A union including non-iterable types is not iterable

src/compiler/checker.ts

@@ -33444,8 +33444,9 @@ namespace ts {
                 if (iterationTypes === noIterationTypes) {
                     if (errorNode) {
                         reportTypeNotIterableError(errorNode, type, !!(use & IterationUse.AllowsAsyncIterablesFlag));
-                        errorNode = undefined;
                     }
+                    setCachedIterationTypes(type, cacheKey, noIterationTypes);
+                    return undefined;
                 }
                 else {
                     allIterationTypes = append(allIterationTypes, iterationTypes);
@@ -33787,6 +33788,28 @@ namespace ts {
                 return methodName === "next" ? anyIterationTypes : undefined;
             }
 
+            // If the method signature comes exclusively from the global iterator or generator type,
+            // create iteration types from its type arguments like `getIterationTypesOfIteratorFast`
+            // does (so as to remove `undefined` from the next and return types). We arrive here when
+            // a contextual type for a generator was not a direct reference to one of those global types,
+            // but looking up `methodType` referred to one of them (and nothing else). E.g., in
+            // `interface SpecialIterator extends Iterator<number> {}`, `SpecialIterator` is not a
+            // reference to `Iterator`, but its `next` member derives exclusively from `Iterator`.
+            if (methodType?.symbol && methodSignatures.length === 1) {
+                const globalGeneratorType = resolver.getGlobalGeneratorType(/*reportErrors*/ false);
+                const globalIteratorType = resolver.getGlobalIteratorType(/*reportErrors*/ false);
+                const isGeneratorMethod = globalGeneratorType.symbol?.members?.get(methodName as __String) === methodType.symbol;
+                const isIteratorMethod = !isGeneratorMethod && globalIteratorType.symbol?.members?.get(methodName as __String) === methodType.symbol;
+                if (isGeneratorMethod || isIteratorMethod) {
+                    const globalType = isGeneratorMethod ? globalGeneratorType : globalIteratorType;
+                    const { mapper } = methodType as AnonymousType;
+                    return createIterationTypes(
+                        getMappedType(globalType.typeParameters![0], mapper!),
+                        getMappedType(globalType.typeParameters![1], mapper!),
+                        methodName === "next" ? getMappedType(globalType.typeParameters![2], mapper!) : undefined);
+                }
+            }
+
             // Extract the first parameter and return type of each signature.
             let methodParameterTypes: Type[] | undefined;
             let methodReturnTypes: Type[] | undefined;

tests/baselines/reference/destructuringArrayBindingPatternAndAssignment4.errors.txt

