Don't error when function has an implicit return but its return type is assignable to undefined

src/compiler/checker.ts

@@ -35660,7 +35660,8 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             const type = returnType && unwrapReturnType(returnType, functionFlags);
 
             // Functions with an explicitly specified 'undefined, 'void' or 'any' return type don't need any return expressions.
-            if (type && maybeTypeOfKind(type, TypeFlags.Undefined | TypeFlags.Void | TypeFlags.Any)) {
+            // 'unknown' is also fine because it is always assignable to 'undefined'.
+            if (type && maybeTypeOfKind(type, TypeFlags.Undefined | TypeFlags.Void | TypeFlags.Any | TypeFlags.Unknown)) {
                 return;
             }
 

tests/baselines/reference/functionsWithImplicitReturnTypeAssignableToUndefined(strictnullchecks=false).js

@@ -0,0 +1,85 @@
+//// [functionsWithImplicitReturnTypeAssignableToUndefined.ts]
+function f1(): unknown {
+    if (Math.random() < 0.5) return true;
+
+    // Implicit return, but undefined is always assignable to unknown.
+}
+
+type MyUnknown = unknown;
+function f2(): unknown {
+    if (Math.random() < 0.5) return true;
+
+    // Implicit return, but undefined is always assignable to unknown.
+}
+
+function f3(): any {
+    // Implicit return, but undefined is always assignable to any.
+}
+
+function f4(): void {
+    // Implicit return, but undefined is always assignable to void.
+}
+
+function f5(): {} {
+    if (Math.random() < 0.5) return {};
+
+    // Implicit return, but undefined is assignable to object when strictNullChecks is off.
+}
+
+function f6(): Record<string, any> {
+    if (Math.random() < 0.5) return { "foo": true };
+
+    // Implicit return, but undefined is assignable to records (which are just fancy objects)
+    // when strictNullChecks is off.
+}
+
+function f7(): null {
+    if (Math.random() < 0.5) return null;
+
+    // Implicit return, but undefined is assignable to null when strictNullChecks is off.
+}
+
+function f8(): string | null {
+    if (Math.random() < 0.5) return "foo";
+
+    // Implicit return, but undefined is assignable to null when strictNullChecks is off.
+}
+
+//// [functionsWithImplicitReturnTypeAssignableToUndefined.js]
+function f1() {
+    if (Math.random() < 0.5)
+        return true;
+    // Implicit return, but undefined is always assignable to unknown.
+}
+function f2() {
+    if (Math.random() < 0.5)
+        return true;
+    // Implicit return, but undefined is always assignable to unknown.
+}
+function f3() {
+    // Implicit return, but undefined is always assignable to any.
+}
+function f4() {
+    // Implicit return, but undefined is always assignable to void.
+}
+function f5() {
+    if (Math.random() < 0.5)
+        return {};
+    // Implicit return, but undefined is assignable to object when strictNullChecks is off.
+}
+function f6() {
+    if (Math.random() < 0.5)
+        return { "foo": true };
+    // Implicit return, but undefined is assignable to records (which are just fancy objects)
+    // when strictNullChecks is off.
+}
+function f7() {
+    if (Math.random() < 0.5)
+        return null;
+    // Implicit return, but undefined is assignable to null when strictNullChecks is off.
+}
+function f8() {
+    if (Math.random() < 0.5)
+        return "foo";
+    // Implicit return, but undefined is assignable to null when strictNullChecks is off.
+}

tests/baselines/reference/functionsWithImplicitReturnTypeAssignableToUndefined(strictnullchecks=false).symbols

