diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 0d4c8c7459df3..d3367776dec35 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -35540,8 +35540,8 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             const functionFlags = getFunctionFlags(func);
             const type = returnType && unwrapReturnType(returnType, functionFlags);
 
-            // Functions with with an explicitly specified 'void' or 'any' return type don't need any return expressions.
-            if (type && maybeTypeOfKind(type, TypeFlags.Any | TypeFlags.Void)) {
+            // 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)) {
                 return;
             }
 
@@ -35560,9 +35560,14 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             else if (type && !hasExplicitReturn) {
                 // minimal check: function has syntactic return type annotation and no explicit return statements in the body
                 // this function does not conform to the specification.
-                error(errorNode, Diagnostics.A_function_whose_declared_type_is_neither_void_nor_any_must_return_a_value);
+                if (strictNullChecks) {
+                    error(errorNode, Diagnostics.A_function_whose_declared_type_is_neither_undefined_void_nor_any_must_return_a_value);
+                }
+                else {
+                    error(errorNode, Diagnostics.A_function_whose_declared_type_is_neither_void_nor_any_must_return_a_value);
+                }
             }
-            else if (type && strictNullChecks && !isTypeAssignableTo(undefinedType, type)) {
+            else if (type && strictNullChecks) {
                 error(errorNode, Diagnostics.Function_lacks_ending_return_statement_and_return_type_does_not_include_undefined);
             }
             else if (compilerOptions.noImplicitReturns) {
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 8a3f57b764378..4562b7e738e3e 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -3611,6 +3611,10 @@
         "category": "Error",
         "code": 2846
     },
+    "A function whose declared type is neither 'undefined', 'void', nor 'any' must return a value.": {
+        "category": "Error",
+        "code": 2847
+    },
 
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",
@@ -6718,7 +6722,7 @@
         "category": "Suggestion",
         "code": 80010
     },
-    
+
     "Add missing 'super()' call": {
         "category": "Message",
         "code": 90001
diff --git a/src/services/codefixes/returnValueCorrect.ts b/src/services/codefixes/returnValueCorrect.ts
index 1492e6377ce26..d3d0945959483 100644
--- a/src/services/codefixes/returnValueCorrect.ts
+++ b/src/services/codefixes/returnValueCorrect.ts
@@ -52,6 +52,7 @@ const fixRemoveBracesFromArrowFunctionBody = "fixRemoveBracesFromArrowFunctionBo
 const fixIdWrapTheBlockWithParen = "fixWrapTheBlockWithParen";
 const errorCodes = [
     Diagnostics.A_function_whose_declared_type_is_neither_void_nor_any_must_return_a_value.code,
+    Diagnostics.A_function_whose_declared_type_is_neither_undefined_void_nor_any_must_return_a_value.code,
     Diagnostics.Type_0_is_not_assignable_to_type_1.code,
     Diagnostics.Argument_of_type_0_is_not_assignable_to_parameter_of_type_1.code
 ];
@@ -214,6 +215,7 @@ function getInfo(checker: TypeChecker, sourceFile: SourceFile, position: number,
     const declaration = findAncestor(node.parent, isFunctionLikeDeclaration);
     switch (errorCode) {
         case Diagnostics.A_function_whose_declared_type_is_neither_void_nor_any_must_return_a_value.code:
+        case Diagnostics.A_function_whose_declared_type_is_neither_undefined_void_nor_any_must_return_a_value.code:
             if (!declaration || !declaration.body || !declaration.type || !rangeContainsRange(declaration.type, node)) return undefined;
             return getFixInfo(checker, declaration, checker.getTypeFromTypeNode(declaration.type), /* isFunctionType */ false);
         case Diagnostics.Argument_of_type_0_is_not_assignable_to_parameter_of_type_1.code:
diff --git a/tests/baselines/reference/controlFlowGenericTypes.errors.txt b/tests/baselines/reference/controlFlowGenericTypes.errors.txt
index 16ec73b1b4a65..4686e36af6091 100644
--- a/tests/baselines/reference/controlFlowGenericTypes.errors.txt
+++ b/tests/baselines/reference/controlFlowGenericTypes.errors.txt
@@ -2,7 +2,7 @@ tests/cases/conformance/controlFlow/controlFlowGenericTypes.ts(49,15): error TS2
 tests/cases/conformance/controlFlow/controlFlowGenericTypes.ts(55,15): error TS2345: Argument of type 'undefined' is not assignable to parameter of type 'Box<unknown>'.
 tests/cases/conformance/controlFlow/controlFlowGenericTypes.ts(81,11): error TS2339: Property 'foo' does not exist on type 'MyUnion'.
