From abc61a0949bbd03684cbd6d5b06b20a31216578c Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Thu, 15 Aug 2019 18:08:01 -0700
Subject: [PATCH] Add InferencePriority.Circularity per CR feedback

---
 src/compiler/checker.ts | 6 +++---
 src/compiler/types.ts   | 1 +
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 0a61d3aa177..b15e4d6afb9 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -15696,7 +15696,7 @@ namespace ts {
                     inferencePriority = Math.min(inferencePriority, status);
                     return;
                 }
-                (visited || (visited = createMap<number>())).set(key, -1);
+                (visited || (visited = createMap<number>())).set(key, InferencePriority.Circularity);
                 const saveInferencePriority = inferencePriority;
                 inferencePriority = InferencePriority.MaxValue;
                 action(source, target);
@@ -15789,7 +15789,7 @@ namespace ts {
                                 inferencePriority = InferencePriority.MaxValue;
                                 inferFromTypes(sources[i], t);
                                 if (inferencePriority === priority) matched[i] = true;
-                                inferenceCircularity = inferenceCircularity || inferencePriority < 0;
+                                inferenceCircularity = inferenceCircularity || inferencePriority === InferencePriority.Circularity;
                                 inferencePriority = Math.min(inferencePriority, saveInferencePriority);
                             }
                         }
@@ -15901,7 +15901,7 @@ namespace ts {
                 const symbol = isNonConstructorObject ? target.symbol : undefined;
                 if (symbol) {
                     if (contains(symbolStack, symbol)) {
-                        inferencePriority = -1;
+                        inferencePriority = InferencePriority.Circularity;
                         return;
                     }
                     (symbolStack || (symbolStack = [])).push(symbol);
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 7d98b7761ac..1d8a6f3eb7f 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -4474,6 +4474,7 @@ namespace ts {
         MaxValue                     = 1 << 8,  // Seed for inference priority tracking
 
         PriorityImpliesCombination = ReturnType | MappedTypeConstraint | LiteralKeyof,  // These priorities imply that the resulting type should be a combination of all candidates
+        Circularity = -1,  // Inference circularity (value less than all other priorities)
     }
 
     /* @internal */