Allow infer types in type argument positions

src/compiler/binder.ts

@@ -1434,6 +1434,8 @@ namespace ts {
                     return ContainerFlags.IsContainer | ContainerFlags.HasLocals;
 
                 case SyntaxKind.ConditionalType:
+                case SyntaxKind.CallExpression:
+                case SyntaxKind.NewExpression:
                     return ContainerFlags.IsInferenceContainer;
 
                 case SyntaxKind.SourceFile:

src/compiler/checker.ts

@@ -3021,6 +3021,11 @@ namespace ts {
                 if (type.flags & TypeFlags.Substitution) {
                     return typeToTypeNodeHelper((<SubstitutionType>type).typeParameter, context);
                 }
+                if (type.flags & TypeFlags.InferType) {
+                    // Infer types only parse as identifiers, so the target should always be a TypeParameter that becomes a TypeReferenceNode
+                    const ref = typeToTypeNodeHelper((<InferType>type).target, context) as TypeReferenceNode;
+                    return createInferTypeNode(createTypeParameterDeclaration(ref.typeName as Identifier));
+                }
 
                 Debug.fail("Should be unreachable.");
 
@@ -3517,7 +3522,7 @@ namespace ts {
                         const params = getTypeParametersOfClassOrInterface(
                             parentSymbol.flags & SymbolFlags.Alias ? resolveAlias(parentSymbol) : parentSymbol
                         );
-                        typeParameterNodes = mapToTypeNodes(map(params, (nextSymbol as TransientSymbol).mapper), context);
+                        typeParameterNodes = mapToTypeNodes(mapIndexless(params, (nextSymbol as TransientSymbol).mapper), context);
                     }
                     else {
                         typeParameterNodes = typeParametersToTypeParameterDeclarations(symbol, context);
@@ -4736,12 +4741,14 @@ namespace ts {
                     case SyntaxKind.JSDocTemplateTag:
                     case SyntaxKind.MappedType:
                     case SyntaxKind.ConditionalType:
+                    case SyntaxKind.CallExpression:
+                    case SyntaxKind.NewExpression:
                         const outerTypeParameters = getOuterTypeParameters(node, includeThisTypes);
                         if (node.kind === SyntaxKind.MappedType) {
                             return append(outerTypeParameters, getDeclaredTypeOfTypeParameter(getSymbolOfNode((<MappedTypeNode>node).typeParameter)));
                         }
-                        else if (node.kind === SyntaxKind.ConditionalType) {
-                            return concatenate(outerTypeParameters, getInferTypeParameters(<ConditionalTypeNode>node));
+                        else if (node.kind === SyntaxKind.ConditionalType || node.kind === SyntaxKind.NewExpression || node.kind === SyntaxKind.CallExpression) {
+                            return concatenate(outerTypeParameters, getInferTypeParameters(<ConditionalTypeNode | CallLikeExpression>node));
                         }
                         const outerAndOwnTypeParameters = appendTypeParameters(outerTypeParameters, getEffectiveTypeParameterDeclarations(<DeclarationWithTypeParameters>node) || emptyArray);
                         const thisType = includeThisTypes &&
@@ -8334,7 +8341,7 @@ namespace ts {
             return type.resolvedFalseType || (type.resolvedFalseType = instantiateType(type.root.falseType, type.mapper));
         }
 
-        function getInferTypeParameters(node: ConditionalTypeNode): TypeParameter[] {
+        function getInferTypeParameters(node: ConditionalTypeNode | CallLikeExpression): TypeParameter[] {
             let result: TypeParameter[];
             if (node.locals) {
                 node.locals.forEach(symbol => {
@@ -8375,10 +8382,16 @@ namespace ts {
             return links.resolvedType;
         }
 
