diff --git a/lib/tsc.js b/lib/tsc.js
index 010f5d28f9c..a0951afd1fa 100644
--- a/lib/tsc.js
+++ b/lib/tsc.js
@@ -41603,7 +41603,7 @@ var ts;
                                 return "continue-outer";
                             if (sourceProperty === targetProperty)
                                 return "continue";
-                            var related = propertyRelatedTo(source, target, sourceProperty, targetProperty, function (_) { return combination[i]; }, false, 0);
+                            var related = propertyRelatedTo(source, target, sourceProperty, targetProperty, function (_) { return combination[i]; }, false, 0, strictNullChecks || relation === comparableRelation);
                             if (!related) {
                                 return "continue-outer";
                             }
@@ -41702,7 +41702,7 @@ var ts;
                     return isRelatedTo(source, addOptionality(getTypeOfSymbol(targetProp), targetIsOptional), reportErrors, undefined, intersectionState);
                 }
             }
-            function propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSourceProperty, reportErrors, intersectionState) {
+            function propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSourceProperty, reportErrors, intersectionState, skipOptional) {
                 var sourcePropFlags = ts.getDeclarationModifierFlagsFromSymbol(sourceProp);
                 var targetPropFlags = ts.getDeclarationModifierFlagsFromSymbol(targetProp);
                 if (sourcePropFlags & 8 || targetPropFlags & 8) {
@@ -41739,7 +41739,7 @@ var ts;
                     }
                     return 0;
                 }
-                if (relation !== comparableRelation && sourceProp.flags & 16777216 && !(targetProp.flags & 16777216)) {
+                if (!skipOptional && sourceProp.flags & 16777216 && !(targetProp.flags & 16777216)) {
                     if (reportErrors) {
                         reportError(ts.Diagnostics.Property_0_is_optional_in_type_1_but_required_in_type_2, symbolToString(targetProp), typeToString(source), typeToString(target));
                     }
@@ -41853,7 +41853,7 @@ var ts;
                     if (!(targetProp.flags & 4194304) && (!numericNamesOnly || isNumericLiteralName(name) || name === "length")) {
                         var sourceProp = getPropertyOfType(source, name);
                         if (sourceProp && sourceProp !== targetProp) {
-                            var related = propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSymbol, reportErrors, intersectionState);
+                            var related = propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSymbol, reportErrors, intersectionState, relation === comparableRelation);
                             if (!related) {
                                 return 0;
                             }
diff --git a/lib/tsserver.js b/lib/tsserver.js
index bbfa3e787cf..9815970212d 100644
--- a/lib/tsserver.js
+++ b/lib/tsserver.js
@@ -49915,7 +49915,7 @@ var ts;
                             if (sourceProperty === targetProperty)
                                 return "continue";
                             // We compare the source property to the target in the context of a single discriminant type.
-                            var related = propertyRelatedTo(source, target, sourceProperty, targetProperty, function (_) { return combination[i]; }, /*reportErrors*/ false, 0 /* None */);
+                            var related = propertyRelatedTo(source, target, sourceProperty, targetProperty, function (_) { return combination[i]; }, /*reportErrors*/ false, 0 /* None */, /*skipOptional*/ strictNullChecks || relation === comparableRelation);
                             // If the target property could not be found, or if the properties were not related,
                             // then this constituent is not a match.
                             if (!related) {
@@ -50023,7 +50023,7 @@ var ts;
                     return isRelatedTo(source, addOptionality(getTypeOfSymbol(targetProp), targetIsOptional), reportErrors, /*headMessage*/ undefined, intersectionState);
                 }
             }
-            function propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSourceProperty, reportErrors, intersectionState) {
+            function propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSourceProperty, reportErrors, intersectionState, skipOptional) {
                 var sourcePropFlags = ts.getDeclarationModifierFlagsFromSymbol(sourceProp);
                 var targetPropFlags = ts.getDeclarationModifierFlagsFromSymbol(targetProp);
                 if (sourcePropFlags & 8 /* Private */ || targetPropFlags & 8 /* Private */) {
@@ -50062,7 +50062,7 @@ var ts;
                     return 0 /* False */;
                 }
                 // When checking for comparability, be more lenient with optional properties.
-                if (relation !== comparableRelation && sourceProp.flags & 16777216 /* Optional */ && !(targetProp.flags & 16777216 /* Optional */)) {
+                if (!skipOptional && sourceProp.flags & 16777216 /* Optional */ && !(targetProp.flags & 16777216 /* Optional */)) {
                     // TypeScript 1.0 spec (April 2014): 3.8.3
                     // S is a subtype of a type T, and T is a supertype of S if ...
