Limit type argument inference from binding patterns

src/compiler/checker.ts

@@ -26517,7 +26517,7 @@ namespace ts {
                 if (result) {
                     return result;
                 }
-                if (!(contextFlags! & ContextFlags.SkipBindingPatterns) && isBindingPattern(declaration.name)) { // This is less a contextual type and more an implied shape - in some cases, this may be undesirable
+                if (!(contextFlags! & ContextFlags.SkipBindingPatterns) && isBindingPattern(declaration.name) && declaration.name.elements.length > 0) {
                     return getTypeFromBindingPattern(declaration.name, /*includePatternInType*/ true, /*reportErrors*/ false);
                 }
             }
@@ -27051,22 +27051,20 @@ namespace ts {
                 const inferenceContext = getInferenceContext(node);
                 // If no inferences have been made, nothing is gained from instantiating as type parameters
                 // would just be replaced with their defaults similar to the apparent type.
-                if (inferenceContext && some(inferenceContext.inferences, hasInferenceCandidates)) {
+                if (inferenceContext && contextFlags! & ContextFlags.Signature && some(inferenceContext.inferences, hasInferenceCandidates)) {
                     // For contextual signatures we incorporate all inferences made so far, e.g. from return
                     // types as well as arguments to the left in a function call.
-                    if (contextFlags && contextFlags & ContextFlags.Signature) {
-                        return instantiateInstantiableTypes(contextualType, inferenceContext.nonFixingMapper);
-                    }
+                    return instantiateInstantiableTypes(contextualType, inferenceContext.nonFixingMapper);
+                }
+                if (inferenceContext?.returnMapper) {
                     // For other purposes (e.g. determining whether to produce literal types) we only
                     // incorporate inferences made from the return type in a function call. We remove
                     // the 'boolean' type from the contextual type such that contextually typed boolean
                     // literals actually end up widening to 'boolean' (see #48363).
-                    if (inferenceContext.returnMapper) {
-                        const type = instantiateInstantiableTypes(contextualType, inferenceContext.returnMapper);
-                        return type.flags & TypeFlags.Union && containsType((type as UnionType).types, regularFalseType) && containsType((type as UnionType).types, regularTrueType) ?
-                            filterType(type, t => t !== regularFalseType && t !== regularTrueType) :
-                            type;
-                    }
+                    const type = instantiateInstantiableTypes(contextualType, inferenceContext.returnMapper);
+                    return type.flags & TypeFlags.Union && containsType((type as UnionType).types, regularFalseType) && containsType((type as UnionType).types, regularTrueType) ?
+                        filterType(type, t => t !== regularFalseType && t !== regularTrueType) :
+                        type;
                 }
             }
             return contextualType;
@@ -29902,29 +29900,43 @@ namespace ts {
             // 'let f: (x: string) => number = wrap(s => s.length)', we infer from the declared type of 'f' to the
             // return type of 'wrap'.
             if (node.kind !== SyntaxKind.Decorator) {
-                const contextualType = getContextualType(node, every(signature.typeParameters, p => !!getDefaultFromTypeParameter(p)) ? ContextFlags.SkipBindingPatterns : ContextFlags.None);
+                const skipBindingPatterns = every(signature.typeParameters, p => !!getDefaultFromTypeParameter(p));
+                const contextualType = getContextualType(node, skipBindingPatterns ? ContextFlags.SkipBindingPatterns : ContextFlags.None);
                 if (contextualType) {
                     const inferenceTargetType = getReturnTypeOfSignature(signature);
                     if (couldContainTypeVariables(inferenceTargetType)) {
-                        // We clone the inference context to avoid disturbing a resolution in progress for an
-                        // outer call expression. Effectively we just want a snapshot of whatever has been
-                        // inferred for any outer call expression so far.
                         const outerContext = getInferenceContext(node);
-                        const outerMapper = getMapperFromContext(cloneInferenceContext(outerContext, InferenceFlags.NoDefault));
-                        const instantiatedType = instantiateType(contextualType, outerMapper);
