From f59f3a27936ff646a376ab6e8756ebdb910dabc8 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Thu, 25 Jan 2018 13:02:35 -0800
Subject: [PATCH] Check 'infer' declarations are in extends clause of
 conditional type

---
 src/compiler/checker.ts              | 9 +++++++++
 src/compiler/diagnosticMessages.json | 4 ++++
 2 files changed, 13 insertions(+)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 0786aee366a..21fac5fe1e3 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -20172,6 +20172,13 @@ namespace ts {
             forEachChild(node, checkSourceElement);
         }
 
+        function checkInferType(node: InferTypeNode) {
+            if (!findAncestor(node, n => n.parent && n.parent.kind === SyntaxKind.ConditionalType && (<ConditionalTypeNode>n.parent).extendsType === n)) {
+                grammarErrorOnNode(node, Diagnostics.infer_declarations_are_only_permitted_in_the_extends_clause_of_a_conditional_type);
+            }
+            checkSourceElement(node.typeParameter);
+        }
+
         function isPrivateWithinAmbient(node: Node): boolean {
             return hasModifier(node, ModifierFlags.Private) && !!(node.flags & NodeFlags.Ambient);
         }
@@ -23874,6 +23881,8 @@ namespace ts {
                     return checkTypeOperator(<TypeOperatorNode>node);
                 case SyntaxKind.ConditionalType:
                     return checkConditionalType(<ConditionalTypeNode>node);
+                case SyntaxKind.InferType:
+                    return checkInferType(<InferTypeNode>node);
                 case SyntaxKind.JSDocAugmentsTag:
                     return checkJSDocAugmentsTag(node as JSDocAugmentsTag);
                 case SyntaxKind.JSDocTypedefTag:
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 6a14aab2aee..bac8bd818c9 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -947,6 +947,10 @@
         "category": "Error",
         "code": 1337
     },
+    "'infer' declarations are only permitted in the 'extends' clause of a conditional type.": {
+        "category": "Error",
+        "code": 1338
+    },
 
     "Duplicate identifier '{0}'.": {
         "category": "Error",