microsoft · rbuckton · Dec 18, 2020 · Oct 31, 2020
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
@@ -30635,8 +30635,17 @@ namespace ts {
                return getIterationTypeOfGeneratorFunctionReturnType(IterationTypeKind.Next, returnType, isAsync)
                    || anyType;
            }
-
-            return getContextualIterationType(IterationTypeKind.Next, func) || anyType;
+            let type = getContextualIterationType(IterationTypeKind.Next, func);
+            if (!type) {
+                type = anyType;
+                if (produceDiagnostics && noImplicitAny && !expressionResultIsUnused(node)) {
+                    const contextualType = getContextualType(node);
+                    if (!contextualType || isTypeAny(contextualType)) {
+                        error(node, Diagnostics.yield_expression_implicitly_results_in_an_any_type_because_its_containing_generator_lacks_a_return_type_annotation);
+                    }
+                }
+            }
+            return type;
        }

        function checkConditionalExpression(node: ConditionalExpression, checkMode?: CheckMode): Type {

diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
@@ -4948,6 +4948,10 @@
        "category": "Error",
        "code": 7056
    },
+    "'yield' expression implicitly results in an 'any' type because its containing generator lacks a return-type annotation.": {
+        "category": "Error",
+        "code": 7057
+    },

    "You cannot rename this element.": {
        "category": "Error",

diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
@@ -6973,4 +6973,42 @@ namespace ts {
            return bindParentToChildIgnoringJSDoc(child, parent) || bindJSDoc(child);
        }
    }
+
+    /**
+     * Indicates whether the result of an `Expression` will be unused.
+     *
+     * NOTE: This requires a node with a valid `parent` pointer.
+     */
+    export function expressionResultIsUnused(node: Expression): boolean {
+        Debug.assertIsDefined(node.parent);
+        while (true) {
+            const parent: Node = node.parent;
+            // walk up parenthesized expressions, but keep a pointer to the top-most parenthesized expression
+            if (isParenthesizedExpression(parent)) {
+                node = parent;
+                continue;
+            }
+            // result is unused in an expression statement, `void` expression, or the initializer or incrementer of a `for` loop
+            if (isExpressionStatement(parent) ||
+                isVoidExpression(parent) ||
+                isForStatement(parent) && (parent.initializer === node || parent.incrementor === node)) {
+                return true;
+            }
+            if (isCommaListExpression(parent)) {
+                // left side of comma is always unused
+                if (node !== last(parent.elements)) return true;
+                // right side of comma is unused if parent is unused
+                node = parent;
+                continue;
+            }
+            if (isBinaryExpression(parent) && parent.operatorToken.kind === SyntaxKind.CommaToken) {
+                // left side of comma is always unused
+                if (node === parent.left) return true;
+                // right side of comma is unused if parent is unused
+                node = parent;
+                continue;
+            }
+            return false;
+        }
+    }
 }
diff --git a/tests/baselines/reference/generatorImplicitAny.errors.txt b/tests/baselines/reference/generatorImplicitAny.errors.txt
@@ -0,0 +1,40 @@
+tests/cases/conformance/generators/generatorImplicitAny.ts(8,19): error TS7057: 'yield' expression implicitly results in an 'any' type because its containing generator lacks a return-type annotation.
+tests/cases/conformance/generators/generatorImplicitAny.ts(26,7): error TS7057: 'yield' expression implicitly results in an 'any' type because its containing generator lacks a return-type annotation.
+
+
+==== tests/cases/conformance/generators/generatorImplicitAny.ts (2 errors) ====
+    function* g() {}
+
+    // https://github.com/microsoft/TypeScript/issues/35105
+    declare function noop(): void;
+    declare function f<T>(value: T): void;
+
+    function* g2() {
+        const value = yield; // error: implicit any
+                      ~~~~~
+!!! error TS7057: 'yield' expression implicitly results in an 'any' type because its containing generator lacks a return-type annotation.
+    }
+
+    function* g3() {
+        const value: string = yield; // ok, contextually typed by `value`.
+    }
+
+    function* g4() {
+        yield; // ok, result is unused
+        yield, noop(); // ok, result is unused
+        noop(), yield, noop(); // ok, result is unused
+        (yield); // ok, result is unused
+        (yield, noop()), noop(); // ok, result is unused
+        for(yield; false; yield); // ok, results are unused
+        void (yield); // ok, results are unused
+    }
+
+    function* g5() {
+        f(yield); // error: implicit any
+          ~~~~~
+!!! error TS7057: 'yield' expression implicitly results in an 'any' type because its containing generator lacks a return-type annotation.
+    }
+
+    function* g6() {
+        f<string>(yield); // ok, contextually typed by f<string>
+    }
diff --git a/tests/baselines/reference/generatorImplicitAny.js b/tests/baselines/reference/generatorImplicitAny.js
@@ -1,5 +1,57 @@
 //// [generatorImplicitAny.ts]
-function* g() {}
+function* g() {}
+
+// https://github.com/microsoft/TypeScript/issues/35105
+declare function noop(): void;
+declare function f<T>(value: T): void;
+
+function* g2() {
+    const value = yield; // error: implicit any
+}
+
+function* g3() {
+    const value: string = yield; // ok, contextually typed by `value`.
+}
+
+function* g4() {
+    yield; // ok, result is unused
+    yield, noop(); // ok, result is unused
+    noop(), yield, noop(); // ok, result is unused
+    (yield); // ok, result is unused
+    (yield, noop()), noop(); // ok, result is unused
+    for(yield; false; yield); // ok, results are unused
+    void (yield); // ok, results are unused
+}
+
+function* g5() {
+    f(yield); // error: implicit any
+}
+
+function* g6() {
+    f<string>(yield); // ok, contextually typed by f<string>
+}

