From bf19d214f32e2a30eced6b896ccc10fff01ec264 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Mon, 6 Aug 2018 16:10:10 -0700
Subject: [PATCH 1/2] Revise logic that computes the contextual type for a
 parameter

---
 src/compiler/checker.ts | 18 ++++--------------
 1 file changed, 4 insertions(+), 14 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index de0da714fcf..130086ca9a2 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -15820,20 +15820,10 @@ namespace ts {
             }
             const contextualSignature = getContextualSignature(func);
             if (contextualSignature) {
-                const funcHasRestParameter = hasRestParameter(func);
-                const len = func.parameters.length - (funcHasRestParameter ? 1 : 0);
-                let indexOfParameter = func.parameters.indexOf(parameter);
-                if (getThisParameter(func) !== undefined && !contextualSignature.thisParameter) {
-                    Debug.assert(indexOfParameter !== 0); // Otherwise we should not have called `getContextuallyTypedParameterType`.
-                    indexOfParameter -= 1;
-                }
-                if (indexOfParameter < len) {
-                    return getTypeAtPosition(contextualSignature, indexOfParameter);
-                }
-                // If last parameter is contextually rest parameter get its type
-                if (funcHasRestParameter && indexOfParameter === len) {
-                    return getRestTypeAtPosition(contextualSignature, indexOfParameter);
-                }
+                const index = func.parameters.indexOf(parameter) - (getThisParameter(func) ? 1 : 0);
+                return parameter.dotDotDotToken && lastOrUndefined(func.parameters) === parameter ?
+                    getRestTypeAtPosition(contextualSignature, index) :
+                    tryGetTypeAtPosition(contextualSignature, index);
             }
         }
 

From 9d71023b5c0613ebb3745838b5928cb9e8a7a3b7 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Mon, 6 Aug 2018 16:33:21 -0700
Subject: [PATCH 2/2] Add fourslash tests

---
 .../fourslash/contextuallyTypedParameters.ts  | 60 +++++++++++++++++++
 1 file changed, 60 insertions(+)
 create mode 100644 tests/cases/fourslash/contextuallyTypedParameters.ts

diff --git a/tests/cases/fourslash/contextuallyTypedParameters.ts b/tests/cases/fourslash/contextuallyTypedParameters.ts
new file mode 100644
index 00000000000..f600180c087
--- /dev/null
+++ b/tests/cases/fourslash/contextuallyTypedParameters.ts
@@ -0,0 +1,60 @@
+/// <reference path='fourslash.ts' />
+
+////declare function foo(cb: (this: any, x: number, y: string, z: boolean) => void): void;
+////
+////foo(function(this, a, ...args) {
+////    a/*10*/;
+////    args/*11*/;
+////});
+////
+////foo(function(this, a, b, ...args) {
+////    a/*20*/;
+////    b/*21*/;
+////    args/*22*/;
+////});
+////
+////foo(function(this, a, b, c, ...args) {
+////    a/*30*/;
+////    b/*31*/;
+////    c/*32*/;
+////    args/*33*/;
+////});
+////
+////foo(function(a, ...args) {
+////    a/*40*/;
+////    args/*41*/;
+////});
+////
+////foo(function(a, b, ...args) {
+////    a/*50*/;
+////    b/*51*/;
+////    args/*52*/;
+////});
+////
+////foo(function(a, b, c, ...args) {
+////    a/*60*/;
+////    b/*61*/;
+////    c/*62*/;
+////    args/*63*/;
+////});
+
+verify.quickInfos({
+    10: "(parameter) a: number",
+    11: "(parameter) args: [string, boolean]",
+    20: "(parameter) a: number",
+    21: "(parameter) b: string",
+    22: "(parameter) args: [boolean]",
+    30: "(parameter) a: number",
+    31: "(parameter) b: string",
+    32: "(parameter) c: boolean",
+    33: "(parameter) args: []",
+    40: "(parameter) a: number",
+    41: "(parameter) args: [string, boolean]",
+    50: "(parameter) a: number",
+    51: "(parameter) b: string",
+    52: "(parameter) args: [boolean]",
+    60: "(parameter) a: number",
+    61: "(parameter) b: string",
+    62: "(parameter) c: boolean",
+    63: "(parameter) args: []",
+});