   Property 'foo' does not exist on type 'AA'.
-tests/cases/conformance/controlFlow/controlFlowGenericTypes.ts(90,44): error TS2355: A function whose declared type is neither 'void' nor 'any' must return a value.
+tests/cases/conformance/controlFlow/controlFlowGenericTypes.ts(90,44): error TS2847: A function whose declared type is neither 'undefined', 'void', nor 'any' must return a value.
 tests/cases/conformance/controlFlow/controlFlowGenericTypes.ts(91,11): error TS2339: Property 'foo' does not exist on type 'MyUnion'.
   Property 'foo' does not exist on type 'AA'.
 tests/cases/conformance/controlFlow/controlFlowGenericTypes.ts(156,16): error TS18048: 'obj' is possibly 'undefined'.
@@ -109,7 +109,7 @@ tests/cases/conformance/controlFlow/controlFlowGenericTypes.ts(168,9): error TS1
     
     const fn2 = <T extends MyUnion>(value: T): MyUnion => {
                                                ~~~~~~~
-!!! error TS2355: A function whose declared type is neither 'void' nor 'any' must return a value.
+!!! error TS2847: A function whose declared type is neither 'undefined', 'void', nor 'any' must return a value.
         value.foo;  // Error
               ~~~
 !!! error TS2339: Property 'foo' does not exist on type 'MyUnion'.
diff --git a/tests/baselines/reference/functionsMissingReturnStatementsAndExpressions.errors.txt b/tests/baselines/reference/functionsMissingReturnStatementsAndExpressions.errors.txt
index efbdcb8290c56..f4466e0b869e4 100644
--- a/tests/baselines/reference/functionsMissingReturnStatementsAndExpressions.errors.txt
+++ b/tests/baselines/reference/functionsMissingReturnStatementsAndExpressions.errors.txt
@@ -1,11 +1,12 @@
 tests/cases/compiler/functionsMissingReturnStatementsAndExpressions.ts(1,16): error TS2355: A function whose declared type is neither 'void' nor 'any' must return a value.
 tests/cases/compiler/functionsMissingReturnStatementsAndExpressions.ts(99,17): error TS2355: A function whose declared type is neither 'void' nor 'any' must return a value.
-tests/cases/compiler/functionsMissingReturnStatementsAndExpressions.ts(104,16): error TS2378: A 'get' accessor must return a value.
-tests/cases/compiler/functionsMissingReturnStatementsAndExpressions.ts(126,15): error TS18050: The value 'undefined' cannot be used here.
-tests/cases/compiler/functionsMissingReturnStatementsAndExpressions.ts(127,5): error TS1003: Identifier expected.
+tests/cases/compiler/functionsMissingReturnStatementsAndExpressions.ts(107,17): error TS2355: A function whose declared type is neither 'void' nor 'any' must return a value.
+tests/cases/compiler/functionsMissingReturnStatementsAndExpressions.ts(112,16): error TS2378: A 'get' accessor must return a value.
+tests/cases/compiler/functionsMissingReturnStatementsAndExpressions.ts(134,15): error TS18050: The value 'undefined' cannot be used here.
+tests/cases/compiler/functionsMissingReturnStatementsAndExpressions.ts(135,5): error TS1003: Identifier expected.
 
 
-==== tests/cases/compiler/functionsMissingReturnStatementsAndExpressions.ts (5 errors) ====
+==== tests/cases/compiler/functionsMissingReturnStatementsAndExpressions.ts (6 errors) ====
     function f1(): string {
                    ~~~~~~
 !!! error TS2355: A function whose declared type is neither 'void' nor 'any' must return a value.