@@ -0,0 +1,84 @@
+=== tests/cases/compiler/functionsWithImplicitReturnTypeAssignableToUndefined.ts ===
+function f1(): unknown {
+>f1 : Symbol(f1, Decl(functionsWithImplicitReturnTypeAssignableToUndefined.ts, 0, 0))
+
+    if (Math.random() < 0.5) return true;
+>Math.random : Symbol(Math.random, Decl(lib.es5.d.ts, --, --))
+>Math : Symbol(Math, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>random : Symbol(Math.random, Decl(lib.es5.d.ts, --, --))
+
+    // Implicit return, but undefined is always assignable to unknown.
+}
+
+type MyUnknown = unknown;
+>MyUnknown : Symbol(MyUnknown, Decl(functionsWithImplicitReturnTypeAssignableToUndefined.ts, 4, 1))
+
+function f2(): unknown {
+>f2 : Symbol(f2, Decl(functionsWithImplicitReturnTypeAssignableToUndefined.ts, 6, 25))
+
+    if (Math.random() < 0.5) return true;
+>Math.random : Symbol(Math.random, Decl(lib.es5.d.ts, --, --))
+>Math : Symbol(Math, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>random : Symbol(Math.random, Decl(lib.es5.d.ts, --, --))
+
+    // Implicit return, but undefined is always assignable to unknown.
+}
+
+function f3(): any {
+>f3 : Symbol(f3, Decl(functionsWithImplicitReturnTypeAssignableToUndefined.ts, 11, 1))
+
+    // Implicit return, but undefined is always assignable to any.
+}
+
+function f4(): void {
+>f4 : Symbol(f4, Decl(functionsWithImplicitReturnTypeAssignableToUndefined.ts, 15, 1))
+
+    // Implicit return, but undefined is always assignable to void.
+}
+
+function f5(): {} {
+>f5 : Symbol(f5, Decl(functionsWithImplicitReturnTypeAssignableToUndefined.ts, 19, 1))
+
+    if (Math.random() < 0.5) return {};
+>Math.random : Symbol(Math.random, Decl(lib.es5.d.ts, --, --))
+>Math : Symbol(Math, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>random : Symbol(Math.random, Decl(lib.es5.d.ts, --, --))
+
+    // Implicit return, but undefined is assignable to object when strictNullChecks is off.
+}
+
+function f6(): Record<string, any> {
+>f6 : Symbol(f6, Decl(functionsWithImplicitReturnTypeAssignableToUndefined.ts, 25, 1))
+>Record : Symbol(Record, Decl(lib.es5.d.ts, --, --))
+
+    if (Math.random() < 0.5) return { "foo": true };
+>Math.random : Symbol(Math.random, Decl(lib.es5.d.ts, --, --))
+>Math : Symbol(Math, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>random : Symbol(Math.random, Decl(lib.es5.d.ts, --, --))
+>"foo" : Symbol("foo", Decl(functionsWithImplicitReturnTypeAssignableToUndefined.ts, 28, 37))
+
+    // Implicit return, but undefined is assignable to records (which are just fancy objects)
+    // when strictNullChecks is off.
+}
+
+function f7(): null {
+>f7 : Symbol(f7, Decl(functionsWithImplicitReturnTypeAssignableToUndefined.ts, 32, 1))
+
+    if (Math.random() < 0.5) return null;
+>Math.random : Symbol(Math.random, Decl(lib.es5.d.ts, --, --))
+>Math : Symbol(Math, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>random : Symbol(Math.random, Decl(lib.es5.d.ts, --, --))
+
+    // Implicit return, but undefined is assignable to null when strictNullChecks is off.
+}
+
+function f8(): string | null {
+>f8 : Symbol(f8, Decl(functionsWithImplicitReturnTypeAssignableToUndefined.ts, 38, 1))
+
+    if (Math.random() < 0.5) return "foo";
+>Math.random : Symbol(Math.random, Decl(lib.es5.d.ts, --, --))
+>Math : Symbol(Math, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>random : Symbol(Math.random, Decl(lib.es5.d.ts, --, --))
+
+    // Implicit return, but undefined is assignable to null when strictNullChecks is off.
+}

tests/baselines/reference/functionsWithImplicitReturnTypeAssignableToUndefined(strictnullchecks=false).types

