Fix bounds check

ahejlsberg · ahejlsberg · commit 720964f729b8 · 2019-10-05T19:14:48.000-07:00
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
@@ -19106,7 +19106,7 @@ namespace ts {
                         const flowType = getTypeAtFlowNode(flow.antecedent);
                         const type = getTypeFromFlowType(flowType);
                         const narrowedType = predicate.type ? narrowTypeByTypePredicate(type, predicate, flow.node, /*assumeTrue*/ true) :
-                            predicate.kind === TypePredicateKind.AssertsIdentifier && predicate.parameterIndex < flow.node.arguments.length ? narrowTypeByAssertion(type, flow.node.arguments[predicate.parameterIndex]) :
+                            predicate.kind === TypePredicateKind.AssertsIdentifier && predicate.parameterIndex >= 0 && predicate.parameterIndex < flow.node.arguments.length ? narrowTypeByAssertion(type, flow.node.arguments[predicate.parameterIndex]) :
                             type;
                         return narrowedType === type ? flowType : createFlowType(narrowedType, isIncomplete(flowType));
                     }

Original file line number	Diff line number	Diff line change
`@@ -19106,7 +19106,7 @@ namespace ts {`
`19106`	`19106`	`const flowType = getTypeAtFlowNode(flow.antecedent);`
`19107`	`19107`	`const type = getTypeFromFlowType(flowType);`
`19108`	`19108`	`const narrowedType = predicate.type ? narrowTypeByTypePredicate(type, predicate, flow.node, /assumeTrue/ true) :`
`19109`		`- predicate.kind === TypePredicateKind.AssertsIdentifier && predicate.parameterIndex < flow.node.arguments.length ? narrowTypeByAssertion(type, flow.node.arguments[predicate.parameterIndex]) :`
	`19109`	`+ predicate.kind === TypePredicateKind.AssertsIdentifier && predicate.parameterIndex >= 0 && predicate.parameterIndex < flow.node.arguments.length ? narrowTypeByAssertion(type, flow.node.arguments[predicate.parameterIndex]) :`
`19110`	`19110`	`type;`
`19111`	`19111`	`return narrowedType === type ? flowType : createFlowType(narrowedType, isIncomplete(flowType));`
`19112`	`19112`	`}`