@@ -40,12 +41,12 @@ tests/cases/compiler/functionsMissingReturnStatementsAndExpressions.ts(127,5): e
         return null;
     }
     
-    function f8(): void {
+    function f8(): any {
         // Fine since are typed any.
         return;
     }
     
-    function f9(): void {
+    function f9(): any {
         // Fine since we are typed any and return undefined
         return undefined;
     }
@@ -112,6 +113,16 @@ tests/cases/compiler/functionsMissingReturnStatementsAndExpressions.ts(127,5): e
         // Not okay; union does not contain void or any
     }
     
+    function f22(): undefined {
+        // Okay; return type allows implicit return of undefined
+    }
+    
+    function f23(): undefined | number {
+                    ~~~~~~~~~~~~~~~~~~
+!!! error TS2355: A function whose declared type is neither 'void' nor 'any' must return a value.
+        // Error; because `undefined | number` becomes `number` without strictNullChecks.
+    }
+    
     class C {
         public get m1() {
                    ~~
@@ -143,4 +154,5 @@ tests/cases/compiler/functionsMissingReturnStatementsAndExpressions.ts(127,5): e
         }
         ~
 !!! error TS1003: Identifier expected.
-    }
\ No newline at end of file
+    }
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/functionsMissingReturnStatementsAndExpressions.js b/tests/baselines/reference/functionsMissingReturnStatementsAndExpressions.js
index a0cfa3f9fcea3..4cc59397c5e06 100644
--- a/tests/baselines/reference/functionsMissingReturnStatementsAndExpressions.js
+++ b/tests/baselines/reference/functionsMissingReturnStatementsAndExpressions.js
@@ -31,12 +31,12 @@ function f7(): void {
     return null;
 }
 
-function f8(): void {
+function f8(): any {
     // Fine since are typed any.
     return;
 }
 
-function f9(): void {
+function f9(): any {
     // Fine since we are typed any and return undefined
     return undefined;
 }
@@ -101,6 +101,14 @@ function f21(): number | string {
     // Not okay; union does not contain void or any
 }
 
+function f22(): undefined {
+    // Okay; return type allows implicit return of undefined
+}
+
+function f23(): undefined | number {
+    // Error; because `undefined | number` becomes `number` without strictNullChecks.
+}
+
 class C {
     public get m1() {
         // Errors; get accessors must return a value.
@@ -126,7 +134,8 @@ class C {
         throw null;
         throw undefined.
     }
-}
+}
+
 
 //// [functionsMissingReturnStatementsAndExpressions.js]
 function f1() {
@@ -209,6 +218,12 @@ function f20() {
 function f21() {
     // Not okay; union does not contain void or any
 }
+function f22() {
+    // Okay; return type allows implicit return of undefined
+}
+function f23() {
+    // Error; because `undefined | number` becomes `number` without strictNullChecks.
+}
 var C = /** @class */ (function () {
     function C() {
     }
diff --git a/tests/baselines/reference/functionsMissingReturnStatementsAndExpressions.symbols b/tests/baselines/reference/functionsMissingReturnStatementsAndExpressions.symbols
index f1bde79daa863..c7048ff46c778 100644
--- a/tests/baselines/reference/functionsMissingReturnStatementsAndExpressions.symbols
+++ b/tests/baselines/reference/functionsMissingReturnStatementsAndExpressions.symbols
@@ -46,14 +46,14 @@ function f7(): void {
     return null;
 }
 
-function f8(): void {
+function f8(): any {
 >f8 : Symbol(f8, Decl(functionsMissingReturnStatementsAndExpressions.ts, 30, 1))
 
     // Fine since are typed any.
     return;
 }
 
-function f9(): void {
+function f9(): any {
 >f9 : Symbol(f9, Decl(functionsMissingReturnStatementsAndExpressions.ts, 35, 1))
 
     // Fine since we are typed any and return undefined
@@ -152,37 +152,49 @@ function f21(): number | string {
     // Not okay; union does not contain void or any
 }
 
+function f22(): undefined {
+>f22 : Symbol(f22, Decl(functionsMissingReturnStatementsAndExpressions.ts, 100, 1))
+
+    // Okay; return type allows implicit return of undefined
+}
+
+function f23(): undefined | number {
+>f23 : Symbol(f23, Decl(functionsMissingReturnStatementsAndExpressions.ts, 104, 1))
+
+    // Error; because `undefined | number` becomes `number` without strictNullChecks.
