Make substitution types even if the substitution base isnt a type variable

src/compiler/checker.ts

@@ -4275,7 +4275,7 @@ namespace ts {
                     return createConditionalTypeNode(checkTypeNode, extendsTypeNode, trueTypeNode, falseTypeNode);
                 }
                 if (type.flags & TypeFlags.Substitution) {
-                    return typeToTypeNodeHelper((<SubstitutionType>type).typeVariable, context);
+                    return typeToTypeNodeHelper((<SubstitutionType>type).baseType, context);
                 }
 
                 return Debug.fail("Should be unreachable.");
@@ -11323,9 +11323,7 @@ namespace ts {
             // Get type from reference to named type that cannot be generic (enum or type parameter)
             const res = tryGetDeclaredTypeOfSymbol(symbol);
             if (res) {
-                return checkNoTypeArguments(node, symbol) ?
-                    res.flags & TypeFlags.TypeParameter ? getConstrainedTypeVariable(<TypeParameter>res, node) : getRegularTypeOfLiteralType(res) :
-                    errorType;
+                return checkNoTypeArguments(node, symbol) ? getRegularTypeOfLiteralType(res) : errorType;
             }
             if (symbol.flags & SymbolFlags.Value && isJSDocTypeReference(node)) {
                 const jsdocType = getTypeFromJSDocValueReference(node, symbol);
@@ -11373,17 +11371,17 @@ namespace ts {
             return links.resolvedJSDocType;
         }
 
-        function getSubstitutionType(typeVariable: TypeVariable, substitute: Type) {
-            if (substitute.flags & TypeFlags.AnyOrUnknown || substitute === typeVariable) {
-                return typeVariable;
+        function getSubstitutionType(baseType: Type, substitute: Type) {
+            if (substitute.flags & TypeFlags.AnyOrUnknown || substitute === baseType) {
+                return baseType;
             }
-            const id = `${getTypeId(typeVariable)}>${getTypeId(substitute)}`;
+            const id = `${getTypeId(baseType)}>${getTypeId(substitute)}`;
             const cached = substitutionTypes.get(id);
             if (cached) {
                 return cached;
             }
             const result = <SubstitutionType>createType(TypeFlags.Substitution);
-            result.typeVariable = typeVariable;
+            result.baseType = baseType;
             result.substitute = substitute;
             substitutionTypes.set(id, result);
             return result;
@@ -11393,25 +11391,25 @@ namespace ts {
             return node.kind === SyntaxKind.TupleType && (<TupleTypeNode>node).elementTypes.length === 1;
         }
 
-        function getImpliedConstraint(typeVariable: TypeVariable, checkNode: TypeNode, extendsNode: TypeNode): Type | undefined {
-            return isUnaryTupleTypeNode(checkNode) && isUnaryTupleTypeNode(extendsNode) ? getImpliedConstraint(typeVariable, (<TupleTypeNode>checkNode).elementTypes[0], (<TupleTypeNode>extendsNode).elementTypes[0]) :
-                getActualTypeVariable(getTypeFromTypeNode(checkNode)) === typeVariable ? getTypeFromTypeNode(extendsNode) :
+        function getImpliedConstraint(type: Type, checkNode: TypeNode, extendsNode: TypeNode): Type | undefined {
+            return isUnaryTupleTypeNode(checkNode) && isUnaryTupleTypeNode(extendsNode) ? getImpliedConstraint(type, (<TupleTypeNode>checkNode).elementTypes[0], (<TupleTypeNode>extendsNode).elementTypes[0]) :
+                getActualTypeVariable(getTypeFromTypeNode(checkNode)) === type ? getTypeFromTypeNode(extendsNode) :
                 undefined;
         }
 