+        function createInferType(target: TypeParameter): InferType {
+            const type = createType(TypeFlags.InferType) as InferType;
+            type.target = target;
+            return type;
+        }
+
         function getTypeFromInferTypeNode(node: InferTypeNode): Type {
             const links = getNodeLinks(node);
             if (!links.resolvedType) {
-                links.resolvedType = getDeclaredTypeOfTypeParameter(getSymbolOfNode(node.typeParameter));
+                links.resolvedType = createInferType(getDeclaredTypeOfTypeParameter(getSymbolOfNode(node.typeParameter)));
             }
             return links.resolvedType;
         }
@@ -8882,7 +8895,7 @@ namespace ts {
                 // mapper to the type parameters to produce the effective list of type arguments, and compute the
                 // instantiation cache key from the type IDs of the type arguments.
                 const combinedMapper = type.objectFlags & ObjectFlags.Instantiated ? combineTypeMappers(type.mapper, mapper) : mapper;
-                const typeArguments = map(typeParameters, combinedMapper);
+                const typeArguments = mapIndexless(typeParameters, combinedMapper);
                 const id = getTypeListId(typeArguments);
                 let result = links.instantiations.get(id);
                 if (!result) {
@@ -8965,7 +8978,7 @@ namespace ts {
                 // We are instantiating a conditional type that has one or more type parameters in scope. Apply the
                 // mapper to the type parameters to produce the effective list of type arguments, and compute the
                 // instantiation cache key from the type IDs of the type arguments.
-                const typeArguments = map(root.outerTypeParameters, mapper);
+                const typeArguments = mapIndexless(root.outerTypeParameters, mapper);
                 const id = getTypeListId(typeArguments);
                 let result = root.instantiations.get(id);
                 if (!result) {
@@ -9036,6 +9049,9 @@ namespace ts {
                 if (type.flags & TypeFlags.Substitution) {
                     return mapper((<SubstitutionType>type).typeParameter);
                 }
+                if (type.flags & TypeFlags.InferType) {
+                    return mapper((<InferType>type).target, /*isInferDeclaration*/ true);
+                }
             }
             return type;
         }
@@ -9642,9 +9658,15 @@ namespace ts {
                 if (source.flags & TypeFlags.Substitution) {
                     source = relation === definitelyAssignableRelation ? (<SubstitutionType>source).typeParameter : (<SubstitutionType>source).substitute;
                 }
+                if (source.flags & TypeFlags.InferType) {
+                    source = (<InferType>source).target;
+                }
                 if (target.flags & TypeFlags.Substitution) {
                     target = (<SubstitutionType>target).typeParameter;
                 }
+                if (target.flags & TypeFlags.InferType) {
+                    target = (<InferType>target).target;
+                }
 
                 // both types are the same - covers 'they are the same primitive type or both are Any' or the same type parameter cases
                 if (source === target) return Ternary.True;
@@ -11587,6 +11609,12 @@ namespace ts {
                 if (!couldContainTypeVariables(target)) {
                     return;
                 }
+                if (source.flags & TypeFlags.InferType) {
+                    source = (source as InferType).target;
+                }
+                if (target.flags & TypeFlags.InferType) {
+                    target = (target as InferType).target;
+                }
                 if (source.flags & TypeFlags.Any) {
                     // We are inferring from an 'any' type. We want to infer this type for every type parameter
                     // referenced in the target type, so we record it as the propagation type and infer from the
@@ -17529,10 +17557,29 @@ namespace ts {
                         candidate = originalCandidate;
                         if (candidate.typeParameters) {
                             let typeArgumentTypes: Type[];
+                            const isJavascript = isInJavaScriptFile(candidate.declaration);
                             if (typeArguments) {
                                 const typeArgumentResult = checkTypeArguments(candidate, typeArguments, /*reportErrors*/ false);
                                 if (typeArgumentResult) {
-                                    typeArgumentTypes = typeArgumentResult;
+                                    if (node.locals) {
+                                        // Call has `infer` arguments that still need to be inferred and instantiated
+                                        const inferParams = getInferTypeParameters(node);
+                                        // Mapper replaces references to infered type parameters with emptyObjectType
+                                        // Causing the original location to be the _only_ inference site
+                                        const preprocessMapper = (p: TypeParameter, isInferDecl: boolean) => {
+                                            if (!isInferDecl && contains(inferParams, p)) return emptyObjectType;
+                                            return p;
+                                        };
+                                        const resultsWithNonInferInferredVarsDefaulted = map(typeArgumentResult, t => instantiateType(t, preprocessMapper));
+                                        const partialCandidate = getSignatureInstantiation(candidate, resultsWithNonInferInferredVarsDefaulted, isJavascript);
+                                        const context = createInferenceContext(inferParams, partialCandidate, InferenceFlags.None);
+                                        const inferences = inferTypeArguments(node, partialCandidate, args, excludeArgument, context);
+                                        const mapper = createTypeMapper(inferParams, inferences);
+                                        typeArgumentTypes = map(typeArgumentResult, t => instantiateType(t, mapper));
+                                    }
+                                    else {
+                                        typeArgumentTypes = typeArgumentResult;
+                                    }
                                 }
                                 else {
                                     candidateForTypeArgumentError = originalCandidate;
@@ -17542,7 +17589,6 @@ namespace ts {
                             else {
                                 typeArgumentTypes = inferTypeArguments(node, candidate, args, excludeArgument, inferenceContext);
                             }
-                            const isJavascript = isInJavaScriptFile(candidate.declaration);
                             candidate = getSignatureInstantiation(candidate, typeArgumentTypes, isJavascript);
                         }
                         if (!checkApplicableSignature(node, args, candidate, relation, excludeArgument, /*reportErrors*/ false)) {
@@ -20544,9 +20590,18 @@ namespace ts {
             forEachChild(node, checkSourceElement);
         }
 