+}
+
 class C {
->C : Symbol(C, Decl(functionsMissingReturnStatementsAndExpressions.ts, 100, 1))
+>C : Symbol(C, Decl(functionsMissingReturnStatementsAndExpressions.ts, 108, 1))
 
     public get m1() {
->m1 : Symbol(C.m1, Decl(functionsMissingReturnStatementsAndExpressions.ts, 102, 9))
+>m1 : Symbol(C.m1, Decl(functionsMissingReturnStatementsAndExpressions.ts, 110, 9))
 
         // Errors; get accessors must return a value.
     }
 
     public get m2() {
->m2 : Symbol(C.m2, Decl(functionsMissingReturnStatementsAndExpressions.ts, 105, 5))
+>m2 : Symbol(C.m2, Decl(functionsMissingReturnStatementsAndExpressions.ts, 113, 5))
 
         // Permissible; returns undefined.
         return;
     }
 
     public get m3() {
->m3 : Symbol(C.m3, Decl(functionsMissingReturnStatementsAndExpressions.ts, 110, 5))
+>m3 : Symbol(C.m3, Decl(functionsMissingReturnStatementsAndExpressions.ts, 118, 5))
 
         return "Okay, because this is a return expression.";
     }
 
     public get m4() {
->m4 : Symbol(C.m4, Decl(functionsMissingReturnStatementsAndExpressions.ts, 114, 5))
+>m4 : Symbol(C.m4, Decl(functionsMissingReturnStatementsAndExpressions.ts, 122, 5))
 
         // Fine since this consists of a single throw statement.
         throw null;
     }
 
     public get m5() {
->m5 : Symbol(C.m5, Decl(functionsMissingReturnStatementsAndExpressions.ts, 119, 5))
+>m5 : Symbol(C.m5, Decl(functionsMissingReturnStatementsAndExpressions.ts, 127, 5))
 
         // Not fine, since we can *only* consist of a single throw statement
         // if no return statements are present but we are a get accessor.
@@ -191,3 +203,4 @@ class C {
 >undefined : Symbol(undefined)
     }
 }
+
diff --git a/tests/baselines/reference/functionsMissingReturnStatementsAndExpressions.types b/tests/baselines/reference/functionsMissingReturnStatementsAndExpressions.types
index 1fc53ae36ef3e..5e6b014bfa2d6 100644
--- a/tests/baselines/reference/functionsMissingReturnStatementsAndExpressions.types
+++ b/tests/baselines/reference/functionsMissingReturnStatementsAndExpressions.types
@@ -47,15 +47,15 @@ function f7(): void {
     return null;
 }
 
-function f8(): void {
->f8 : () => void
+function f8(): any {
+>f8 : () => any
 
     // Fine since are typed any.
     return;
 }
 
-function f9(): void {
->f9 : () => void
+function f9(): any {
+>f9 : () => any
 
     // Fine since we are typed any and return undefined
     return undefined;
@@ -154,6 +154,18 @@ function f21(): number | string {
     // Not okay; union does not contain void or any
 }
 
+function f22(): undefined {
+>f22 : () => undefined
+
+    // Okay; return type allows implicit return of undefined
+}
+
+function f23(): undefined | number {
+>f23 : () => undefined | number
+
+    // Error; because `undefined | number` becomes `number` without strictNullChecks.
+}
+
 class C {
 >C : C
 
@@ -196,3 +208,4 @@ class C {
     }
 > : any
 }
+
diff --git a/tests/baselines/reference/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.errors.txt b/tests/baselines/reference/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.errors.txt
new file mode 100644
index 0000000000000..a570470cf2ea0
--- /dev/null
+++ b/tests/baselines/reference/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.errors.txt
@@ -0,0 +1,25 @@
+tests/cases/compiler/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts(5,16): error TS2847: A function whose declared type is neither 'undefined', 'void', nor 'any' must return a value.
+tests/cases/compiler/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts(13,22): error TS2847: A function whose declared type is neither 'undefined', 'void', nor 'any' must return a value.
+
+
+==== tests/cases/compiler/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts (2 errors) ====
+    function f1(): undefined | number {
+        // Okay; return type allows implicit return of undefined
+    }
+    
+    function f2(): number {
+                   ~~~~~~
+!!! error TS2847: A function whose declared type is neither 'undefined', 'void', nor 'any' must return a value.
