PoC for #36165 'inherit' keyword

tsc/internal/bundled/libs/lib.es5.d.ts

@@ -1577,6 +1577,46 @@ interface ArrayLike<T> {
     readonly [n: number]: T;
 }
 
+declare namespace inherit {
+type _Func = (...args: any[]) => any;
+
+// given number of normal parameters before the rest parameter, this gives the appropriate type for the rest parameter.
+// there might be a way to define this recursively but for now it is only well defined for 0 to 4 arguments.
+// otherwise this returns a string constant explaining limitation in implementation.
+type _GetRestParam<F extends _Func, P extends number> = P extends 0 ? F extends ((...args: infer A)=>any) ? A : never
+    : P extends 1 ? F extends ((x0: any, ...args: infer A) => any) ? A : never
+    : P extends 2 ? F extends ((x0: any, x1: any, ...args: infer A) => any) ? A : never
+    : P extends 3 ? F extends ((x0: any, x1: any, x2:any, ...args: infer A) => any) ? A : never
+    : P extends 4 ? F extends ((x0: any, x1: any, x2:any, x3:any, ...args: infer A) => any) ? A : never
+    : "REST PARAMETERS ONLY IMPLEMENTED FOR UP TO 4 PRIOR ARGUMENTS"
+
+type _KeysOfMethods<BaseType> = { [K in keyof BaseType]: BaseType[K] extends _Func ? K : never }[keyof BaseType];
+
+}
+/**
+ * demonstration of how the `inherit` keyword would work as working with explicit type arguments.
+ * @param BaseType - intersection of extended base class and all implemented interfaces
+ * @param Field - the field name or method name being used. if Pos is not "Field" this must point to a method.
+ * @param Pos - one of:
+ *              "Field" for the type of the defined field (default if not given)
+ *              "Return" for the return type of the method
+ *              "Param" for a parameter of the method (Idx specifies position)
+ *              "Rest" for a rest parameter (...args).
+ * @param Idx - when Pos is "Param" this is the position of the parameter,
+ *              when Pos is "Rest" this is the number of parameters before the rest parameter.
+ */
+type inherit<BaseType, Field extends (Pos extends ("Param" | "Return" | "Rest") ? inherit._KeysOfMethods<BaseType> : keyof BaseType),
+    Pos extends "Field" | "Param" | "Return" | "Rest" = "Field",
+    Idx extends (Pos extends "Param" | "Rest" ? number : never) = never
+    > = Pos extends "Field" ? BaseType[Field]
+    : BaseType[Field] extends inherit._Func
+    ? Pos extends "Return" ? ReturnType<BaseType[Field]>
+    : Pos extends "Param" ? Parameters<BaseType[Field]>[Idx]
+    : Pos extends "Rest" ? inherit._GetRestParam<BaseType[Field], Idx>
+    : never // case where Pos is not one of the 4 choices
+    : never; // case where BaseType[Field] is not a function for one of the choices that require a function
+
+
 /**
  * Make all properties in T optional
  */

tsc/internal/checker/checker.go

@@ -24138,6 +24138,139 @@ func (c *Checker) getTupleElementType(t *Type, index int) *Type {
 	return nil
 }
 