                     // S' and T are object types and, for each member M in T..
@@ -50187,7 +50187,7 @@ var ts;
                     if (!(targetProp.flags & 4194304 /* Prototype */) && (!numericNamesOnly || isNumericLiteralName(name) || name === "length")) {
                         var sourceProp = getPropertyOfType(source, name);
                         if (sourceProp && sourceProp !== targetProp) {
-                            var related = propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSymbol, reportErrors, intersectionState);
+                            var related = propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSymbol, reportErrors, intersectionState, relation === comparableRelation);
                             if (!related) {
                                 return 0 /* False */;
                             }
diff --git a/lib/tsserverlibrary.js b/lib/tsserverlibrary.js
index 6a69a6e8a20..b39b619c55c 100644
--- a/lib/tsserverlibrary.js
+++ b/lib/tsserverlibrary.js
@@ -50065,7 +50065,7 @@ var ts;
                             if (sourceProperty === targetProperty)
                                 return "continue";
                             // We compare the source property to the target in the context of a single discriminant type.
-                            var related = propertyRelatedTo(source, target, sourceProperty, targetProperty, function (_) { return combination[i]; }, /*reportErrors*/ false, 0 /* None */);
+                            var related = propertyRelatedTo(source, target, sourceProperty, targetProperty, function (_) { return combination[i]; }, /*reportErrors*/ false, 0 /* None */, /*skipOptional*/ strictNullChecks || relation === comparableRelation);
                             // If the target property could not be found, or if the properties were not related,
                             // then this constituent is not a match.
                             if (!related) {
@@ -50173,7 +50173,7 @@ var ts;
                     return isRelatedTo(source, addOptionality(getTypeOfSymbol(targetProp), targetIsOptional), reportErrors, /*headMessage*/ undefined, intersectionState);
                 }
             }
-            function propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSourceProperty, reportErrors, intersectionState) {
+            function propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSourceProperty, reportErrors, intersectionState, skipOptional) {
                 var sourcePropFlags = ts.getDeclarationModifierFlagsFromSymbol(sourceProp);
                 var targetPropFlags = ts.getDeclarationModifierFlagsFromSymbol(targetProp);
                 if (sourcePropFlags & 8 /* Private */ || targetPropFlags & 8 /* Private */) {
@@ -50212,7 +50212,7 @@ var ts;
                     return 0 /* False */;
                 }
                 // When checking for comparability, be more lenient with optional properties.
-                if (relation !== comparableRelation && sourceProp.flags & 16777216 /* Optional */ && !(targetProp.flags & 16777216 /* Optional */)) {
+                if (!skipOptional && sourceProp.flags & 16777216 /* Optional */ && !(targetProp.flags & 16777216 /* Optional */)) {
                     // TypeScript 1.0 spec (April 2014): 3.8.3
                     // S is a subtype of a type T, and T is a supertype of S if ...
                     // S' and T are object types and, for each member M in T..
@@ -50337,7 +50337,7 @@ var ts;
                     if (!(targetProp.flags & 4194304 /* Prototype */) && (!numericNamesOnly || isNumericLiteralName(name) || name === "length")) {
                         var sourceProp = getPropertyOfType(source, name);
                         if (sourceProp && sourceProp !== targetProp) {
-                            var related = propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSymbol, reportErrors, intersectionState);
+                            var related = propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSymbol, reportErrors, intersectionState, relation === comparableRelation);
                             if (!related) {
                                 return 0 /* False */;
                             }
diff --git a/lib/typescript.js b/lib/typescript.js
index ff8f7841af7..78cc5c394cb 100644
--- a/lib/typescript.js
+++ b/lib/typescript.js
@@ -50065,7 +50065,7 @@ var ts;
                             if (sourceProperty === targetProperty)
                                 return "continue";
                             // We compare the source property to the target in the context of a single discriminant type.
-                            var related = propertyRelatedTo(source, target, sourceProperty, targetProperty, function (_) { return combination[i]; }, /*reportErrors*/ false, 0 /* None */);
+                            var related = propertyRelatedTo(source, target, sourceProperty, targetProperty, function (_) { return combination[i]; }, /*reportErrors*/ false, 0 /* None */, /*skipOptional*/ strictNullChecks || relation === comparableRelation);
                             // If the target property could not be found, or if the properties were not related,
                             // then this constituent is not a match.