 //// [generatorImplicitAny.js]
 function* g() { }
+function* g2() {
+    const value = yield; // error: implicit any
+}
+function* g3() {
+    const value = yield; // ok, contextually typed by `value`.
+}
+function* g4() {
+    yield; // ok, result is unused
+    yield, noop(); // ok, result is unused
+    noop(), yield, noop(); // ok, result is unused
+    (yield); // ok, result is unused
+    (yield, noop()), noop(); // ok, result is unused
+    for (yield; false; yield)
+        ; // ok, results are unused
+    void (yield); // ok, results are unused
+}
+function* g5() {
+    f(yield); // error: implicit any
+}
+function* g6() {
+    f(yield); // ok, contextually typed by f<string>
+}
diff --git a/tests/baselines/reference/generatorImplicitAny.symbols b/tests/baselines/reference/generatorImplicitAny.symbols
@@ -2,3 +2,60 @@
 function* g() {}
 >g : Symbol(g, Decl(generatorImplicitAny.ts, 0, 0))

+// https://github.com/microsoft/TypeScript/issues/35105
+declare function noop(): void;
+>noop : Symbol(noop, Decl(generatorImplicitAny.ts, 0, 16))
+
+declare function f<T>(value: T): void;
+>f : Symbol(f, Decl(generatorImplicitAny.ts, 3, 30))
+>T : Symbol(T, Decl(generatorImplicitAny.ts, 4, 19))
+>value : Symbol(value, Decl(generatorImplicitAny.ts, 4, 22))
+>T : Symbol(T, Decl(generatorImplicitAny.ts, 4, 19))
+
+function* g2() {
+>g2 : Symbol(g2, Decl(generatorImplicitAny.ts, 4, 38))
+
+    const value = yield; // error: implicit any
+>value : Symbol(value, Decl(generatorImplicitAny.ts, 7, 9))
+}
+
+function* g3() {
+>g3 : Symbol(g3, Decl(generatorImplicitAny.ts, 8, 1))
+
+    const value: string = yield; // ok, contextually typed by `value`.
+>value : Symbol(value, Decl(generatorImplicitAny.ts, 11, 9))
+}
+
+function* g4() {
+>g4 : Symbol(g4, Decl(generatorImplicitAny.ts, 12, 1))
+
+    yield; // ok, result is unused
+    yield, noop(); // ok, result is unused
+>noop : Symbol(noop, Decl(generatorImplicitAny.ts, 0, 16))
+
+    noop(), yield, noop(); // ok, result is unused
+>noop : Symbol(noop, Decl(generatorImplicitAny.ts, 0, 16))
+>noop : Symbol(noop, Decl(generatorImplicitAny.ts, 0, 16))
+
+    (yield); // ok, result is unused
+    (yield, noop()), noop(); // ok, result is unused
+>noop : Symbol(noop, Decl(generatorImplicitAny.ts, 0, 16))
+>noop : Symbol(noop, Decl(generatorImplicitAny.ts, 0, 16))
+
+    for(yield; false; yield); // ok, results are unused
+    void (yield); // ok, results are unused
