From e96ec8c2c7bb6ea85610c9462bf6b07ed8b4c7cc Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Tue, 16 Jan 2018 12:51:24 -0800
Subject: [PATCH] Erase substitution types in type references and type alias
 instantiations

---
 src/compiler/checker.ts | 10 ++++++++++
 src/compiler/types.ts   |  4 ++++
 2 files changed, 14 insertions(+)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 6abe6ad04b0..4f3a6225d88 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -7316,7 +7316,16 @@ namespace ts {
             return result & TypeFlags.PropagatingFlags;
         }
 
+        // This function replaces substitution types in the given array with their underlying type parameter.
+        // We do this when creating type references and type alias instantiations because subsitution types are
+        // no longer necessary once the type arguments have been validated against their corresponding type
+        // parameter constraints.
+        function eraseSubstitutionTypes(types: Type[]) {
+            return sameMap(types, t => t.flags & TypeFlags.Substitution ? (<SubstitutionType>t).typeParameter : t);
+        }
+
         function createTypeReference(target: GenericType, typeArguments: Type[]): TypeReference {
+            typeArguments = eraseSubstitutionTypes(typeArguments);
             const id = getTypeListId(typeArguments);
             let type = target.instantiations.get(id);
             if (!type) {
@@ -7383,6 +7392,7 @@ namespace ts {
         }
 
         function getTypeAliasInstantiation(symbol: Symbol, typeArguments: Type[]): Type {
+            typeArguments = eraseSubstitutionTypes(typeArguments);
             const type = getDeclaredTypeOfSymbol(symbol);
             const links = getSymbolLinks(symbol);
             const typeParameters = links.typeParameters;
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index f426370002b..c544334b7dd 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -3703,6 +3703,10 @@ namespace ts {
     }
 
     // Type parameter substitution (TypeFlags.Substitution)
+    // Substitution types are created for type parameter references that occur in the true branch
+    // of a conditional type. For example, in 'T extends string ? Foo<T> : Bar<T>', the reference to
+    // T in Foo<T> is resolved as a substitution type that substitutes 'string & T' for T. Thus, if
+    // Foo<T> has a 'string' constraint on its type parameter, T will satisfy it.
     export interface SubstitutionType extends InstantiableType {
         typeParameter: TypeParameter;  // Target type parameter
         substitute: Type;              // Type to substitute for type parameter