@@ -0,0 +1,107 @@
+=== tests/cases/compiler/functionsWithImplicitReturnTypeAssignableToUndefined.ts ===
+function f1(): unknown {
+>f1 : () => unknown
+
+    if (Math.random() < 0.5) return true;
+>Math.random() < 0.5 : boolean
+>Math.random() : number
+>Math.random : () => number
+>Math : Math
+>random : () => number
+>0.5 : 0.5
+>true : true
+
+    // Implicit return, but undefined is always assignable to unknown.
+}
+
+type MyUnknown = unknown;
+>MyUnknown : unknown
+
+function f2(): unknown {
+>f2 : () => unknown
+
+    if (Math.random() < 0.5) return true;
+>Math.random() < 0.5 : boolean
+>Math.random() : number
+>Math.random : () => number
+>Math : Math
+>random : () => number
+>0.5 : 0.5
+>true : true
+
+    // Implicit return, but undefined is always assignable to unknown.
+}
+
+function f3(): any {
+>f3 : () => any
+
+    // Implicit return, but undefined is always assignable to any.
+}
+
+function f4(): void {
+>f4 : () => void
+
+    // Implicit return, but undefined is always assignable to void.
+}
+
+function f5(): {} {
+>f5 : () => {}
+
+    if (Math.random() < 0.5) return {};
+>Math.random() < 0.5 : boolean
+>Math.random() : number
+>Math.random : () => number
+>Math : Math
+>random : () => number
+>0.5 : 0.5
+>{} : {}
+
+    // Implicit return, but undefined is assignable to object when strictNullChecks is off.
+}
+
+function f6(): Record<string, any> {
+>f6 : () => Record<string, any>
+
+    if (Math.random() < 0.5) return { "foo": true };
+>Math.random() < 0.5 : boolean
+>Math.random() : number
+>Math.random : () => number
+>Math : Math
+>random : () => number
+>0.5 : 0.5
+>{ "foo": true } : { foo: boolean; }
+>"foo" : boolean
+>true : true
+
+    // Implicit return, but undefined is assignable to records (which are just fancy objects)
+    // when strictNullChecks is off.
+}
+
+function f7(): null {
+>f7 : () => null
+
+    if (Math.random() < 0.5) return null;
+>Math.random() < 0.5 : boolean
+>Math.random() : number
+>Math.random : () => number
+>Math : Math
+>random : () => number
+>0.5 : 0.5
+
+    // Implicit return, but undefined is assignable to null when strictNullChecks is off.
+}
+
+function f8(): string | null {
+>f8 : () => string | null
+
+    if (Math.random() < 0.5) return "foo";
+>Math.random() < 0.5 : boolean
+>Math.random() : number
+>Math.random : () => number
+>Math : Math
+>random : () => number
+>0.5 : 0.5
+>"foo" : "foo"
+
+    // Implicit return, but undefined is assignable to null when strictNullChecks is off.
+}

tests/baselines/reference/functionsWithImplicitReturnTypeAssignableToUndefined(strictnullchecks=true).errors.txt

@@ -0,0 +1,60 @@
+tests/cases/compiler/functionsWithImplicitReturnTypeAssignableToUndefined.ts(22,16): error TS2366: Function lacks ending return statement and return type does not include 'undefined'.
+tests/cases/compiler/functionsWithImplicitReturnTypeAssignableToUndefined.ts(28,16): error TS2366: Function lacks ending return statement and return type does not include 'undefined'.
+tests/cases/compiler/functionsWithImplicitReturnTypeAssignableToUndefined.ts(35,16): error TS2366: Function lacks ending return statement and return type does not include 'undefined'.
+tests/cases/compiler/functionsWithImplicitReturnTypeAssignableToUndefined.ts(41,16): error TS2366: Function lacks ending return statement and return type does not include 'undefined'.
+
+
+==== tests/cases/compiler/functionsWithImplicitReturnTypeAssignableToUndefined.ts (4 errors) ====
+    function f1(): unknown {
+        if (Math.random() < 0.5) return true;
+
+        // Implicit return, but undefined is always assignable to unknown.
+    }
+
+    type MyUnknown = unknown;
+    function f2(): unknown {
+        if (Math.random() < 0.5) return true;
+
+        // Implicit return, but undefined is always assignable to unknown.
+    }
+
+    function f3(): any {
+        // Implicit return, but undefined is always assignable to any.
+    }
+
+    function f4(): void {
+        // Implicit return, but undefined is always assignable to void.
+    }
+
+    function f5(): {} {
+                   ~~
+!!! error TS2366: Function lacks ending return statement and return type does not include 'undefined'.
+        if (Math.random() < 0.5) return {};
+
+        // Implicit return, but undefined is assignable to object when strictNullChecks is off.
+    }
+
+    function f6(): Record<string, any> {
+                   ~~~~~~~~~~~~~~~~~~~
+!!! error TS2366: Function lacks ending return statement and return type does not include 'undefined'.
+        if (Math.random() < 0.5) return { "foo": true };
+
+        // Implicit return, but undefined is assignable to records (which are just fancy objects)
+        // when strictNullChecks is off.
+    }
+
+    function f7(): null {
+                   ~~~~
+!!! error TS2366: Function lacks ending return statement and return type does not include 'undefined'.
+        if (Math.random() < 0.5) return null;
+
+        // Implicit return, but undefined is assignable to null when strictNullChecks is off.
+    }
+
+    function f8(): string | null {
+                   ~~~~~~~~~~~~~
+!!! error TS2366: Function lacks ending return statement and return type does not include 'undefined'.
+        if (Math.random() < 0.5) return "foo";
+
+        // Implicit return, but undefined is assignable to null when strictNullChecks is off.
+    }