+// getInheritTypeArguments implements the `inherit` proof-of-concept. Given a `TypeReferenceNode`
+// that refers to the global `inherit` type alias and was written with no explicit type arguments,
+// it synthesizes the type arguments the alias expects based on where `inherit` appears:
+//
+//	inherit<BaseType, Field, Pos, Idx>
+//
+// where:
+//   - BaseType is the intersection of the enclosing class's extends type and all implemented interfaces,
+//   - Field is a string-literal type of the member name,
+//   - Pos is one of "Field" | "Param" | "Return" | "Rest",
+//   - Idx is a number-literal type (parameter index for "Param", count of leading params for "Rest").
+//
+// It returns nil when `inherit` is used in a position that is not supported (e.g. outside a class, or
+// not in a field/parameter/return type), so that the caller reports the normal diagnostic.
+func (c *Checker) getInheritTypeArguments(node *ast.Node) []*Type {
+	// Determine the enclosing class and its base-type intersection.
+	classNode := ast.GetContainingClass(node)
+	if classNode == nil {
+		return nil
+	}
+	baseType := c.getInheritBaseType(classNode)
+	if baseType == nil {
+		return nil
+	}
+
+	// Classify the position of `inherit`. `inherit` may be the direct type of a declaration
+	// (`foo: inherit`) or nested inside a composed type (`foo: Readonly<inherit>`,
+	// `foo: inherit | "x"`). Walk up through any enclosing type nodes so that composition works;
+	// the walk stops at the first non-type ancestor, which should be the owning declaration.
+	cur := node
+	for cur.Parent != nil && ast.IsTypeNode(cur.Parent) {
+		cur = cur.Parent
+	}
+	parent := cur.Parent
+	if parent == nil {
+		return nil
+	}
+	stringArg := func(s string) *Type { return c.getStringLiteralType(s) }
+
+	switch {
+	case ast.IsPropertyDeclaration(parent) && parent.Type() == cur:
+		// `public foo: inherit` -> inherit<Base, "foo", "Field">
+		name := c.getInheritMemberName(parent)
+		if name == "" {
+			return nil
+		}
+		return []*Type{baseType, stringArg(name), stringArg("Field")}
+
+	case ast.IsParameterDeclaration(parent) && parent.Type() == cur:
+		// Parameter type: either a normal parameter or a rest parameter of a method.
+		paramNode := parent
+		method := paramNode.Parent
+		if method == nil || !ast.IsFunctionLikeDeclaration(method) {
+			return nil
+		}
+		name := c.getInheritMemberName(method)
+		if name == "" {
+			return nil
+		}
+		// A setter's parameter inherits the field type of the member, not a method parameter type.
+		// `set foo(v: inherit)` -> inherit<Base, "foo", "Field">
+		if ast.IsSetAccessorDeclaration(method) {
+			return []*Type{baseType, stringArg(name), stringArg("Field")}
+		}
+		params := method.Parameters()
+		index := -1
+		for i, p := range params {
+			if p == paramNode {
+				index = i
+				break
+			}
+		}
+		if index < 0 {
+			return nil
+		}
+		idxArg := c.getNumberLiteralType(jsnum.Number(index))
+		if paramNode.AsParameterDeclaration().DotDotDotToken != nil {
+			// `...rest: inherit` -> inherit<Base, name, "Rest", index>
+			return []*Type{baseType, stringArg(name), stringArg("Rest"), idxArg}
+		}
+		// `param: inherit` -> inherit<Base, name, "Param", index>
