Add clarifying comment about checkExpressionCached

JsonFreeman · JsonFreeman · commit 903c1d8dd8b1 · 2015-03-26T15:15:02.000-07:00
diff --git a/src/compiler/checker.ts b/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);
         }

Original file line number	Diff line number	Diff line change
`@@ -5949,6 +5949,12 @@ module ts {`
`5949`	`5949`	`}`
`5950`	`5950`
`5951`	`5951`	`function checkSpreadElementExpression(node: SpreadElementExpression, contextualMapper?: TypeMapper): Type {`
	`5952`	`+ // It is usually not safe to call checkExpressionCached if we can be contextually typing.`
	`5953`	`+ // You can tell that we are contextually typing because of the contextualMapper parameter.`
	`5954`	`+ // While it is true that a spread element can have a contextual type, it does not do anything`
	`5955`	`+ // with this type. It is neither affected by it, nor does it propagate it to its operand.`
	`5956`	`+ // So the fact that contextualMapper is passed is not important, because the operand of a spread`
	`5957`	`+ // element is not contextually typed.`
`5952`	`5958`	`let arrayOrIterableType = checkExpressionCached(node.expression, contextualMapper);`
`5953`	`5959`	`return checkIteratedTypeOrElementType(arrayOrIterableType, node.expression, /allowStringInput/ false);`
`5954`	`5960`	`}`