From dc3eb3c27eb0723cd8d8ea93c200dab3445719cc Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Wed, 28 Mar 2018 13:16:11 -0700
Subject: [PATCH] Erase 'infer T' locations in conditional type constraints

---
 src/compiler/checker.ts | 14 ++++++++++++--
 src/compiler/types.ts   |  4 ++++
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 89c799db46d..4c5f9b93967 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -6233,7 +6233,7 @@ namespace ts {
         }
 
         function getDefaultConstraintOfConditionalType(type: ConditionalType) {
-            return getUnionType([getTrueTypeFromConditionalType(type), getFalseTypeFromConditionalType(type)]);
+            return getUnionType([getInferredTrueTypeFromConditionalType(type), getFalseTypeFromConditionalType(type)]);
         }
 
         function getConstraintOfDistributiveConditionalType(type: ConditionalType): Type {
@@ -8343,16 +8343,19 @@ namespace ts {
             // If this is a distributive conditional type and the check type is generic we need to defer
             // resolution of the conditional type such that a later instantiation will properly distribute
             // over union types.
-            if (!root.isDistributive || !maybeTypeOfKind(checkType, TypeFlags.Instantiable)) {
+            const isDeferred = root.isDistributive && maybeTypeOfKind(checkType, TypeFlags.Instantiable);
                 let combinedMapper: TypeMapper;
                 if (root.inferTypeParameters) {
                     const context = createInferenceContext(root.inferTypeParameters, /*signature*/ undefined, InferenceFlags.None);
+                if (!isDeferred) {
                     // We don't want inferences from constraints as they may cause us to eagerly resolve the
                     // conditional type instead of deferring resolution. Also, we always want strict function
                     // types rules (i.e. proper contravariance) for inferences.
                     inferTypes(context.inferences, checkType, extendsType, InferencePriority.NoConstraints | InferencePriority.AlwaysStrict);
+                }
                     combinedMapper = combineTypeMappers(mapper, context);
                 }
+            if (!isDeferred) {
                 // Return union of trueType and falseType for 'any' since it matches anything
                 if (checkType.flags & TypeFlags.Any) {
                     return getUnionType([instantiateType(root.trueType, combinedMapper || mapper), instantiateType(root.falseType, mapper)]);
@@ -8381,6 +8384,7 @@ namespace ts {
             result.checkType = erasedCheckType;
             result.extendsType = extendsType;
             result.mapper = mapper;
+            result.combinedMapper = combinedMapper;
             result.aliasSymbol = root.aliasSymbol;
             result.aliasTypeArguments = instantiateTypes(root.aliasTypeArguments, mapper);
             return result;
@@ -8394,6 +8398,12 @@ namespace ts {
             return type.resolvedFalseType || (type.resolvedFalseType = instantiateType(type.root.falseType, type.mapper));
         }
 
+        function getInferredTrueTypeFromConditionalType(type: ConditionalType) {
+            return type.combinedMapper ?
+                type.resolvedInferredTrueType || (type.resolvedInferredTrueType = instantiateType(type.root.trueType, type.combinedMapper)) :
+                getTrueTypeFromConditionalType(type);
+        }
+
         function getInferTypeParameters(node: ConditionalTypeNode): TypeParameter[] {
             let result: TypeParameter[];
             if (node.locals) {
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 51b1d984bf6..7775543dee6 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -3878,7 +3878,11 @@ namespace ts {
         resolvedTrueType?: Type;
         resolvedFalseType?: Type;
         /* @internal */
+        resolvedInferredTrueType?: Type;
+        /* @internal */
         mapper?: TypeMapper;
+        /* @internal */
+        combinedMapper?: TypeMapper;
     }
 
     // Type parameter substitution (TypeFlags.Substitution)