+		return []*Type{baseType, stringArg(name), stringArg("Param"), idxArg}
+
+	case ast.IsFunctionLikeDeclaration(parent) && parent.Type() == cur:
+		name := c.getInheritMemberName(parent)
+		if name == "" {
+			return nil
+		}
+		// A getter's return type inherits the field type of the member, not a method return type.
+		// `get foo(): inherit` -> inherit<Base, "foo", "Field">
+		if ast.IsGetAccessorDeclaration(parent) {
+			return []*Type{baseType, stringArg(name), stringArg("Field")}
+		}
+		// Method return type: `foo(): inherit` -> inherit<Base, name, "Return">
+		return []*Type{baseType, stringArg(name), stringArg("Return")}
+	}
+
+	return nil
+}
+
+// getInheritMemberName returns the text of a declaration's name, or "" if it is not a simple
+// identifier/string name (e.g. a computed property name, which `inherit` does not support).
+func (c *Checker) getInheritMemberName(decl *ast.Node) string {
+	name := decl.Name()
+	if name == nil {
+		return ""
+	}
+	switch name.Kind {
+	case ast.KindIdentifier, ast.KindStringLiteral, ast.KindNumericLiteral, ast.KindPrivateIdentifier:
+		return name.Text()
+	}
+	return ""
+}
+
+// getInheritBaseType computes the BaseType argument for `inherit`: the intersection of the class's
+// extends type (if any) and all implemented interface types. Returns nil if the class neither
+// extends nor implements anything.
+func (c *Checker) getInheritBaseType(classNode *ast.Node) *Type {
+	classType := c.getDeclaredTypeOfSymbol(c.getSymbolOfDeclaration(classNode))
+	var baseTypes []*Type
+	baseTypes = append(baseTypes, c.getBaseTypes(classType)...)
+	for _, typeRefNode := range ast.GetImplementsHeritageClauseElements(classNode) {
+		t := c.getReducedType(c.getTypeFromTypeNode(typeRefNode))
+		if !c.isErrorType(t) {
+			baseTypes = append(baseTypes, t)
+		}
+	}
+	if len(baseTypes) == 0 {
+		return nil
+	}
+	return c.getIntersectionType(baseTypes)
+}
+
 /**
  * Get type from reference to type alias. When a type alias is generic, the declared type of the type alias may include
  * references to the type parameters of the alias. We replace those with the actual type arguments by instantiating the
@@ -24160,6 +24293,16 @@ func (c *Checker) getTypeFromTypeAliasReference(node *ast.Node, symbol *ast.Symb
 	typeParameters := c.typeAliasLinks.Get(symbol).typeParameters
 	if len(typeParameters) != 0 {
 		numTypeArguments := len(typeArguments)
+		// `inherit` proof-of-concept: when the global `inherit` type alias is used with no explicit
+		// type arguments, synthesize them from the syntactic position in the enclosing class (base
+		// type(s), member name, position kind, and index) and instantiate the alias with those.
+		// If the position is not a valid use of `inherit`, fall through so the normal
+		// "requires N type arguments" diagnostic is reported.
+		if numTypeArguments == 0 && symbol.Name() == "inherit" {
+			if inheritArgs := c.getInheritTypeArguments(node); inheritArgs != nil {
+				return c.getTypeAliasInstantiation(symbol, inheritArgs, nil)
+			}
+		}
 		minTypeArgumentCount := c.getMinTypeArgumentCount(typeParameters)
 		if numTypeArguments < minTypeArgumentCount || numTypeArguments > len(typeParameters) {
 			message := core.IfElse(minTypeArgumentCount == len(typeParameters),