+        // Error; return type does not include undefined
+    }
+    
+    async function f3(): Promise<undefined | number> {
+        // Okay; return type allows implicit return of undefined
+    }
+    
+    async function f4(): Promise<number> {
+                         ~~~~~~~~~~~~~~~
+!!! error TS2847: A function whose declared type is neither 'undefined', 'void', nor 'any' must return a value.
+        // Error; return type does not include undefined
+    }
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.js b/tests/baselines/reference/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.js
new file mode 100644
index 0000000000000..3ca9a8cc8353d
--- /dev/null
+++ b/tests/baselines/reference/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.js
@@ -0,0 +1,31 @@
+//// [functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts]
+function f1(): undefined | number {
+    // Okay; return type allows implicit return of undefined
+}
+
+function f2(): number {
+    // Error; return type does not include undefined
+}
+
+async function f3(): Promise<undefined | number> {
+    // Okay; return type allows implicit return of undefined
+}
+
+async function f4(): Promise<number> {
+    // Error; return type does not include undefined
+}
+
+
+//// [functionsMissingReturnStatementsAndExpressionsStrictNullChecks.js]
+function f1() {
+    // Okay; return type allows implicit return of undefined
+}
+function f2() {
+    // Error; return type does not include undefined
+}
+async function f3() {
+    // Okay; return type allows implicit return of undefined
+}
+async function f4() {
+    // Error; return type does not include undefined
+}
diff --git a/tests/baselines/reference/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.symbols b/tests/baselines/reference/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.symbols
new file mode 100644
index 0000000000000..befceb738b18a
--- /dev/null
+++ b/tests/baselines/reference/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.symbols
@@ -0,0 +1,27 @@
+=== tests/cases/compiler/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts ===
+function f1(): undefined | number {
+>f1 : Symbol(f1, Decl(functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts, 0, 0))
+
+    // Okay; return type allows implicit return of undefined
+}
+
+function f2(): number {
+>f2 : Symbol(f2, Decl(functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts, 2, 1))
+
+    // Error; return type does not include undefined
+}
+
+async function f3(): Promise<undefined | number> {
+>f3 : Symbol(f3, Decl(functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts, 6, 1))
+>Promise : Symbol(Promise, Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.promise.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2018.promise.d.ts, --, --))
+
+    // Okay; return type allows implicit return of undefined
+}
+
+async function f4(): Promise<number> {
+>f4 : Symbol(f4, Decl(functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts, 10, 1))
+>Promise : Symbol(Promise, Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.promise.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2018.promise.d.ts, --, --))
+
+    // Error; return type does not include undefined
+}
+
diff --git a/tests/baselines/reference/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.types b/tests/baselines/reference/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.types
new file mode 100644
index 0000000000000..beb7ae82be089
--- /dev/null
+++ b/tests/baselines/reference/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.types
@@ -0,0 +1,25 @@
+=== tests/cases/compiler/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts ===
+function f1(): undefined | number {
+>f1 : () => undefined | number
+
+    // Okay; return type allows implicit return of undefined
+}
+
+function f2(): number {
+>f2 : () => number
+
+    // Error; return type does not include undefined
+}
+
+async function f3(): Promise<undefined | number> {
+>f3 : () => Promise<undefined | number>
+
+    // Okay; return type allows implicit return of undefined
+}
+
+async function f4(): Promise<number> {
+>f4 : () => Promise<number>
+
+    // Error; return type does not include undefined
+}
+
diff --git a/tests/baselines/reference/jsdocFunction_missingReturn.errors.txt b/tests/baselines/reference/jsdocFunction_missingReturn.errors.txt
index 646e03fcd6f57..8e102d797ec7c 100644
--- a/tests/baselines/reference/jsdocFunction_missingReturn.errors.txt
+++ b/tests/baselines/reference/jsdocFunction_missingReturn.errors.txt
@@ -1,9 +1,9 @@
-/a.js(1,24): error TS2355: A function whose declared type is neither 'void' nor 'any' must return a value.
+/a.js(1,24): error TS2847: A function whose declared type is neither 'undefined', 'void', nor 'any' must return a value.