-                        // If the contextual type is a generic function type with a single call signature, we
-                        // instantiate the type with its own type parameters and type arguments. This ensures that
-                        // the type parameters are not erased to type any during type inference such that they can
-                        // be inferred as actual types from the contextual type. For example:
-                        //   declare function arrayMap<T, U>(f: (x: T) => U): (a: T[]) => U[];
-                        //   const boxElements: <A>(a: A[]) => { value: A }[] = arrayMap(value => ({ value }));
-                        // Above, the type of the 'value' parameter is inferred to be 'A'.
-                        const contextualSignature = getSingleCallSignature(instantiatedType);
-                        const inferenceSourceType = contextualSignature && contextualSignature.typeParameters ?
-                            getOrCreateTypeFromSignature(getSignatureInstantiationWithoutFillingInTypeArguments(contextualSignature, contextualSignature.typeParameters)) :
-                            instantiatedType;
-                        // Inferences made from return types have lower priority than all other inferences.
-                        inferTypes(context.inferences, inferenceSourceType, inferenceTargetType, InferencePriority.ReturnType);
+                        const isFromBindingPattern = !skipBindingPatterns && getContextualType(node, ContextFlags.SkipBindingPatterns) !== contextualType;
+                        // A return type inference from a binding pattern can be used in instantiating the contextual
+                        // type of an argument later in inference, but cannot stand on its own as the final return type.
+                        // It is incorporated into `context.returnMapper` which is used in `instantiateContextualType`,
+                        // but doesn't need to go into `context.inferences`. This allows a an array binding pattern to
+                        // produce a tuple for `T` in
+                        //   declare function f<T>(cb: () => T): T;
+                        //   const [e1, e2, e3] = f(() => [1, "hi", true]);
+                        // but does not produce any inference for `T` in
+                        //   declare function f<T>(): T;
+                        //   const [e1, e2, e3] = f();
+                        if (!isFromBindingPattern) {
+                            // We clone the inference context to avoid disturbing a resolution in progress for an
+                            // outer call expression. Effectively we just want a snapshot of whatever has been
+                            // inferred for any outer call expression so far.
+                            const outerMapper = getMapperFromContext(cloneInferenceContext(outerContext, InferenceFlags.NoDefault));
+                            const instantiatedType = instantiateType(contextualType, outerMapper);
+                            // If the contextual type is a generic function type with a single call signature, we
+                            // instantiate the type with its own type parameters and type arguments. This ensures that
+                            // the type parameters are not erased to type any during type inference such that they can
+                            // be inferred as actual types from the contextual type. For example:
+                            //   declare function arrayMap<T, U>(f: (x: T) => U): (a: T[]) => U[];
+                            //   const boxElements: <A>(a: A[]) => { value: A }[] = arrayMap(value => ({ value }));
+                            // Above, the type of the 'value' parameter is inferred to be 'A'.
+                            const contextualSignature = getSingleCallSignature(instantiatedType);
+                            const inferenceSourceType = contextualSignature && contextualSignature.typeParameters ?
+                                getOrCreateTypeFromSignature(getSignatureInstantiationWithoutFillingInTypeArguments(contextualSignature, contextualSignature.typeParameters)) :
+                                instantiatedType;
+                            // Inferences made from return types have lower priority than all other inferences.
+                            inferTypes(context.inferences, inferenceSourceType, inferenceTargetType, InferencePriority.ReturnType);
+                        }
                         // Create a type mapper for instantiating generic contextual types using the inferences made
                         // from the return type. We need a separate inference pass here because (a) instantiation of
                         // the source type uses the outer context's return mapper (which excludes inferences made from

