Improve logic that chooses co- vs. contra-variant inferences (#52123)

Author: ahejlsberg

src/compiler/checker.ts

@@ -24504,11 +24504,15 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             if (signature) {
                 const inferredCovariantType = inference.candidates ? getCovariantInference(inference, signature) : undefined;
                 if (inference.contraCandidates) {
-                    // If we have both co- and contra-variant inferences, we prefer the contra-variant inference
-                    // unless the co-variant inference is a subtype of some contra-variant inference and not 'never'.
-                    inferredType = inferredCovariantType && !(inferredCovariantType.flags & TypeFlags.Never) &&
-                        some(inference.contraCandidates, t => isTypeSubtypeOf(inferredCovariantType, t)) ?
-                        inferredCovariantType : getContravariantInference(inference);
+                    // If we have both co- and contra-variant inferences, we use the co-variant inference if it is not 'never',
+                    // it is a subtype of some contra-variant inference, and no other type parameter is constrained to this type
+                    // parameter and has inferences that would conflict. Otherwise, we use the contra-variant inference.
+                    const useCovariantType = inferredCovariantType && !(inferredCovariantType.flags & TypeFlags.Never) &&
+                        some(inference.contraCandidates, t => isTypeSubtypeOf(inferredCovariantType, t)) &&
+                        every(context.inferences, other => other === inference ||
+                            getConstraintOfTypeParameter(other.typeParameter) !== inference.typeParameter ||
+                            every(other.candidates, t => isTypeSubtypeOf(t, inferredCovariantType)));
+                    inferredType = useCovariantType ? inferredCovariantType : getContravariantInference(inference);
                 }
                 else if (inferredCovariantType) {
                     inferredType = inferredCovariantType;

tests/baselines/reference/coAndContraVariantInferences2.js

@@ -0,0 +1,140 @@
+//// [coAndContraVariantInferences2.ts]
+interface A { a: string }
+interface B extends A { b: string }
+interface C extends A { c: string }
+
+declare function cast<T, U extends T>(x: T, test: (x: T) => x is U): U;
+
+declare function isC(x: A): x is C;
+
+function f1(a: A, b: B) {
+    const x1 = cast(a, isC);  // cast<A, C>
+    const x2 = cast(b, isC);  // cast<A, C>
+}
+
+declare function useA(a: A): void;
+
+declare function consume<T, U extends T>(t: T, u: U, f: (x: T) => void): void;
+
+function f2(b: B, c: C) {
+    consume(b, c, useA);  // consume<A, C>
+    consume(c, b, useA);  // consume<A, B>
+    consume(b, b, useA);  // consume<B, B>
+    consume(c, c, useA);  // consume<C, C>
+}
+
+// Repro from #52111
+
+enum SyntaxKind {
+    Block,
+    Identifier,
+    CaseClause,
+    FunctionExpression,
+    FunctionDeclaration,
+}
+
+interface Node { kind: SyntaxKind; }
+interface Expression extends Node { _expressionBrand: any; }
+interface Declaration extends Node { _declarationBrand: any; }
+interface Block extends Node { kind: SyntaxKind.Block; }
+interface Identifier extends Expression, Declaration { kind: SyntaxKind.Identifier; }
+interface CaseClause extends Node { kind: SyntaxKind.CaseClause; }
+interface FunctionDeclaration extends Declaration { kind: SyntaxKind.FunctionDeclaration; }
+
+type HasLocals = Block | FunctionDeclaration;
+declare function canHaveLocals(node: Node): node is HasLocals;
+
+declare function assertNode<T extends Node, U extends T>(node: T | undefined, test: (node: T) => node is U): asserts node is U;
+declare function assertNode(node: Node | undefined, test: ((node: Node) => boolean) | undefined): void;
+
+function foo(node: FunctionDeclaration | CaseClause) {
+    assertNode(node, canHaveLocals);  // assertNode<Node, HasLocals>
+    node;  // FunctionDeclaration
+}
+
+declare function isExpression(node: Node): node is Expression;
+
+declare function tryCast<TOut extends TIn, TIn = any>(value: TIn | undefined, test: (value: TIn) => value is TOut): TOut;
+
+function bar(node: Identifier | FunctionDeclaration) {
+    const a = tryCast(node, isExpression);  // tryCast<Expression, Node>
+}
+
+// Repro from #49924
+
+const enum SyntaxKind1 {
+    ClassExpression,
+    ClassStatement,
+}
+
+interface Node1 {
+    kind: SyntaxKind1;
+}
+
+interface Statement1 extends Node1 {
+    _statementBrand: any;
+}
+
+interface ClassExpression1 extends Node1 {
+    kind: SyntaxKind1.ClassExpression;
+}
+
+interface ClassStatement1 extends Statement1 {
+    kind: SyntaxKind1.ClassStatement;
+}
+
+type ClassLike1 = ClassExpression1 | ClassStatement1;
+
+declare function isClassLike(node: Node1): node is ClassLike1;
+
+declare const statement: Statement1 | undefined;
+
+const maybeClassStatement = tryCast(statement, isClassLike);  // ClassLike1
+
+// Repro from #49924
+
+interface TypeNode extends Node {
+    typeInfo: string;
+}
+
+interface NodeArray<T extends Node> extends Array<T> {
+    someProp: string;
+}
+
+declare function isNodeArray<T extends Node>(array: readonly T[]): array is NodeArray<T>;
+
+declare const types: readonly TypeNode[];
+
+const x = tryCast(types, isNodeArray);  // NodeAray<TypeNode>
+
+
+//// [coAndContraVariantInferences2.js]
+"use strict";
+function f1(a, b) {
+    var x1 = cast(a, isC); // cast<A, C>
+    var x2 = cast(b, isC); // cast<A, C>
+}
+function f2(b, c) {
+    consume(b, c, useA); // consume<A, C>
+    consume(c, b, useA); // consume<A, B>
+    consume(b, b, useA); // consume<B, B>
+    consume(c, c, useA); // consume<C, C>
+}
+// Repro from #52111
+var SyntaxKind;
+(function (SyntaxKind) {
+    SyntaxKind[SyntaxKind["Block"] = 0] = "Block";
+    SyntaxKind[SyntaxKind["Identifier"] = 1] = "Identifier";
+    SyntaxKind[SyntaxKind["CaseClause"] = 2] = "CaseClause";
+    SyntaxKind[SyntaxKind["FunctionExpression"] = 3] = "FunctionExpression";
+    SyntaxKind[SyntaxKind["FunctionDeclaration"] = 4] = "FunctionDeclaration";
+})(SyntaxKind || (SyntaxKind = {}));
+function foo(node) {
+    assertNode(node, canHaveLocals); // assertNode<Node, HasLocals>
+    node; // FunctionDeclaration
+}
+function bar(node) {
+    var a = tryCast(node, isExpression); // tryCast<Expression, Node>
+}
+var maybeClassStatement = tryCast(statement, isClassLike); // ClassLike1
+var x = tryCast(types, isNodeArray); // NodeAray<TypeNode>