 
 
 ==== /a.js (1 errors) ====
     /** @type {function(): number} */
                            ~~~~~~
-!!! error TS2355: A function whose declared type is neither 'void' nor 'any' must return a value.
+!!! error TS2847: A function whose declared type is neither 'undefined', 'void', nor 'any' must return a value.
     function f() {}
     
\ No newline at end of file
diff --git a/tests/baselines/reference/parseInvalidNonNullableTypes.errors.txt b/tests/baselines/reference/parseInvalidNonNullableTypes.errors.txt
index a5b8dbef915bd..378419d288544 100644
--- a/tests/baselines/reference/parseInvalidNonNullableTypes.errors.txt
+++ b/tests/baselines/reference/parseInvalidNonNullableTypes.errors.txt
@@ -4,9 +4,9 @@ tests/cases/compiler/parseInvalidNonNullableTypes.ts(9,16): error TS17019: '!' a
 tests/cases/compiler/parseInvalidNonNullableTypes.ts(10,16): error TS17019: '!' at the end of a type is not valid TypeScript syntax. Did you mean to write 'number'?
 tests/cases/compiler/parseInvalidNonNullableTypes.ts(12,16): error TS17020: '!' at the start of a type is not valid TypeScript syntax. Did you mean to write 'string'?
 tests/cases/compiler/parseInvalidNonNullableTypes.ts(13,16): error TS17020: '!' at the start of a type is not valid TypeScript syntax. Did you mean to write 'number'?
-tests/cases/compiler/parseInvalidNonNullableTypes.ts(15,16): error TS2355: A function whose declared type is neither 'void' nor 'any' must return a value.
+tests/cases/compiler/parseInvalidNonNullableTypes.ts(15,16): error TS2847: A function whose declared type is neither 'undefined', 'void', nor 'any' must return a value.
 tests/cases/compiler/parseInvalidNonNullableTypes.ts(15,16): error TS17019: '!' at the end of a type is not valid TypeScript syntax. Did you mean to write 'string'?
-tests/cases/compiler/parseInvalidNonNullableTypes.ts(16,16): error TS2355: A function whose declared type is neither 'void' nor 'any' must return a value.
+tests/cases/compiler/parseInvalidNonNullableTypes.ts(16,16): error TS2847: A function whose declared type is neither 'undefined', 'void', nor 'any' must return a value.
 tests/cases/compiler/parseInvalidNonNullableTypes.ts(16,16): error TS17020: '!' at the start of a type is not valid TypeScript syntax. Did you mean to write 'string'?
 tests/cases/compiler/parseInvalidNonNullableTypes.ts(18,16): error TS17019: '!' at the end of a type is not valid TypeScript syntax. Did you mean to write 'any'?
 tests/cases/compiler/parseInvalidNonNullableTypes.ts(19,10): error TS17019: '!' at the end of a type is not valid TypeScript syntax. Did you mean to write 'number'?
@@ -43,12 +43,12 @@ tests/cases/compiler/parseInvalidNonNullableTypes.ts(22,10): error TS17020: '!'
     
     function f7(): string! {}
                    ~~~~~~~
-!!! error TS2355: A function whose declared type is neither 'void' nor 'any' must return a value.
+!!! error TS2847: A function whose declared type is neither 'undefined', 'void', nor 'any' must return a value.
                    ~~~~~~~
 !!! error TS17019: '!' at the end of a type is not valid TypeScript syntax. Did you mean to write 'string'?
     function f8(): !string {}
                    ~~~~~~~
-!!! error TS2355: A function whose declared type is neither 'void' nor 'any' must return a value.
+!!! error TS2847: A function whose declared type is neither 'undefined', 'void', nor 'any' must return a value.
                    ~~~~~~~
 !!! error TS17020: '!' at the start of a type is not valid TypeScript syntax. Did you mean to write 'string'?
