From 5702f61562f3dc2719394c25efd2e737f733be1a Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Wed, 31 Jan 2018 07:48:15 -0800
Subject: [PATCH] Always use strict rules for contravariant inferences

---
 src/compiler/checker.ts | 7 ++++---
 src/compiler/types.ts   | 1 +
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 1584038430f..49885fffb1c 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -8129,8 +8129,9 @@ namespace ts {
             if (inferTypeParameters) {
                 const inferences = map(inferTypeParameters, createInferenceInfo);
                 // We don't want inferences from constraints as they may cause us to eagerly resolve the
-                // conditional type instead of deferring resolution.
-                inferTypes(inferences, checkType, extendsType, InferencePriority.NoConstraints);
+                // conditional type instead of deferring resolution. Also, we always want strict function
+                // types rules (i.e. proper contravariance) for inferences.
+                inferTypes(inferences, checkType, extendsType, InferencePriority.NoConstraints | InferencePriority.AlwaysStrict);
                 // We infer 'never' when there are no candidates for a type parameter
                 const inferredTypes = map(inferences, inference => getTypeFromInference(inference) || neverType);
                 const inferenceMapper = createTypeMapper(inferTypeParameters, inferredTypes);
@@ -11510,7 +11511,7 @@ namespace ts {
             }
 
             function inferFromContravariantTypes(source: Type, target: Type) {
-                if (strictFunctionTypes) {
+                if (strictFunctionTypes || priority & InferencePriority.AlwaysStrict) {
                     contravariant = !contravariant;
                     inferFromTypes(source, target);
                     contravariant = !contravariant;
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index f9b06bc059c..260ac6e744e 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -3884,6 +3884,7 @@ namespace ts {
         MappedType        = 1 << 1,  // Reverse inference for mapped type
         ReturnType        = 1 << 2,  // Inference made from return type of generic function
         NoConstraints     = 1 << 3,  // Don't infer from constraints of instantiable types
+        AlwaysStrict      = 1 << 4,  // Always use strict rules for contravariant inferences
     }
 
     export interface InferenceInfo {