The iteration type of overloaded iterator signatures derives from the intersection of their return types (microsoft#39722)

Author: andrewbranch

src/compiler/checker.ts

@@ -33624,7 +33624,7 @@ namespace ts {
                 return setCachedIterationTypes(type, resolver.iterableCacheKey, noIterationTypes);
             }
 
-            const iteratorType = getUnionType(map(signatures, getReturnTypeOfSignature), UnionReduction.Subtype);
+            const iteratorType = getIntersectionType(map(signatures, getReturnTypeOfSignature));
             const iterationTypes = getIterationTypesOfIterator(iteratorType, resolver, errorNode) ?? noIterationTypes;
             return setCachedIterationTypes(type, resolver.iterableCacheKey, iterationTypes);
         }
@@ -33830,7 +33830,7 @@ namespace ts {
 
             // Resolve the *yield* and *return* types from the return type of the method (i.e. `IteratorResult`)
             let yieldType: Type;
-            const methodReturnType = methodReturnTypes ? getUnionType(methodReturnTypes, UnionReduction.Subtype) : neverType;
+            const methodReturnType = methodReturnTypes ? getIntersectionType(methodReturnTypes) : neverType;
             const resolvedMethodReturnType = resolver.resolveIterationType(methodReturnType, errorNode) || anyType;
             const iterationTypes = getIterationTypesOfIteratorResult(resolvedMethodReturnType);
             if (iterationTypes === noIterationTypes) {

tests/baselines/reference/for-of58.js

@@ -0,0 +1,17 @@
+//// [for-of58.ts]
+type X = { x: 'x' };
+type Y = { y: 'y' };
+
+declare const arr: X[] & Y[];
+
+for (const item of arr) {
+    item.x;
+    item.y;
+}
+
+
+//// [for-of58.js]
+for (const item of arr) {
+    item.x;
+    item.y;
+}

tests/baselines/reference/for-of58.symbols

@@ -0,0 +1,29 @@
+=== tests/cases/conformance/es6/for-ofStatements/for-of58.ts ===
+type X = { x: 'x' };
+>X : Symbol(X, Decl(for-of58.ts, 0, 0))
+>x : Symbol(x, Decl(for-of58.ts, 0, 10))
+
+type Y = { y: 'y' };
+>Y : Symbol(Y, Decl(for-of58.ts, 0, 20))
+>y : Symbol(y, Decl(for-of58.ts, 1, 10))
+
+declare const arr: X[] & Y[];
+>arr : Symbol(arr, Decl(for-of58.ts, 3, 13))
+>X : Symbol(X, Decl(for-of58.ts, 0, 0))
+>Y : Symbol(Y, Decl(for-of58.ts, 0, 20))
+
+for (const item of arr) {
+>item : Symbol(item, Decl(for-of58.ts, 5, 10))
+>arr : Symbol(arr, Decl(for-of58.ts, 3, 13))
+
+    item.x;
+>item.x : Symbol(x, Decl(for-of58.ts, 0, 10))
+>item : Symbol(item, Decl(for-of58.ts, 5, 10))
+>x : Symbol(x, Decl(for-of58.ts, 0, 10))
+
+    item.y;
+>item.y : Symbol(y, Decl(for-of58.ts, 1, 10))
+>item : Symbol(item, Decl(for-of58.ts, 5, 10))
+>y : Symbol(y, Decl(for-of58.ts, 1, 10))
+}
+

tests/baselines/reference/for-of58.types

@@ -0,0 +1,27 @@
+=== tests/cases/conformance/es6/for-ofStatements/for-of58.ts ===
+type X = { x: 'x' };
+>X : X
+>x : "x"
+
+type Y = { y: 'y' };
+>Y : Y
+>y : "y"
+
+declare const arr: X[] & Y[];
+>arr : X[] & Y[]
+
+for (const item of arr) {
+>item : X & Y
+>arr : X[] & Y[]
+
+    item.x;
+>item.x : "x"
+>item : X & Y
+>x : "x"
+
+    item.y;
+>item.y : "y"
+>item : X & Y
+>y : "y"
+}
+

tests/cases/conformance/es6/for-ofStatements/for-of58.ts

@@ -0,0 +1,10 @@
+// @target: es6
+type X = { x: 'x' };
+type Y = { y: 'y' };
+
+declare const arr: X[] & Y[];
+
+for (const item of arr) {
+    item.x;
+    item.y;
+}