+}
+
+function* g5() {
+>g5 : Symbol(g5, Decl(generatorImplicitAny.ts, 22, 1))
+
+    f(yield); // error: implicit any
+>f : Symbol(f, Decl(generatorImplicitAny.ts, 3, 30))
+}
+
+function* g6() {
+>g6 : Symbol(g6, Decl(generatorImplicitAny.ts, 26, 1))
+
+    f<string>(yield); // ok, contextually typed by f<string>
+>f : Symbol(f, Decl(generatorImplicitAny.ts, 3, 30))
+}
diff --git a/tests/baselines/reference/generatorImplicitAny.types b/tests/baselines/reference/generatorImplicitAny.types
@@ -2,3 +2,90 @@
 function* g() {}
 >g : () => Generator<never, void, unknown>

+// https://github.com/microsoft/TypeScript/issues/35105
+declare function noop(): void;
+>noop : () => void
+
+declare function f<T>(value: T): void;
+>f : <T>(value: T) => void
+>value : T
+
+function* g2() {
+>g2 : () => Generator<undefined, void, unknown>
+
+    const value = yield; // error: implicit any
+>value : any
+>yield : any
+}
+
+function* g3() {
+>g3 : () => Generator<undefined, void, string>
+
+    const value: string = yield; // ok, contextually typed by `value`.
+>value : string
+>yield : any
+}
+
+function* g4() {
+>g4 : () => Generator<undefined, void, unknown>
+
+    yield; // ok, result is unused
+>yield : any
+
+    yield, noop(); // ok, result is unused
+>yield, noop() : void
+>yield : any
+>noop() : void
+>noop : () => void
+
+    noop(), yield, noop(); // ok, result is unused
+>noop(), yield, noop() : void
+>noop(), yield : any
+>noop() : void
+>noop : () => void
+>yield : any
+>noop() : void
+>noop : () => void
+
+    (yield); // ok, result is unused
+>(yield) : any
+>yield : any
+
+    (yield, noop()), noop(); // ok, result is unused
+>(yield, noop()), noop() : void
+>(yield, noop()) : void
+>yield, noop() : void
+>yield : any
+>noop() : void
+>noop : () => void
+>noop() : void
+>noop : () => void
+
+    for(yield; false; yield); // ok, results are unused
+>yield : any
+>false : false
+>yield : any
+
+    void (yield); // ok, results are unused
+>void (yield) : undefined
+>(yield) : any
+>yield : any
+}
+
+function* g5() {
+>g5 : () => Generator<undefined, void, any>
+
+    f(yield); // error: implicit any
+>f(yield) : void
+>f : <T>(value: T) => void
+>yield : any
+}
+
+function* g6() {
+>g6 : () => Generator<undefined, void, string>
+
+    f<string>(yield); // ok, contextually typed by f<string>
+>f<string>(yield) : void
+>f : <T>(value: T) => void
+>yield : any
+}