                             if (!related) {
@@ -50173,7 +50173,7 @@ var ts;
                     return isRelatedTo(source, addOptionality(getTypeOfSymbol(targetProp), targetIsOptional), reportErrors, /*headMessage*/ undefined, intersectionState);
                 }
             }
-            function propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSourceProperty, reportErrors, intersectionState) {
+            function propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSourceProperty, reportErrors, intersectionState, skipOptional) {
                 var sourcePropFlags = ts.getDeclarationModifierFlagsFromSymbol(sourceProp);
                 var targetPropFlags = ts.getDeclarationModifierFlagsFromSymbol(targetProp);
                 if (sourcePropFlags & 8 /* Private */ || targetPropFlags & 8 /* Private */) {
@@ -50212,7 +50212,7 @@ var ts;
                     return 0 /* False */;
                 }
                 // When checking for comparability, be more lenient with optional properties.
-                if (relation !== comparableRelation && sourceProp.flags & 16777216 /* Optional */ && !(targetProp.flags & 16777216 /* Optional */)) {
+                if (!skipOptional && sourceProp.flags & 16777216 /* Optional */ && !(targetProp.flags & 16777216 /* Optional */)) {
                     // TypeScript 1.0 spec (April 2014): 3.8.3
                     // S is a subtype of a type T, and T is a supertype of S if ...
                     // S' and T are object types and, for each member M in T..
@@ -50337,7 +50337,7 @@ var ts;
                     if (!(targetProp.flags & 4194304 /* Prototype */) && (!numericNamesOnly || isNumericLiteralName(name) || name === "length")) {
                         var sourceProp = getPropertyOfType(source, name);
                         if (sourceProp && sourceProp !== targetProp) {
-                            var related = propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSymbol, reportErrors, intersectionState);
+                            var related = propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSymbol, reportErrors, intersectionState, relation === comparableRelation);
                             if (!related) {
                                 return 0 /* False */;
                             }
diff --git a/lib/typescriptServices.js b/lib/typescriptServices.js
index d73da5100e8..d466177408b 100644
--- a/lib/typescriptServices.js
+++ b/lib/typescriptServices.js
@@ -50065,7 +50065,7 @@ var ts;
                             if (sourceProperty === targetProperty)
                                 return "continue";
                             // We compare the source property to the target in the context of a single discriminant type.
-                            var related = propertyRelatedTo(source, target, sourceProperty, targetProperty, function (_) { return combination[i]; }, /*reportErrors*/ false, 0 /* None */);
+                            var related = propertyRelatedTo(source, target, sourceProperty, targetProperty, function (_) { return combination[i]; }, /*reportErrors*/ false, 0 /* None */, /*skipOptional*/ strictNullChecks || relation === comparableRelation);
                             // If the target property could not be found, or if the properties were not related,
                             // then this constituent is not a match.
                             if (!related) {
@@ -50173,7 +50173,7 @@ var ts;
                     return isRelatedTo(source, addOptionality(getTypeOfSymbol(targetProp), targetIsOptional), reportErrors, /*headMessage*/ undefined, intersectionState);
                 }
             }
-            function propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSourceProperty, reportErrors, intersectionState) {
+            function propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSourceProperty, reportErrors, intersectionState, skipOptional) {
                 var sourcePropFlags = ts.getDeclarationModifierFlagsFromSymbol(sourceProp);
                 var targetPropFlags = ts.getDeclarationModifierFlagsFromSymbol(targetProp);
                 if (sourcePropFlags & 8 /* Private */ || targetPropFlags & 8 /* Private */) {
@@ -50212,7 +50212,7 @@ var ts;
                     return 0 /* False */;
                 }
                 // When checking for comparability, be more lenient with optional properties.
-                if (relation !== comparableRelation && sourceProp.flags & 16777216 /* Optional */ && !(targetProp.flags & 16777216 /* Optional */)) {
+                if (!skipOptional && sourceProp.flags & 16777216 /* Optional */ && !(targetProp.flags & 16777216 /* Optional */)) {
                     // TypeScript 1.0 spec (April 2014): 3.8.3
                     // S is a subtype of a type T, and T is a supertype of S if ...
                     // S' and T are object types and, for each member M in T..