src/compiler/types.ts

@@ -5867,7 +5867,7 @@ namespace ts {
         ReturnType                   = 1 << 7,  // Inference made from return type of generic function
         LiteralKeyof                 = 1 << 8,  // Inference made from a string literal to a keyof T
         NoConstraints                = 1 << 9,  // Don't infer from constraints of instantiable types
-        AlwaysStrict                 = 1 << 10,  // Always use strict rules for contravariant inferences
+        AlwaysStrict                 = 1 << 10, // Always use strict rules for contravariant inferences
         MaxValue                     = 1 << 11, // Seed for inference priority tracking
 
         PriorityImpliesCombination = ReturnType | MappedTypeConstraint | LiteralKeyof,  // These priorities imply that the resulting type should be a combination of all candidates

tests/baselines/reference/bindingPatternCannotBeOnlyInferenceSource.errors.txt

@@ -0,0 +1,44 @@
+tests/cases/compiler/bindingPatternCannotBeOnlyInferenceSource.ts(2,7): error TS2571: Object is of type 'unknown'.
+tests/cases/compiler/bindingPatternCannotBeOnlyInferenceSource.ts(3,9): error TS2339: Property 'p1' does not exist on type 'unknown'.
+tests/cases/compiler/bindingPatternCannotBeOnlyInferenceSource.ts(4,7): error TS2461: Type 'unknown' is not an array type.
+tests/cases/compiler/bindingPatternCannotBeOnlyInferenceSource.ts(4,7): error TS2571: Object is of type 'unknown'.
+tests/cases/compiler/bindingPatternCannotBeOnlyInferenceSource.ts(5,7): error TS2461: Type 'unknown' is not an array type.
+
+
+==== tests/cases/compiler/bindingPatternCannotBeOnlyInferenceSource.ts (5 errors) ====
+    declare function f<T>(): T;
+    const {} = f();       // error (only in strictNullChecks)
+          ~~
+!!! error TS2571: Object is of type 'unknown'.
+    const { p1 } = f();   // error
+            ~~
+!!! error TS2339: Property 'p1' does not exist on type 'unknown'.
+    const [] = f();       // error
+          ~~
+!!! error TS2461: Type 'unknown' is not an array type.
+          ~~
+!!! error TS2571: Object is of type 'unknown'.
+    const [e1, e2] = f(); // error
+          ~~~~~~~~
+!!! error TS2461: Type 'unknown' is not an array type.
+
+    // Repro from #43605
+    type Dispatch<A = { type: any; [extraProps: string]: any }> = { <T extends A>(action: T): T };
+    type IFuncs = { readonly [key: string]: (...p: any) => void };
+    type IDestructuring<T extends IFuncs> = { readonly [key in keyof T]?: (...p: Parameters<T[key]>) => void };
+    type Destructuring<T extends IFuncs, U extends IDestructuring<T>> = (dispatch: Dispatch<any>, funcs: T) => U;
+    const funcs1 = {
+        funcA: (a: boolean): void => {},
+        funcB: (b: string, bb: string): void => {},
+        funcC: (c: number, cc: number, ccc: boolean): void => {},
+    };
+    type TFuncs1 = typeof funcs1;
+    declare function useReduxDispatch1<T extends IDestructuring<TFuncs1>>(destructuring: Destructuring<TFuncs1, T>): T;
+    const {} = useReduxDispatch1(
+        (d, f) => ({
+            funcA: (...p) => d(f.funcA(...p)), // p should be inferrable
+            funcB: (...p) => d(f.funcB(...p)),
+            funcC: (...p) => d(f.funcC(...p)),
+        })
+    );
+

tests/baselines/reference/bindingPatternCannotBeOnlyInferenceSource.js

@@ -0,0 +1,61 @@
+//// [bindingPatternCannotBeOnlyInferenceSource.ts]
+declare function f<T>(): T;
+const {} = f();       // error (only in strictNullChecks)
+const { p1 } = f();   // error
+const [] = f();       // error
+const [e1, e2] = f(); // error
+
+// Repro from #43605
+type Dispatch<A = { type: any; [extraProps: string]: any }> = { <T extends A>(action: T): T };
+type IFuncs = { readonly [key: string]: (...p: any) => void };
+type IDestructuring<T extends IFuncs> = { readonly [key in keyof T]?: (...p: Parameters<T[key]>) => void };
+type Destructuring<T extends IFuncs, U extends IDestructuring<T>> = (dispatch: Dispatch<any>, funcs: T) => U;
+const funcs1 = {
+    funcA: (a: boolean): void => {},
+    funcB: (b: string, bb: string): void => {},
+    funcC: (c: number, cc: number, ccc: boolean): void => {},
+};
+type TFuncs1 = typeof funcs1;
+declare function useReduxDispatch1<T extends IDestructuring<TFuncs1>>(destructuring: Destructuring<TFuncs1, T>): T;
+const {} = useReduxDispatch1(
+    (d, f) => ({
+        funcA: (...p) => d(f.funcA(...p)), // p should be inferrable
+        funcB: (...p) => d(f.funcB(...p)),
+        funcC: (...p) => d(f.funcC(...p)),
+    })
+);
+
+
+//// [bindingPatternCannotBeOnlyInferenceSource.js]
+var _a = f(); // error (only in strictNullChecks)
+var p1 = f().p1; // error
+var _b = f(); // error
+var _c = f(), e1 = _c[0], e2 = _c[1]; // error
+var funcs1 = {
+    funcA: function (a) { },
+    funcB: function (b, bb) { },
+    funcC: function (c, cc, ccc) { }
+};
+var _d = useReduxDispatch1(function (d, f) { return ({
+    funcA: function () {
+        var p = [];
+        for (var _i = 0; _i < arguments.length; _i++) {
+            p[_i] = arguments[_i];
+        }
+        return d(f.funcA.apply(f, p));
+    },
+    funcB: function () {
+        var p = [];
+        for (var _i = 0; _i < arguments.length; _i++) {
+            p[_i] = arguments[_i];
+        }
+        return d(f.funcB.apply(f, p));
+    },
+    funcC: function () {
+        var p = [];
+        for (var _i = 0; _i < arguments.length; _i++) {
+            p[_i] = arguments[_i];
+        }
+        return d(f.funcC.apply(f, p));
+    }
+}); });