tsc/internal/fourslash/tests/util/util.go

@@ -937,6 +937,11 @@ var CompletionGlobalTypeDecls = []fourslash.CompletionsExpectedItem{
 		Kind:     new(lsproto.CompletionItemKindClass),
 		SortText: new(string(ls.SortTextGlobalsOrKeywords)),
 	},
+	&lsproto.CompletionItem{
+		Label:    "inherit",
+		Kind:     new(lsproto.CompletionItemKindClass),
+		SortText: new(string(ls.SortTextGlobalsOrKeywords)),
+	},
 	&lsproto.CompletionItem{
 		Label:    "ArrayLike",
 		Kind:     new(lsproto.CompletionItemKindInterface),

tsc/testdata/baselines/reference/compiler/inheritTypeAliasPoC.errors.txt

@@ -0,0 +1,178 @@
+inheritTypeAliasPoC.ts(76,5): error TS2322: Type '"square"' is not assignable to type '"circle"'.
+inheritTypeAliasPoC.ts(119,9): error TS2322: Type '"triangle"' is not assignable to type '"circle" | "square"'.
+inheritTypeAliasPoC.ts(128,5): error TS2322: Type '"triangle"' is not assignable to type '"circle" | "square"'.
+inheritTypeAliasPoC.ts(134,44): error TS2322: Type 'string' is not assignable to type 'boolean'.
+inheritTypeAliasPoC.ts(140,15): error TS2322: Type 'number' is not assignable to type 'string'.
+inheritTypeAliasPoC.ts(150,8): error TS2707: Generic type 'inherit' requires between 2 and 4 type arguments.
+inheritTypeAliasPoC.ts(153,21): error TS2707: Generic type 'inherit' requires between 2 and 4 type arguments.
+
+
+==== inheritTypeAliasPoC.ts (7 errors) ====
+    // Proof-of-concept test suite for the `inherit` type alias (issue #36165).
+    //
+    // `inherit` is a global type alias (defined in lib.es5.d.ts) that, when used with no explicit
+    // type arguments, is resolved contextually by the checker to the matching member type of the
+    // enclosing class's base class and/or implemented interfaces:
+    //
+    //   - Instance field          -> Base[Field]
+    //   - Getter return / setter param -> Base[Field] (same as a field)
+    //   - Method parameter         -> Parameters<Base[Field]>[Index]
+    //   - Method rest parameter    -> trailing tuple after `Index` leading params
+    //   - Method return value      -> ReturnType<Base[Field]>
+    //
+    // where Base is the intersection of the extends type and all implemented interfaces.
+    //
+    // This file is both a demonstration and an executable spec. Cases marked "SHOULD RESOLVE" are the
+    // intended behavior; cases marked "SHOULD ERROR" document intentional constraints. Where the
+    // current PoC does not yet match the intended behavior (notably `inherit` nested inside a composed
+    // type such as `Readonly<inherit>` or `inherit | "x"`), the baseline will reveal the gap.
+
+    interface Base {
+        field: "square" | "circle";
+        foo(a: number, b: string): boolean;
+        bar(a: number, b: string, c?: any): boolean;
+    }
+
+    // ---------------------------------------------------------------------------
+    // 1a. implements an interface.
+    // ---------------------------------------------------------------------------
+    class AllPositions implements Base {
+        field: inherit = "square";                               // SHOULD RESOLVE: "square" | "circle"
+        foo(a: inherit, b: inherit): inherit {                   // SHOULD RESOLVE: a:number, b:string, ret:boolean
+            return true;
+        }
+        bar(a: inherit, ...rest: inherit): inherit {             // SHOULD RESOLVE: a:number, rest:[string, any?], ret:boolean
+            return true;
+        }
+    }
+
+
+    // ---------------------------------------------------------------------------
+    // 1b. extends base class.
+    // ---------------------------------------------------------------------------
+    class Geometry {
+        model: "square" | "circle" = "square";
+        render(scale: number): void {}
+    }
+    class Canvas extends Geometry {
+        model: inherit = "square";                               // SHOULD RESOLVE: "square" | "circle"
+        render(scale: inherit): inherit {}                       // SHOULD RESOLVE: scale:number, ret:void
+    }
+
+    // ---------------------------------------------------------------------------
+    // 2. Multiple bases: extends + implements, intersected as (Base & A & B).
+    // ---------------------------------------------------------------------------
+    interface A { a: number; shared: string; }
+    interface B { b: "x" | "y"; shared: string; }
+    class BaseC { c: boolean = true; }
+    class Multi extends BaseC implements A, B {
+        a: inherit = 1;                                          // SHOULD RESOLVE: number
+        b: inherit = "x";                                        // SHOULD RESOLVE: "x" | "y"
+        c: inherit = false;                                      // SHOULD RESOLVE: boolean (from extends)
+        shared: inherit = "s";                                   // SHOULD RESOLVE: string (A & B agree)
+    }
+
+    // ---------------------------------------------------------------------------
+    // 3. Conflicting member types across interfaces narrow to their intersection.
+    //     Foo.shape = "square" | "circle", Bar.shape = "circle" | "triangle"
+    //     => (Foo & Bar)["shape"] = "circle"