@@ -0,0 +1,12 @@
+tests/cases/conformance/es6/destructuring/destructuringArrayBindingPatternAndAssignment4.ts(5,7): error TS2548: Type 'number[] | null' is not an array type or does not have a '[Symbol.iterator]()' method that returns an iterator.
+
+
+==== tests/cases/conformance/es6/destructuring/destructuringArrayBindingPatternAndAssignment4.ts (1 errors) ====
+    // #35497
+
+
+    declare const data: number[] | null;
+    const [value] = data; // Error
+          ~~~~~~~
+!!! error TS2548: Type 'number[] | null' is not an array type or does not have a '[Symbol.iterator]()' method that returns an iterator.
+

tests/baselines/reference/destructuringArrayBindingPatternAndAssignment4.js

@@ -0,0 +1,28 @@
+//// [destructuringArrayBindingPatternAndAssignment4.ts]
+// #35497
+
+
+declare const data: number[] | null;
+const [value] = data; // Error
+
+
+//// [destructuringArrayBindingPatternAndAssignment4.js]
+"use strict";
+// #35497
+var __read = (this && this.__read) || function (o, n) {
+    var m = typeof Symbol === "function" && o[Symbol.iterator];
+    if (!m) return o;
+    var i = m.call(o), r, ar = [], e;
+    try {
+        while ((n === void 0 || n-- > 0) && !(r = i.next()).done) ar.push(r.value);
+    }
+    catch (error) { e = { error: error }; }
+    finally {
+        try {
+            if (r && !r.done && (m = i["return"])) m.call(i);
+        }
+        finally { if (e) throw e.error; }
+    }
+    return ar;
+};
+var _a = __read(data, 1), value = _a[0]; // Error

tests/baselines/reference/destructuringArrayBindingPatternAndAssignment4.symbols

@@ -0,0 +1,11 @@
+=== tests/cases/conformance/es6/destructuring/destructuringArrayBindingPatternAndAssignment4.ts ===
+// #35497
+
+
+declare const data: number[] | null;
+>data : Symbol(data, Decl(destructuringArrayBindingPatternAndAssignment4.ts, 3, 13))
+
+const [value] = data; // Error
+>value : Symbol(value, Decl(destructuringArrayBindingPatternAndAssignment4.ts, 4, 7))
+>data : Symbol(data, Decl(destructuringArrayBindingPatternAndAssignment4.ts, 3, 13))
+

tests/baselines/reference/destructuringArrayBindingPatternAndAssignment4.types

@@ -0,0 +1,12 @@
+=== tests/cases/conformance/es6/destructuring/destructuringArrayBindingPatternAndAssignment4.ts ===
+// #35497
+
+
+declare const data: number[] | null;
+>data : number[] | null
+>null : null
+
+const [value] = data; // Error
+>value : any
+>data : number[] | null
+

tests/baselines/reference/generatorReturnTypeIndirectReferenceToGlobalType.js

@@ -0,0 +1,15 @@
+//// [generatorReturnTypeIndirectReferenceToGlobalType.ts]
+interface I1 extends Iterator<0, 1, 2> {}
+
+function* f1(): I1 {
+  const a = yield 0;
+  return 1;
+}
+
+
+//// [generatorReturnTypeIndirectReferenceToGlobalType.js]
+"use strict";
+function* f1() {
+    const a = yield 0;
+    return 1;
+}

tests/baselines/reference/generatorReturnTypeIndirectReferenceToGlobalType.symbols

@@ -0,0 +1,15 @@
+=== tests/cases/conformance/generators/generatorReturnTypeIndirectReferenceToGlobalType.ts ===
+interface I1 extends Iterator<0, 1, 2> {}
+>I1 : Symbol(I1, Decl(generatorReturnTypeIndirectReferenceToGlobalType.ts, 0, 0))
+>Iterator : Symbol(Iterator, Decl(lib.es2015.iterable.d.ts, --, --))
+
+function* f1(): I1 {
+>f1 : Symbol(f1, Decl(generatorReturnTypeIndirectReferenceToGlobalType.ts, 0, 41))
+>I1 : Symbol(I1, Decl(generatorReturnTypeIndirectReferenceToGlobalType.ts, 0, 0))
+
+  const a = yield 0;
+>a : Symbol(a, Decl(generatorReturnTypeIndirectReferenceToGlobalType.ts, 3, 7))
+
+  return 1;
+}
+

tests/baselines/reference/generatorReturnTypeIndirectReferenceToGlobalType.types

@@ -0,0 +1,15 @@
+=== tests/cases/conformance/generators/generatorReturnTypeIndirectReferenceToGlobalType.ts ===
+interface I1 extends Iterator<0, 1, 2> {}
+
+function* f1(): I1 {
+>f1 : () => I1
+
+  const a = yield 0;
+>a : 2
+>yield 0 : 2
+>0 : 0
+
+  return 1;
+>1 : 1
+}
+

tests/baselines/reference/generatorYieldContextualType.types

@@ -25,11 +25,11 @@ declare function f2<T, R, S>(gen: () => Generator<R, T, S> | AsyncGenerator<R, T
 f2<0, 0, 1>(async function* () {
 >f2<0, 0, 1>(async function* () {	const a = yield 0;	return 0;}) : void
 >f2 : <T, R, S>(gen: () => Generator<R, T, S> | AsyncGenerator<R, T, S>) => void
->async function* () {	const a = yield 0;	return 0;} : () => AsyncGenerator<0, 0, 1>
+>async function* () {	const a = yield 0;	return 0;} : () => AsyncGenerator<0, 0, 1 | undefined>
 
 	const a = yield 0;
->a : 1
->yield 0 : 1
+>a : 1 | undefined
+>yield 0 : 1 | undefined
 >0 : 0
 
 	return 0;

tests/baselines/reference/types.asyncGenerators.es2018.2.errors.txt

@@ -44,7 +44,7 @@ tests/cases/conformance/types/asyncGenerators/types.asyncGenerators.es2018.2.ts(
 tests/cases/conformance/types/asyncGenerators/types.asyncGenerators.es2018.2.ts(70,42): error TS2322: Type 'AsyncGenerator<number, any, undefined>' is not assignable to type 'Iterator<number, any, undefined>'.
   The types returned by 'next(...)' are incompatible between these types.
     Type 'Promise<IteratorResult<number, any>>' is not assignable to type 'IteratorResult<number, any>'.
-      Property 'value' is missing in type 'Promise<IteratorResult<number, any>>' but required in type 'IteratorYieldResult<number>'.
+      Type 'Promise<IteratorResult<number, any>>' is missing the following properties from type 'IteratorReturnResult<any>': done, value
 tests/cases/conformance/types/asyncGenerators/types.asyncGenerators.es2018.2.ts(74,12): error TS2504: Type '{}' must have a '[Symbol.asyncIterator]()' method that returns an async iterator.
 
 
@@ -191,8 +191,7 @@ tests/cases/conformance/types/asyncGenerators/types.asyncGenerators.es2018.2.ts(
 !!! error TS2322: Type 'AsyncGenerator<number, any, undefined>' is not assignable to type 'Iterator<number, any, undefined>'.
 !!! error TS2322:   The types returned by 'next(...)' are incompatible between these types.
 !!! error TS2322:     Type 'Promise<IteratorResult<number, any>>' is not assignable to type 'IteratorResult<number, any>'.
-!!! error TS2322:       Property 'value' is missing in type 'Promise<IteratorResult<number, any>>' but required in type 'IteratorYieldResult<number>'.
-!!! related TS2728 /.ts/lib.es2015.iterable.d.ts:33:5: 'value' is declared here.
+!!! error TS2322:       Type 'Promise<IteratorResult<number, any>>' is missing the following properties from type 'IteratorReturnResult<any>': done, value
         yield 1;
     }
     async function * yieldStar() {

tests/cases/conformance/es6/destructuring/destructuringArrayBindingPatternAndAssignment4.ts

@@ -0,0 +1,9 @@
+// #35497
+
+// @target: es5
+// @downlevelIteration: true
+// @lib: es6
+// @strict: true
+
+declare const data: number[] | null;
+const [value] = data; // Error

tests/cases/conformance/generators/generatorReturnTypeIndirectReferenceToGlobalType.ts

@@ -0,0 +1,9 @@
+// @strict: true
+// @target: esnext
+
+interface I1 extends Iterator<0, 1, 2> {}
+
+function* f1(): I1 {
+  const a = yield 0;
+  return 1;
+}