-        function getConstrainedTypeVariable(typeVariable: TypeVariable, node: Node) {
+        function getConditionalFlowTypeOfType(type: Type, node: Node) {
             let constraints: Type[] | undefined;
             while (node && !isStatement(node) && node.kind !== SyntaxKind.JSDocComment) {
                 const parent = node.parent;
                 if (parent.kind === SyntaxKind.ConditionalType && node === (<ConditionalTypeNode>parent).trueType) {
-                    const constraint = getImpliedConstraint(typeVariable, (<ConditionalTypeNode>parent).checkType, (<ConditionalTypeNode>parent).extendsType);
+                    const constraint = getImpliedConstraint(type, (<ConditionalTypeNode>parent).checkType, (<ConditionalTypeNode>parent).extendsType);
                     if (constraint) {
                         constraints = append(constraints, constraint);
                     }
                 }
                 node = parent;
             }
-            return constraints ? getSubstitutionType(typeVariable, getIntersectionType(append(constraints, typeVariable))) : typeVariable;
+            return constraints ? getSubstitutionType(type, getIntersectionType(append(constraints, type))) : type;
         }
 
         function isJSDocTypeReference(node: Node): node is TypeReferenceNode {
@@ -12854,7 +12852,7 @@ namespace ts {
                 links.resolvedType = resolved.flags & TypeFlags.IndexedAccess &&
                     (<IndexedAccessType>resolved).objectType === objectType &&
                     (<IndexedAccessType>resolved).indexType === indexType ?
-                    getConstrainedTypeVariable(<IndexedAccessType>resolved, node) : resolved;
+                    getConditionalFlowTypeOfType(resolved, node) : resolved;
             }
             return links.resolvedType;
         }
@@ -12876,7 +12874,7 @@ namespace ts {
 
         function getActualTypeVariable(type: Type): Type {
             if (type.flags & TypeFlags.Substitution) {
-                return (<SubstitutionType>type).typeVariable;
+                return (<SubstitutionType>type).baseType;
             }
             if (type.flags & TypeFlags.IndexedAccess && (
                 (<IndexedAccessType>type).objectType.flags & TypeFlags.Substitution ||
@@ -13424,6 +13422,10 @@ namespace ts {
         }
 
         function getTypeFromTypeNode(node: TypeNode): Type {
+            return getConditionalFlowTypeOfType(getTypeFromTypeNodeWorker(node), node);
+        }
+
+        function getTypeFromTypeNodeWorker(node: TypeNode): Type {
             switch (node.kind) {
                 case SyntaxKind.AnyKeyword:
                 case SyntaxKind.JSDocAllType:
@@ -13758,7 +13760,7 @@ namespace ts {
                         return !!tp.isThisType;
                     case SyntaxKind.Identifier:
                         return !tp.isThisType && isPartOfTypeNode(node) && maybeTypeParameterReference(node) &&
-                            getTypeFromTypeNode(<TypeNode>node) === tp;
+                            getTypeFromTypeNodeWorker(<TypeNode>node) === tp; // use worker because we're looking for === equality
                     case SyntaxKind.TypeQuery:
                         return true;
                 }
@@ -13959,7 +13961,7 @@ namespace ts {
                 return getConditionalTypeInstantiation(<ConditionalType>type, combineTypeMappers((<ConditionalType>type).mapper, mapper));
             }
             if (flags & TypeFlags.Substitution) {
-                const maybeVariable = instantiateType((<SubstitutionType>type).typeVariable, mapper);
+                const maybeVariable = instantiateType((<SubstitutionType>type).baseType, mapper);
                 if (maybeVariable.flags & TypeFlags.TypeVariable) {
                     return getSubstitutionType(maybeVariable as TypeVariable, instantiateType((<SubstitutionType>type).substitute, mapper));
                 }
@@ -14968,7 +14970,7 @@ namespace ts {
                 const t = isFreshLiteralType(type) ? (<FreshableType>type).regularType :
                     getObjectFlags(type) & ObjectFlags.Reference && (<TypeReference>type).node ? createTypeReference((<TypeReference>type).target, getTypeArguments(<TypeReference>type)) :
                     type.flags & TypeFlags.UnionOrIntersection ? getReducedType(type) :
-                    type.flags & TypeFlags.Substitution ? writing ? (<SubstitutionType>type).typeVariable : (<SubstitutionType>type).substitute :
+                    type.flags & TypeFlags.Substitution ? writing ? (<SubstitutionType>type).baseType : (<SubstitutionType>type).substitute :
                     type.flags & TypeFlags.Simplifiable ? getSimplifiedType(type, writing) :
                     type;
                 if (t === type) break;

src/compiler/types.ts

@@ -4808,8 +4808,8 @@ namespace ts {
     // Thus, if Foo has a 'string' constraint on its type parameter, T will satisfy it. Substitution
     // types disappear upon instantiation (just like type parameters).
     export interface SubstitutionType extends InstantiableType {
-        typeVariable: TypeVariable;  // Target type variable
-        substitute: Type;            // Type to substitute for type parameter
+        baseType: Type;     // Target type
+        substitute: Type;   // Type to substitute for type parameter
     }
 
