Use undefinedWideningType

Author: Andy Hanson

src/compiler/checker.ts

@@ -18389,7 +18389,7 @@ namespace ts {
 
         function getYieldedTypeOfYieldExpression(node: YieldExpression, isAsync: boolean, checkMode?: CheckMode): Type {
             const errorNode = node.expression || node;
-            const expressionType = node.expression ? checkExpressionCached(node.expression, checkMode) : undefinedType;
+            const expressionType = node.expression ? checkExpressionCached(node.expression, checkMode) : undefinedWideningType;
             // A yield* expression effectively yields everything that its operand yields
             const yieldedType = node.asteriskToken ? checkIteratedTypeOrElementType(expressionType, errorNode, /*allowStringInput*/ false, isAsync) : expressionType;
             return !isAsync ? yieldedType : getAwaitedType(yieldedType, errorNode, node.asteriskToken

tests/baselines/reference/generatorES6InAMDModule.types

@@ -1,6 +1,6 @@
 === tests/cases/compiler/generatorES6InAMDModule.ts ===
 export function* foo() {
->foo : () => IterableIterator<undefined>
+>foo : () => IterableIterator<any>
 
     yield
 >yield : any

tests/baselines/reference/generatorES6_1.types

@@ -1,6 +1,6 @@
 === tests/cases/compiler/generatorES6_1.ts ===
 function* foo() {
->foo : () => IterableIterator<undefined>
+>foo : () => IterableIterator<any>
 
     yield
 >yield : any

tests/baselines/reference/generatorTypeCheck48.errors.txt

@@ -0,0 +1,17 @@
+tests/cases/conformance/es6/yieldExpressions/generatorTypeCheck48.ts(1,11): error TS7010: 'g', which lacks return-type annotation, implicitly has an 'any' return type.
+tests/cases/conformance/es6/yieldExpressions/generatorTypeCheck48.ts(5,11): error TS7010: 'h', which lacks return-type annotation, implicitly has an 'any' return type.
+
+
+==== tests/cases/conformance/es6/yieldExpressions/generatorTypeCheck48.ts (2 errors) ====
+    function* g() {
+              ~
+!!! error TS7010: 'g', which lacks return-type annotation, implicitly has an 'any' return type.
+        yield;
+    }
+    
+    function* h() {
+              ~
+!!! error TS7010: 'h', which lacks return-type annotation, implicitly has an 'any' return type.
+        yield undefined;
+    }
+    

tests/baselines/reference/generatorTypeCheck48.js

@@ -1,9 +1,17 @@
 //// [generatorTypeCheck48.ts]
 function* g() {
     yield;
-}
+}
+
+function* h() {
+    yield undefined;
+}
+
 
 //// [generatorTypeCheck48.js]
 function* g() {
     yield;
 }
+function* h() {
+    yield undefined;
+}

tests/baselines/reference/generatorTypeCheck48.symbols

@@ -4,3 +4,11 @@ function* g() {
 
     yield;
 }
+
+function* h() {
+>h : Symbol(h, Decl(generatorTypeCheck48.ts, 2, 1))
+
+    yield undefined;
+>undefined : Symbol(undefined)
+}
+

tests/baselines/reference/generatorTypeCheck48.types

@@ -1,7 +1,16 @@
 === tests/cases/conformance/es6/yieldExpressions/generatorTypeCheck48.ts ===
 function* g() {
->g : () => IterableIterator<undefined>
+>g : () => IterableIterator<any>
 
     yield;
 >yield : any
 }
+
+function* h() {
+>h : () => IterableIterator<any>
+
+    yield undefined;
+>yield undefined : any
+>undefined : undefined
+}
+

tests/cases/conformance/es6/yieldExpressions/generatorTypeCheck48.ts

@@ -3,4 +3,8 @@
 
 function* g() {
     yield;
-}
+}
+
+function* h() {
+    yield undefined;
+}