Remove undefined from optional spread properties

Fixes #16509 by making the change from #15938 less strict. This is
technically a hole, but it's not as big a hole as before #15938.

src/compiler/checker.ts

@@ -7716,7 +7716,7 @@ namespace ts {
                         const declarations: Declaration[] = concatenate(leftProp.declarations, rightProp.declarations);
                         const flags = SymbolFlags.Property | (leftProp.flags & SymbolFlags.Optional);
                         const result = createSymbol(flags, leftProp.name);
-                        result.type = getUnionType([getTypeOfSymbol(leftProp), rightType]);
+                        result.type = getUnionType([getTypeOfSymbol(leftProp), getTypeWithFacts(rightType, TypeFacts.NEUndefined)]);
                         result.leftSpread = leftProp;
                         result.rightSpread = rightProp;
                         result.declarations = declarations;

tests/baselines/reference/objectSpreadStrictNull.errors.txt

@@ -1,11 +1,3 @@
-tests/cases/conformance/types/spread/objectSpreadStrictNull.ts(9,9): error TS2322: Type '{ sn: string | number | undefined; }' is not assignable to type '{ sn: string | number; }'.
-  Types of property 'sn' are incompatible.
-    Type 'string | number | undefined' is not assignable to type 'string | number'.
-      Type 'undefined' is not assignable to type 'string | number'.
-tests/cases/conformance/types/spread/objectSpreadStrictNull.ts(10,9): error TS2322: Type '{ sn: string | number | undefined; }' is not assignable to type '{ sn: string | number; }'.
-  Types of property 'sn' are incompatible.
-    Type 'string | number | undefined' is not assignable to type 'string | number'.
-      Type 'undefined' is not assignable to type 'string | number'.
 tests/cases/conformance/types/spread/objectSpreadStrictNull.ts(14,9): error TS2322: Type '{ sn: number | undefined; }' is not assignable to type '{ sn: string | number; }'.
   Types of property 'sn' are incompatible.
     Type 'number | undefined' is not assignable to type 'string | number'.
@@ -21,9 +13,13 @@ tests/cases/conformance/types/spread/objectSpreadStrictNull.ts(18,9): error TS23
 tests/cases/conformance/types/spread/objectSpreadStrictNull.ts(28,7): error TS2322: Type '{ title: undefined; yearReleased: number; }' is not assignable to type 'Movie'.
   Types of property 'title' are incompatible.
     Type 'undefined' is not assignable to type 'string'.
+tests/cases/conformance/types/spread/objectSpreadStrictNull.ts(42,5): error TS2322: Type '{ foo: number | undefined; bar: string | undefined; }' is not assignable to type 'Fields'.
+  Types of property 'foo' are incompatible.
+    Type 'number | undefined' is not assignable to type 'number'.
+      Type 'undefined' is not assignable to type 'number'.
 
 
-==== tests/cases/conformance/types/spread/objectSpreadStrictNull.ts (6 errors) ====
+==== tests/cases/conformance/types/spread/objectSpreadStrictNull.ts (5 errors) ====
     function f(
         definiteBoolean: { sn: boolean },
         definiteString: { sn: string },
@@ -33,17 +29,7 @@ tests/cases/conformance/types/spread/objectSpreadStrictNull.ts(28,7): error TS23
         undefinedNumber: { sn: number | undefined }) {
         // optional
         let optionalUnionStops: { sn: string | number } = { ...definiteBoolean, ...definiteString, ...optionalNumber };
-            ~~~~~~~~~~~~~~~~~~
-!!! error TS2322: Type '{ sn: string | number | undefined; }' is not assignable to type '{ sn: string | number; }'.
-!!! error TS2322:   Types of property 'sn' are incompatible.
-!!! error TS2322:     Type 'string | number | undefined' is not assignable to type 'string | number'.
-!!! error TS2322:       Type 'undefined' is not assignable to type 'string | number'.
         let optionalUnionDuplicates: { sn: string | number } = { ...definiteBoolean, ...definiteString, ...optionalString, ...optionalNumber };
-            ~~~~~~~~~~~~~~~~~~~~~~~
-!!! error TS2322: Type '{ sn: string | number | undefined; }' is not assignable to type '{ sn: string | number; }'.
-!!! error TS2322:   Types of property 'sn' are incompatible.
-!!! error TS2322:     Type 'string | number | undefined' is not assignable to type 'string | number'.
-!!! error TS2322:       Type 'undefined' is not assignable to type 'string | number'.
         let allOptional: { sn?: string | number } = { ...optionalString, ...optionalNumber };
     