+        function isConditionalTypeExtendsClause(n: Node) {
+            return n.parent && n.parent.kind === SyntaxKind.ConditionalType && (<ConditionalTypeNode>n.parent).extendsType === n;
+        }
+
+        function isCallOrNewExpressionTypeArgument(n: Node) {
+            return n.parent && (n.parent.kind === SyntaxKind.CallExpression || n.parent.kind === SyntaxKind.NewExpression)
+                && contains((<CallExpression | NewExpression>n.parent).typeArguments, n);
+        }
+
         function checkInferType(node: InferTypeNode) {
-            if (!findAncestor(node, n => n.parent && n.parent.kind === SyntaxKind.ConditionalType && (<ConditionalTypeNode>n.parent).extendsType === n)) {
-                grammarErrorOnNode(node, Diagnostics.infer_declarations_are_only_permitted_in_the_extends_clause_of_a_conditional_type);
+            if (!findAncestor(node, n => isConditionalTypeExtendsClause(n) || isCallOrNewExpressionTypeArgument(n))) {
+                grammarErrorOnNode(node, Diagnostics.infer_declarations_are_only_permitted_in_the_extends_clause_of_a_conditional_type_or_in_call_or_new_expression_type_argument_lists);
             }
             checkSourceElement(node.typeParameter);
         }

src/compiler/core.ts

@@ -414,6 +414,17 @@ namespace ts {
         array.length = 0;
     }
 
+    export function mapIndexless<T, U>(array: ReadonlyArray<T>, f: (x: T) => U): U[] {
+        let result: U[];
+        if (array) {
+            result = [];
+            for (const elem of array) {
+                result.push(f(elem));
+            }
+        }
+        return result;
+    }
+
     export function map<T, U>(array: ReadonlyArray<T>, f: (x: T, i: number) => U): U[] {
         let result: U[];
         if (array) {

src/compiler/diagnosticMessages.json

@@ -947,7 +947,7 @@
         "category": "Error",
         "code": 1337
     },
-    "'infer' declarations are only permitted in the 'extends' clause of a conditional type.": {
+    "'infer' declarations are only permitted in the 'extends' clause of a conditional type or in call or new expression type argument lists.": {
         "category": "Error",
         "code": 1338
     },

src/compiler/types.ts

@@ -3538,6 +3538,7 @@ namespace ts {
         /* @internal */
         ContainsAnyFunctionType = 1 << 26,  // Type is or contains the anyFunctionType
         NonPrimitive            = 1 << 27,  // intrinsic object type
+        InferType               = 1 << 28,  // A type whose concrete value upon instantiation will be inferred at a given site
         /* @internal */
         GenericMappedType       = 1 << 29,  // Flag used by maybeTypeOfKind
 
@@ -3562,7 +3563,7 @@ namespace ts {
         ESSymbolLike = ESSymbol | UniqueESSymbol,
         UnionOrIntersection = Union | Intersection,
         StructuredType = Object | Union | Intersection,
-        TypeVariable = TypeParameter | IndexedAccess,
+        TypeVariable = TypeParameter | IndexedAccess | InferType,
         InstantiableNonPrimitive = TypeVariable | Conditional | Substitution,
         InstantiablePrimitive = Index,
         Instantiable = InstantiableNonPrimitive | InstantiablePrimitive,
@@ -3817,6 +3818,11 @@ namespace ts {
         resolvedDefaultType?: Type;
     }
 
+    // Infer Types (TypeFlags.InferType)
+    export interface InferType extends Type {
+        target: TypeParameter;
+    }
+
     // Indexed access types (TypeFlags.IndexedAccess)
     // Possible forms are T[xxx], xxx[T], or xxx[keyof T], where T is a type variable
     export interface IndexedAccessType extends InstantiableType {
@@ -3919,7 +3925,7 @@ namespace ts {
     }
 
     /* @internal */
-    export type TypeMapper = (t: TypeParameter) => Type;
+    export type TypeMapper = (t: TypeParameter, isInferDeclaration?: boolean) => Type;
 
     export const enum InferencePriority {
         NakedTypeVariable           = 1 << 0,  // Naked type variable in union or intersection type

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

@@ -2074,6 +2074,7 @@ declare namespace ts {
         Conditional = 2097152,
         Substitution = 4194304,
         NonPrimitive = 134217728,
+        InferType = 268435456,
         Literal = 224,
         Unit = 13536,
         StringOrNumberLiteral = 96,
@@ -2085,12 +2086,12 @@ declare namespace ts {
         ESSymbolLike = 1536,
         UnionOrIntersection = 393216,
         StructuredType = 458752,
-        TypeVariable = 1081344,
-        InstantiableNonPrimitive = 7372800,
+        TypeVariable = 269516800,
+        InstantiableNonPrimitive = 275808256,
         InstantiablePrimitive = 524288,
-        Instantiable = 7897088,
-        StructuredOrInstantiable = 8355840,
-        Narrowable = 142575359,
+        Instantiable = 276332544,
+        StructuredOrInstantiable = 276791296,
+        Narrowable = 411010815,
         NotUnionOrUnit = 134283777,
     }
     type DestructuringPattern = BindingPattern | ObjectLiteralExpression | ArrayLiteralExpression;
@@ -2184,6 +2185,9 @@ declare namespace ts {
     }
     interface TypeParameter extends InstantiableType {
     }
+    interface InferType extends Type {
+        target: TypeParameter;
+    }
     interface IndexedAccessType extends InstantiableType {
         objectType: Type;
         indexType: Type;