     
diff --git a/tests/baselines/reference/unknownType1.errors.txt b/tests/baselines/reference/unknownType1.errors.txt
index 2a46fbeac4a59..32034c8e2b9d9 100644
--- a/tests/baselines/reference/unknownType1.errors.txt
+++ b/tests/baselines/reference/unknownType1.errors.txt
@@ -20,7 +20,7 @@ tests/cases/conformance/types/unknown/unknownType1.ts(128,5): error TS2322: Type
 tests/cases/conformance/types/unknown/unknownType1.ts(129,5): error TS2322: Type 'number' is not assignable to type '{ [x: string]: unknown; }'.
 tests/cases/conformance/types/unknown/unknownType1.ts(143,29): error TS2698: Spread types may only be created from object types.
 tests/cases/conformance/types/unknown/unknownType1.ts(144,29): error TS2698: Spread types may only be created from object types.
-tests/cases/conformance/types/unknown/unknownType1.ts(150,17): error TS2355: A function whose declared type is neither 'void' nor 'any' must return a value.
+tests/cases/conformance/types/unknown/unknownType1.ts(150,17): error TS2847: A function whose declared type is neither 'undefined', 'void', nor 'any' must return a value.
 tests/cases/conformance/types/unknown/unknownType1.ts(156,14): error TS2700: Rest types may only be created from object types.
 tests/cases/conformance/types/unknown/unknownType1.ts(162,5): error TS2564: Property 'a' has no initializer and is not definitely assigned in the constructor.
 tests/cases/conformance/types/unknown/unknownType1.ts(170,9): error TS2322: Type 'T' is not assignable to type '{}'.
@@ -223,7 +223,7 @@ tests/cases/conformance/types/unknown/unknownType1.ts(181,5): error TS2322: Type
     
     function f27(): unknown {
                     ~~~~~~~
-!!! error TS2355: A function whose declared type is neither 'void' nor 'any' must return a value.
+!!! error TS2847: A function whose declared type is neither 'undefined', 'void', nor 'any' must return a value.
     }
     
     // Rest type cannot be created from unknown
diff --git a/tests/cases/compiler/functionsMissingReturnStatementsAndExpressions.ts b/tests/cases/compiler/functionsMissingReturnStatementsAndExpressions.ts
index 582940038c5e5..cf5eb31ecad58 100644
--- a/tests/cases/compiler/functionsMissingReturnStatementsAndExpressions.ts
+++ b/tests/cases/compiler/functionsMissingReturnStatementsAndExpressions.ts
@@ -34,12 +34,12 @@ function f7(): void {
     return null;
 }
 
-function f8(): void {
+function f8(): any {
     // Fine since are typed any.
     return;
 }
 
-function f9(): void {
+function f9(): any {
     // Fine since we are typed any and return undefined
     return undefined;
 }
@@ -104,6 +104,14 @@ function f21(): number | string {
     // Not okay; union does not contain void or any
 }
 
+function f22(): undefined {
+    // Okay; return type allows implicit return of undefined
+}
+
+function f23(): undefined | number {
+    // Error; because `undefined | number` becomes `number` without strictNullChecks.
+}
+
 class C {
     public get m1() {
         // Errors; get accessors must return a value.
@@ -129,4 +137,4 @@ class C {
         throw null;
         throw undefined.
     }
-}
\ No newline at end of file
+}
diff --git a/tests/cases/compiler/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts b/tests/cases/compiler/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts
new file mode 100644
index 0000000000000..89d3d5866221a
--- /dev/null
+++ b/tests/cases/compiler/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.ts
@@ -0,0 +1,19 @@
+// @strictNullChecks: true
+
+// @target: es2018
+
+function f1(): undefined | number {
+    // Okay; return type allows implicit return of undefined
+}
+
+function f2(): number {
+    // Error; return type does not include undefined
+}
+
+async function f3(): Promise<undefined | number> {
+    // Okay; return type allows implicit return of undefined
+}
+
+async function f4(): Promise<number> {
+    // Error; return type does not include undefined
+}
diff --git a/tests/cases/fourslash/codeFixCorrectReturnValue6.ts b/tests/cases/fourslash/codeFixCorrectReturnValue6.ts
index 693145f8c61b8..a7dbffae98cf3 100644
--- a/tests/cases/fourslash/codeFixCorrectReturnValue6.ts
+++ b/tests/cases/fourslash/codeFixCorrectReturnValue6.ts
@@ -4,6 +4,4 @@
 ////     undefined
 //// }
 
-verify.codeFixAvailable([
-    { description: 'Add a return statement' },
-]);
+verify.not.codeFixAvailable();