     /* @internal */

tests/baselines/reference/api/tsserverlibrary.d.ts

@@ -2523,7 +2523,7 @@ declare namespace ts {
         resolvedFalseType: Type;
     }
     export interface SubstitutionType extends InstantiableType {
-        typeVariable: TypeVariable;
+        baseType: Type;
         substitute: Type;
     }
     export enum SignatureKind {

tests/baselines/reference/api/typescript.d.ts

@@ -2523,7 +2523,7 @@ declare namespace ts {
         resolvedFalseType: Type;
     }
     export interface SubstitutionType extends InstantiableType {
-        typeVariable: TypeVariable;
+        baseType: Type;
         substitute: Type;
     }
     export enum SignatureKind {

tests/baselines/reference/inlineConditionalHasSimilarAssignability.js

@@ -0,0 +1,29 @@
+//// [inlineConditionalHasSimilarAssignability.ts]
+type MyExtract<T, U> = T extends U ? T : never
+
+function foo<T>(a: T) {
+  const b: Extract<any[], T> = 0 as any;
+  a = b; // ok
+
+  const c: (any[] extends T ? any[] : never) = 0 as any;
+  a = c;
+
+  const d: MyExtract<any[], T> = 0 as any;
+  a = d; // ok
+
+  type CustomType = any[] extends T ? any[] : never;
+  const e: CustomType = 0 as any;
+  a = e;
+}
+
+//// [inlineConditionalHasSimilarAssignability.js]
+function foo(a) {
+    var b = 0;
+    a = b; // ok
+    var c = 0;
+    a = c;
+    var d = 0;
+    a = d; // ok
+    var e = 0;
+    a = e;
+}

tests/baselines/reference/inlineConditionalHasSimilarAssignability.symbols

@@ -0,0 +1,53 @@
+=== tests/cases/compiler/inlineConditionalHasSimilarAssignability.ts ===
+type MyExtract<T, U> = T extends U ? T : never
+>MyExtract : Symbol(MyExtract, Decl(inlineConditionalHasSimilarAssignability.ts, 0, 0))
+>T : Symbol(T, Decl(inlineConditionalHasSimilarAssignability.ts, 0, 15))
+>U : Symbol(U, Decl(inlineConditionalHasSimilarAssignability.ts, 0, 17))
+>T : Symbol(T, Decl(inlineConditionalHasSimilarAssignability.ts, 0, 15))
+>U : Symbol(U, Decl(inlineConditionalHasSimilarAssignability.ts, 0, 17))
+>T : Symbol(T, Decl(inlineConditionalHasSimilarAssignability.ts, 0, 15))
+
+function foo<T>(a: T) {
+>foo : Symbol(foo, Decl(inlineConditionalHasSimilarAssignability.ts, 0, 46))
+>T : Symbol(T, Decl(inlineConditionalHasSimilarAssignability.ts, 2, 13))
+>a : Symbol(a, Decl(inlineConditionalHasSimilarAssignability.ts, 2, 16))
+>T : Symbol(T, Decl(inlineConditionalHasSimilarAssignability.ts, 2, 13))
+
+  const b: Extract<any[], T> = 0 as any;
+>b : Symbol(b, Decl(inlineConditionalHasSimilarAssignability.ts, 3, 7))
+>Extract : Symbol(Extract, Decl(lib.es5.d.ts, --, --))
+>T : Symbol(T, Decl(inlineConditionalHasSimilarAssignability.ts, 2, 13))
+
+  a = b; // ok
+>a : Symbol(a, Decl(inlineConditionalHasSimilarAssignability.ts, 2, 16))
+>b : Symbol(b, Decl(inlineConditionalHasSimilarAssignability.ts, 3, 7))
+
+  const c: (any[] extends T ? any[] : never) = 0 as any;
+>c : Symbol(c, Decl(inlineConditionalHasSimilarAssignability.ts, 6, 7))
+>T : Symbol(T, Decl(inlineConditionalHasSimilarAssignability.ts, 2, 13))
+
+  a = c;
+>a : Symbol(a, Decl(inlineConditionalHasSimilarAssignability.ts, 2, 16))
+>c : Symbol(c, Decl(inlineConditionalHasSimilarAssignability.ts, 6, 7))
+
+  const d: MyExtract<any[], T> = 0 as any;
+>d : Symbol(d, Decl(inlineConditionalHasSimilarAssignability.ts, 9, 7))
+>MyExtract : Symbol(MyExtract, Decl(inlineConditionalHasSimilarAssignability.ts, 0, 0))
+>T : Symbol(T, Decl(inlineConditionalHasSimilarAssignability.ts, 2, 13))
+
+  a = d; // ok
+>a : Symbol(a, Decl(inlineConditionalHasSimilarAssignability.ts, 2, 16))
+>d : Symbol(d, Decl(inlineConditionalHasSimilarAssignability.ts, 9, 7))
+
+  type CustomType = any[] extends T ? any[] : never;
+>CustomType : Symbol(CustomType, Decl(inlineConditionalHasSimilarAssignability.ts, 10, 8))
+>T : Symbol(T, Decl(inlineConditionalHasSimilarAssignability.ts, 2, 13))
+
+  const e: CustomType = 0 as any;
+>e : Symbol(e, Decl(inlineConditionalHasSimilarAssignability.ts, 13, 7))
+>CustomType : Symbol(CustomType, Decl(inlineConditionalHasSimilarAssignability.ts, 10, 8))
+
+  a = e;
+>a : Symbol(a, Decl(inlineConditionalHasSimilarAssignability.ts, 2, 16))
+>e : Symbol(e, Decl(inlineConditionalHasSimilarAssignability.ts, 13, 7))
+}

