            // element is not contextually typed.
             let arrayOrIterableType = checkExpressionCached(node.expression, contextualMapper);
             return checkIteratedTypeOrElementType(arrayOrIterableType, node.expression, /*allowStringInput*/ false);
         }