@@ -50337,7 +50337,7 @@ var ts;
                     if (!(targetProp.flags & 4194304 /* Prototype */) && (!numericNamesOnly || isNumericLiteralName(name) || name === "length")) {
                         var sourceProp = getPropertyOfType(source, name);
                         if (sourceProp && sourceProp !== targetProp) {
-                            var related = propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSymbol, reportErrors, intersectionState);
+                            var related = propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSymbol, reportErrors, intersectionState, relation === comparableRelation);
                             if (!related) {
                                 return 0 /* False */;
                             }
diff --git a/lib/typingsInstaller.js b/lib/typingsInstaller.js
index d2593265096..84de64a8576 100644
--- a/lib/typingsInstaller.js
+++ b/lib/typingsInstaller.js
@@ -49904,7 +49904,7 @@ var ts;
                             if (sourceProperty === targetProperty)
                                 return "continue";
                             // We compare the source property to the target in the context of a single discriminant type.
-                            var related = propertyRelatedTo(source, target, sourceProperty, targetProperty, function (_) { return combination[i]; }, /*reportErrors*/ false, 0 /* None */);
+                            var related = propertyRelatedTo(source, target, sourceProperty, targetProperty, function (_) { return combination[i]; }, /*reportErrors*/ false, 0 /* None */, /*skipOptional*/ strictNullChecks || relation === comparableRelation);
                             // If the target property could not be found, or if the properties were not related,
                             // then this constituent is not a match.
                             if (!related) {
@@ -50012,7 +50012,7 @@ var ts;
                     return isRelatedTo(source, addOptionality(getTypeOfSymbol(targetProp), targetIsOptional), reportErrors, /*headMessage*/ undefined, intersectionState);
                 }
             }
-            function propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSourceProperty, reportErrors, intersectionState) {
+            function propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSourceProperty, reportErrors, intersectionState, skipOptional) {
                 var sourcePropFlags = ts.getDeclarationModifierFlagsFromSymbol(sourceProp);
                 var targetPropFlags = ts.getDeclarationModifierFlagsFromSymbol(targetProp);
                 if (sourcePropFlags & 8 /* Private */ || targetPropFlags & 8 /* Private */) {
@@ -50051,7 +50051,7 @@ var ts;
                     return 0 /* False */;
                 }
                 // When checking for comparability, be more lenient with optional properties.
-                if (relation !== comparableRelation && sourceProp.flags & 16777216 /* Optional */ && !(targetProp.flags & 16777216 /* Optional */)) {
+                if (!skipOptional && sourceProp.flags & 16777216 /* Optional */ && !(targetProp.flags & 16777216 /* Optional */)) {
                     // TypeScript 1.0 spec (April 2014): 3.8.3
                     // S is a subtype of a type T, and T is a supertype of S if ...
                     // S' and T are object types and, for each member M in T..
@@ -50176,7 +50176,7 @@ var ts;
                     if (!(targetProp.flags & 4194304 /* Prototype */) && (!numericNamesOnly || isNumericLiteralName(name) || name === "length")) {
                         var sourceProp = getPropertyOfType(source, name);
                         if (sourceProp && sourceProp !== targetProp) {
-                            var related = propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSymbol, reportErrors, intersectionState);
+                            var related = propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSymbol, reportErrors, intersectionState, relation === comparableRelation);
                             if (!related) {
                                 return 0 /* False */;
                             }
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 0bf27e3f07b..25e25e43757 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -16639,7 +16639,7 @@ namespace ts {
                             if (!targetProperty) continue outer;
                             if (sourceProperty === targetProperty) continue;
                             // We compare the source property to the target in the context of a single discriminant type.
-                            const related = propertyRelatedTo(source, target, sourceProperty, targetProperty, _ => combination[i], /*reportErrors*/ false, IntersectionState.None);
+                            const related = propertyRelatedTo(source, target, sourceProperty, targetProperty, _ => combination[i], /*reportErrors*/ false, IntersectionState.None, /*skipOptional*/ strictNullChecks || relation === comparableRelation);
                             // If the target property could not be found, or if the properties were not related,
                             // then this constituent is not a match.