tests/baselines/reference/inlineConditionalHasSimilarAssignability.types

@@ -0,0 +1,51 @@
+=== tests/cases/compiler/inlineConditionalHasSimilarAssignability.ts ===
+type MyExtract<T, U> = T extends U ? T : never
+>MyExtract : MyExtract<T, U>
+
+function foo<T>(a: T) {
+>foo : <T>(a: T) => void
+>a : T
+
+  const b: Extract<any[], T> = 0 as any;
+>b : Extract<any[], T>
+>0 as any : any
+>0 : 0
+
+  a = b; // ok
+>a = b : Extract<any[], T>
+>a : T
+>b : Extract<any[], T>
+
+  const c: (any[] extends T ? any[] : never) = 0 as any;
+>c : any[] extends T ? any[] : never
+>0 as any : any
+>0 : 0
+
+  a = c;
+>a = c : any[] extends T ? any[] : never
+>a : T
+>c : any[] extends T ? any[] : never
+
+  const d: MyExtract<any[], T> = 0 as any;
+>d : MyExtract<any[], T>
+>0 as any : any
+>0 : 0
+
+  a = d; // ok
+>a = d : MyExtract<any[], T>
+>a : T
+>d : MyExtract<any[], T>
+
+  type CustomType = any[] extends T ? any[] : never;
+>CustomType : any[] extends T ? any[] : never
+
+  const e: CustomType = 0 as any;
+>e : any[] extends T ? any[] : never
+>0 as any : any
+>0 : 0
+
+  a = e;
+>a = e : any[] extends T ? any[] : never
+>a : T
+>e : any[] extends T ? any[] : never
+}

tests/cases/compiler/inlineConditionalHasSimilarAssignability.ts

@@ -0,0 +1,16 @@
+type MyExtract<T, U> = T extends U ? T : never
+
+function foo<T>(a: T) {
+  const b: Extract<any[], T> = 0 as any;
+  a = b; // ok
+
+  const c: (any[] extends T ? any[] : never) = 0 as any;
+  a = c;
+
+  const d: MyExtract<any[], T> = 0 as any;
+  a = d; // ok
+
+  type CustomType = any[] extends T ? any[] : never;
+  const e: CustomType = 0 as any;
+  a = e;
+}