+    // ---------------------------------------------------------------------------
+    interface Foo { shape: "square" | "circle"; }
+    interface Bar { shape: "circle" | "triangle"; }
+    class IntersectNarrow implements Foo, Bar {
+        shape: inherit = "circle";                               // SHOULD RESOLVE: "circle" (only common member)
+    }
+    class IntersectNarrowBad implements Foo, Bar {
+        shape: inherit = "square";                               // SHOULD ERROR: "square" not in the intersection "circle"
+        ~~~~~
+!!! error TS2322: Type '"square"' is not assignable to type '"circle"'.
+    }
+
+    // ---------------------------------------------------------------------------
+    // 4. Composition: inherit nested inside other type constructors.
+    // ---------------------------------------------------------------------------
+    interface Robot {
+        sensors: {sight?: boolean, hearing?: boolean, touch?: boolean};
+        doAction(action: "walk" | "turn"): void;
+    }
+    class FlyingRobot implements Robot {
+        sensors: Required<inherit> = {sight: true, hearing: false, touch: false};  // SHOULD RESOLVE: {sight: boolean, hearing: boolean, touch: boolean};
+        doAction(action: inherit | "fly"): inherit {             // SHOULD RESOLVE: action:"walk"|"turn"|"fly"
+        }
+    }
+
+    // ---------------------------------------------------------------------------
+    // 5. Arrow-function field: field-level inherit, params inferred by assignment.
+    // ---------------------------------------------------------------------------
+    class ArrowField implements Base {
+        field: inherit = "square";
+        foo: inherit = (a, b) => true;                           // SHOULD RESOLVE: foo:(a:number,b:string)=>boolean
+        bar: inherit = (a, b, c) => true;
+    }
+
+    // ---------------------------------------------------------------------------
+    // 6. Accessors: a getter's return and a setter's parameter inherit the FIELD type.
+    //     (Issue lists "Getter return value" and "Setter parameter" as supported positions.)
+    // ---------------------------------------------------------------------------
+    abstract class ShapeBase {
+        abstract kind: "square" | "circle";
+    }
+    class ShapeImpl extends ShapeBase {
+        private _kind: "square" | "circle" = "square";
+        get kind(): inherit {                                    // SHOULD RESOLVE: "square" | "circle" (field type)
+            return this._kind;
+        }
+        set kind(v: inherit) {                                   // SHOULD RESOLVE: "square" | "circle" (field type)
+            this._kind = v;
+        }
+    }
+    class ShapeImplBadGetter extends ShapeBase {
+        get kind(): inherit {
+            return "triangle";                                   // SHOULD ERROR: "triangle" not in "square" | "circle"
+            ~~~~~~
+!!! error TS2322: Type '"triangle"' is not assignable to type '"circle" | "square"'.
+        }
+        set kind(v: inherit) {}
+    }
+
+    // ---------------------------------------------------------------------------
+    // 7. Negative / assignment checks: inherit must produce the NARROW type, not `any`.
+    // ---------------------------------------------------------------------------
+    class WrongField implements Base {
+        field: inherit = "triangle";                             // SHOULD ERROR: not "square" | "circle"
+        ~~~~~
+!!! error TS2322: Type '"triangle"' is not assignable to type '"circle" | "square"'.
+        foo(a: inherit, b: inherit): inherit { return true; }
+        bar(a: inherit, ...rest: inherit): inherit { return true; }
+    }
+    class WrongReturn implements Base {
+        field: inherit = "square";
+        foo(a: inherit, b: inherit): inherit { return "nope"; }  // SHOULD ERROR: string not assignable to boolean
+                                               ~~~~~~
+!!! error TS2322: Type 'string' is not assignable to type 'boolean'.
+        bar(a: inherit, ...rest: inherit): inherit { return true; }
+    }
+    class WrongParamUse implements Base {
+        field: inherit = "square";
+        foo(a: inherit, b: inherit): inherit {
+            const s: string = a;                                 // SHOULD ERROR: number not assignable to string
+                  ~
+!!! error TS2322: Type 'number' is not assignable to type 'string'.
+            return true;
+        }
+        bar(a: inherit, ...rest: inherit): inherit { return true; }
+    }
+
+    // ---------------------------------------------------------------------------
+    // 8. Invalid positions / constraints.
+    // ---------------------------------------------------------------------------
+    class NoBase {
+        x: inherit = 1;                                          // SHOULD ERROR: class has no base/interfaces
+           ~~~~~~~
+!!! error TS2707: Generic type 'inherit' requires between 2 and 4 type arguments.
+    }
+
+    type OutsideClass = inherit;                                 // SHOULD ERROR: inherit used outside a class
+                        ~~~~~~~
+!!! error TS2707: Generic type 'inherit' requires between 2 and 4 type arguments.
+