From 0e273c3e07666d201bb434a6c822a2efbaec5a77 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Thu, 11 Jul 2019 10:45:19 -1000
Subject: [PATCH] Fix type parameter inference cache invalidation

---
 src/compiler/checker.ts | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 5d5d99f62c..b840417db3 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -15229,8 +15229,8 @@ namespace ts {
                 const inference = inferences[i];
                 if (t === inference.typeParameter) {
                     if (fix && !inference.isFixed) {
+                        clearCachedInferences(inferences);
                         inference.isFixed = true;
-                        inference.inferredType = undefined;
                     }
                     return getInferredType(context, i);
                 }
@@ -15238,6 +15238,14 @@ namespace ts {
             return t;
         }
 
+        function clearCachedInferences(inferences: InferenceInfo[]) {
+            for (const inference of inferences) {
+                if (!inference.isFixed) {
+                    inference.inferredType = undefined;
+                }
+            }
+        }
+
         function createInferenceInfo(typeParameter: TypeParameter): InferenceInfo {
             return {
                 typeParameter,
@@ -15517,17 +15525,17 @@ namespace ts {
                                 if (contravariant && !bivariant) {
                                     if (!contains(inference.contraCandidates, candidate)) {
                                         inference.contraCandidates = append(inference.contraCandidates, candidate);
-                                        inference.inferredType = undefined;
+                                        clearCachedInferences(inferences);
                                     }
                                 }
                                 else if (!contains(inference.candidates, candidate)) {
                                     inference.candidates = append(inference.candidates, candidate);
-                                    inference.inferredType = undefined;
+                                    clearCachedInferences(inferences);
                                 }
                             }
                             if (!(priority & InferencePriority.ReturnType) && target.flags & TypeFlags.TypeParameter && inference.topLevel && !isTypeParameterAtTopLevel(originalTarget, <TypeParameter>target)) {
                                 inference.topLevel = false;
-                                inference.inferredType = undefined;
+                                clearCachedInferences(inferences);
                             }
                         }
                         return;