                             if (!related) {
@@ -16737,7 +16737,7 @@ namespace ts {
                 }
             }
 
-            function propertyRelatedTo(source: Type, target: Type, sourceProp: Symbol, targetProp: Symbol, getTypeOfSourceProperty: (sym: Symbol) => Type, reportErrors: boolean, intersectionState: IntersectionState): Ternary {
+            function propertyRelatedTo(source: Type, target: Type, sourceProp: Symbol, targetProp: Symbol, getTypeOfSourceProperty: (sym: Symbol) => Type, reportErrors: boolean, intersectionState: IntersectionState, skipOptional: boolean): Ternary {
                 const sourcePropFlags = getDeclarationModifierFlagsFromSymbol(sourceProp);
                 const targetPropFlags = getDeclarationModifierFlagsFromSymbol(targetProp);
                 if (sourcePropFlags & ModifierFlags.Private || targetPropFlags & ModifierFlags.Private) {
@@ -16780,7 +16780,7 @@ namespace ts {
                     return Ternary.False;
                 }
                 // When checking for comparability, be more lenient with optional properties.
-                if (relation !== comparableRelation && sourceProp.flags & SymbolFlags.Optional && !(targetProp.flags & SymbolFlags.Optional)) {
+                if (!skipOptional && sourceProp.flags & SymbolFlags.Optional && !(targetProp.flags & SymbolFlags.Optional)) {
                     // TypeScript 1.0 spec (April 2014): 3.8.3
                     // S is a subtype of a type T, and T is a supertype of S if ...
                     // S' and T are object types and, for each member M in T..
@@ -16910,7 +16910,7 @@ namespace ts {
                     if (!(targetProp.flags & SymbolFlags.Prototype) && (!numericNamesOnly || isNumericLiteralName(name) || name === "length")) {
                         const sourceProp = getPropertyOfType(source, name);
                         if (sourceProp && sourceProp !== targetProp) {
-                            const related = propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSymbol, reportErrors, intersectionState);
+                            const related = propertyRelatedTo(source, target, sourceProp, targetProp, getTypeOfSymbol, reportErrors, intersectionState, relation === comparableRelation);
                             if (!related) {
                                 return Ternary.False;
                             }
diff --git a/tests/baselines/reference/unionRelationshipCheckPasses.js b/tests/baselines/reference/unionRelationshipCheckPasses.js
new file mode 100644
index 00000000000..65e90ed34e8
--- /dev/null
+++ b/tests/baselines/reference/unionRelationshipCheckPasses.js
@@ -0,0 +1,7 @@
+//// [unionRelationshipCheckPasses.ts]
+const item: { foo?: undefined } | { foo: number } = null as any as { foo?: number | undefined };
+
+
+//// [unionRelationshipCheckPasses.js]
+"use strict";
+var item = null;
diff --git a/tests/baselines/reference/unionRelationshipCheckPasses.symbols b/tests/baselines/reference/unionRelationshipCheckPasses.symbols
new file mode 100644
index 00000000000..3fda4578bdb
--- /dev/null
+++ b/tests/baselines/reference/unionRelationshipCheckPasses.symbols
@@ -0,0 +1,7 @@
+=== tests/cases/compiler/unionRelationshipCheckPasses.ts ===
+const item: { foo?: undefined } | { foo: number } = null as any as { foo?: number | undefined };
+>item : Symbol(item, Decl(unionRelationshipCheckPasses.ts, 0, 5))
+>foo : Symbol(foo, Decl(unionRelationshipCheckPasses.ts, 0, 13))
+>foo : Symbol(foo, Decl(unionRelationshipCheckPasses.ts, 0, 35))
+>foo : Symbol(foo, Decl(unionRelationshipCheckPasses.ts, 0, 68))
+
diff --git a/tests/baselines/reference/unionRelationshipCheckPasses.types b/tests/baselines/reference/unionRelationshipCheckPasses.types
new file mode 100644
index 00000000000..e0e582916bb
--- /dev/null
+++ b/tests/baselines/reference/unionRelationshipCheckPasses.types
@@ -0,0 +1,10 @@
+=== tests/cases/compiler/unionRelationshipCheckPasses.ts ===
+const item: { foo?: undefined } | { foo: number } = null as any as { foo?: number | undefined };
+>item : { foo?: undefined; } | { foo: number; }
+>foo : undefined
+>foo : number
+>null as any as { foo?: number | undefined } : { foo?: number | undefined; }
+>null as any : any
+>null : null
+>foo : number | undefined
+
diff --git a/tests/cases/compiler/unionRelationshipCheckPasses.ts b/tests/cases/compiler/unionRelationshipCheckPasses.ts
new file mode 100644
index 00000000000..cec4cffbf08
--- /dev/null
+++ b/tests/cases/compiler/unionRelationshipCheckPasses.ts
@@ -0,0 +1,2 @@
+// @strict: true
+const item: { foo?: undefined } | { foo: number } = null as any as { foo?: number | undefined };