         // undefined
@@ -81,4 +67,24 @@ tests/cases/conformance/types/spread/objectSpreadStrictNull.ts(28,7): error TS23
 !!! error TS2322: Type '{ title: undefined; yearReleased: number; }' is not assignable to type 'Movie'.
 !!! error TS2322:   Types of property 'title' are incompatible.
 !!! error TS2322:     Type 'undefined' is not assignable to type 'string'.
+    
+    interface Fields {
+        foo: number;
+        bar: string;
+    }
+    interface NearlyPartialFields {
+        foo: number | undefined;
+        bar: string | undefined;
+    }
+    function g(fields: Fields, partialFields: Partial<Fields>, nearlyPartialFields: NearlyPartialFields) {
+        // ok, undefined is stripped from optional properties when spread
+        fields = { ...fields, ...partialFields };
+        // error: not optional, undefined remains
+        fields = { ...fields, ...nearlyPartialFields };
+        ~~~~~~
+!!! error TS2322: Type '{ foo: number | undefined; bar: string | undefined; }' is not assignable to type 'Fields'.
+!!! error TS2322:   Types of property 'foo' are incompatible.
+!!! error TS2322:     Type 'number | undefined' is not assignable to type 'number'.
+!!! error TS2322:       Type 'undefined' is not assignable to type 'number'.
+    }
     

tests/baselines/reference/objectSpreadStrictNull.js

@@ -27,6 +27,21 @@ type Movie = {
 const m = { title: "The Matrix", yearReleased: 1999 };
 // should error here because title: undefined is not assignable to string
 const x: Movie = { ...m, title: undefined };
+
+interface Fields {
+    foo: number;
+    bar: string;
+}
+interface NearlyPartialFields {
+    foo: number | undefined;
+    bar: string | undefined;
+}
+function g(fields: Fields, partialFields: Partial<Fields>, nearlyPartialFields: NearlyPartialFields) {
+    // ok, undefined is stripped from optional properties when spread
+    fields = { ...fields, ...partialFields };
+    // error: not optional, undefined remains
+    fields = { ...fields, ...nearlyPartialFields };
+}
 
 
 //// [objectSpreadStrictNull.js]
@@ -52,3 +67,9 @@ function f(definiteBoolean, definiteString, optionalString, optionalNumber, unde
 var m = { title: "The Matrix", yearReleased: 1999 };
 // should error here because title: undefined is not assignable to string
 var x = __assign({}, m, { title: undefined });
+function g(fields, partialFields, nearlyPartialFields) {
+    // ok, undefined is stripped from optional properties when spread
+    fields = __assign({}, fields, partialFields);
+    // error: not optional, undefined remains
+    fields = __assign({}, fields, nearlyPartialFields);
+}

tests/cases/conformance/types/spread/objectSpreadStrictNull.ts

@@ -28,3 +28,18 @@ type Movie = {
 const m = { title: "The Matrix", yearReleased: 1999 };
 // should error here because title: undefined is not assignable to string
 const x: Movie = { ...m, title: undefined };
+
+interface Fields {
+    foo: number;
+    bar: string;
+}
+interface NearlyPartialFields {
+    foo: number | undefined;
+    bar: string | undefined;
+}
+function g(fields: Fields, partialFields: Partial<Fields>, nearlyPartialFields: NearlyPartialFields) {
+    // ok, undefined is stripped from optional properties when spread
+    fields = { ...fields, ...partialFields };
+    // error: not optional, undefined remains
+    fields = { ...fields, ...nearlyPartialFields };
+}