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

@@ -2074,6 +2074,7 @@ declare namespace ts {
         Conditional = 2097152,
         Substitution = 4194304,
         NonPrimitive = 134217728,
+        InferType = 268435456,
         Literal = 224,
         Unit = 13536,
         StringOrNumberLiteral = 96,
@@ -2085,12 +2086,12 @@ declare namespace ts {
         ESSymbolLike = 1536,
         UnionOrIntersection = 393216,
         StructuredType = 458752,
-        TypeVariable = 1081344,
-        InstantiableNonPrimitive = 7372800,
+        TypeVariable = 269516800,
+        InstantiableNonPrimitive = 275808256,
         InstantiablePrimitive = 524288,
-        Instantiable = 7897088,
-        StructuredOrInstantiable = 8355840,
-        Narrowable = 142575359,
+        Instantiable = 276332544,
+        StructuredOrInstantiable = 276791296,
+        Narrowable = 411010815,
         NotUnionOrUnit = 134283777,
     }
     type DestructuringPattern = BindingPattern | ObjectLiteralExpression | ArrayLiteralExpression;
@@ -2184,6 +2185,9 @@ declare namespace ts {
     }
     interface TypeParameter extends InstantiableType {
     }
+    interface InferType extends Type {
+        target: TypeParameter;
+    }
     interface IndexedAccessType extends InstantiableType {
         objectType: Type;
         indexType: Type;

tests/baselines/reference/inferTypeArgumentKeyword.errors.txt

@@ -0,0 +1,24 @@
+tests/cases/conformance/types/typeParameters/typeArgumentLists/inferTypeArgumentKeyword.ts(7,51): error TS2345: Argument of type '{ z: number; }' is not assignable to parameter of type '{ z: { y: number; }; }'.
+  Types of property 'z' are incompatible.
+    Type 'number' is not assignable to type '{ y: number; }'.
+tests/cases/conformance/types/typeParameters/typeArgumentLists/inferTypeArgumentKeyword.ts(10,30): error TS2345: Argument of type '{ y: number; }' is not assignable to parameter of type 'number'.
+
+
+==== tests/cases/conformance/types/typeParameters/typeArgumentLists/inferTypeArgumentKeyword.ts (2 errors) ====
+    declare function foo<A, B, C>(x: A, y: B, z: { z: C }): A & B & C;
+
+    // good
+    foo<infer A, {x: string}, A>({y: 12}, {x: "yes"}, {z: {y: 12}});
+
+    // error on 3rd arg
+    foo<infer A, {x: string}, A>({y: 12}, {x: "yes"}, {z: 12});
+                                                      ~~~~~~~
+!!! error TS2345: Argument of type '{ z: number; }' is not assignable to parameter of type '{ z: { y: number; }; }'.
+!!! error TS2345:   Types of property 'z' are incompatible.
+!!! error TS2345:     Type 'number' is not assignable to type '{ y: number; }'.
+
+    // error on first arg
+    foo<A, {x: string}, infer A>({y: 12}, {x: "yes"}, {z: 12});
+                                 ~~~~~~~
+!!! error TS2345: Argument of type '{ y: number; }' is not assignable to parameter of type 'number'.
+

tests/baselines/reference/inferTypeArgumentKeyword.js

@@ -0,0 +1,20 @@
+//// [inferTypeArgumentKeyword.ts]
+declare function foo<A, B, C>(x: A, y: B, z: { z: C }): A & B & C;
+
+// good
+foo<infer A, {x: string}, A>({y: 12}, {x: "yes"}, {z: {y: 12}});
+
+// error on 3rd arg
+foo<infer A, {x: string}, A>({y: 12}, {x: "yes"}, {z: 12});
+
+// error on first arg
+foo<A, {x: string}, infer A>({y: 12}, {x: "yes"}, {z: 12});
+
+
+//// [inferTypeArgumentKeyword.js]
+// good
+foo({ y: 12 }, { x: "yes" }, { z: { y: 12 } });
+// error on 3rd arg
+foo({ y: 12 }, { x: "yes" }, { z: 12 });
+// error on first arg
+foo({ y: 12 }, { x: "yes" }, { z: 12 });