Allow partial union props to contain the undefined type

Author: weswigham

src/compiler/checker.ts

@@ -13201,7 +13201,8 @@ namespace ts {
                                 }
                                 return Ternary.False;
                             }
-                            const related = isRelatedTo(getTypeOfSymbol(sourceProp), getTypeOfSymbol(targetProp), reportErrors);
+                            // If the target comes from a partial union prop, allow `undefined` in the target type
+                            const related = isRelatedTo(getTypeOfSymbol(sourceProp), addOptionality(getTypeOfSymbol(targetProp), !!(getCheckFlags(targetProp) & CheckFlags.Partial)), reportErrors);
                             if (!related) {
                                 if (reportErrors) {
                                     reportError(Diagnostics.Types_of_property_0_are_incompatible, symbolToString(targetProp));

tests/baselines/reference/unionExcessPropsWithPartialMember.js

@@ -0,0 +1,31 @@
+//// [unionExcessPropsWithPartialMember.ts]
+interface A {
+    unused?: string;
+    x: string;
+}
+
+interface B {
+    x: string;
+    y: string;
+}
+
+declare var ab: A | B;
+declare var a: A;
+
+ab = {...a, y: (null as any as string | undefined)}; // Should be allowed, since `y` is missing on `A`
+
+
+//// [unionExcessPropsWithPartialMember.js]
+"use strict";
+var __assign = (this && this.__assign) || function () {
+    __assign = Object.assign || function(t) {
+        for (var s, i = 1, n = arguments.length; i < n; i++) {
+            s = arguments[i];
+            for (var p in s) if (Object.prototype.hasOwnProperty.call(s, p))
+                t[p] = s[p];
+        }
+        return t;
+    };
+    return __assign.apply(this, arguments);
+};
+ab = __assign({}, a, { y: null }); // Should be allowed, since `y` is missing on `A`

tests/baselines/reference/unionExcessPropsWithPartialMember.symbols

@@ -0,0 +1,35 @@
+=== tests/cases/compiler/unionExcessPropsWithPartialMember.ts ===
+interface A {
+>A : Symbol(A, Decl(unionExcessPropsWithPartialMember.ts, 0, 0))
+
+    unused?: string;
+>unused : Symbol(A.unused, Decl(unionExcessPropsWithPartialMember.ts, 0, 13))
+
+    x: string;
+>x : Symbol(A.x, Decl(unionExcessPropsWithPartialMember.ts, 1, 20))
+}
+
+interface B {
+>B : Symbol(B, Decl(unionExcessPropsWithPartialMember.ts, 3, 1))
+
+    x: string;
+>x : Symbol(B.x, Decl(unionExcessPropsWithPartialMember.ts, 5, 13))
+
+    y: string;
+>y : Symbol(B.y, Decl(unionExcessPropsWithPartialMember.ts, 6, 14))
+}
+
+declare var ab: A | B;
+>ab : Symbol(ab, Decl(unionExcessPropsWithPartialMember.ts, 10, 11))
+>A : Symbol(A, Decl(unionExcessPropsWithPartialMember.ts, 0, 0))
+>B : Symbol(B, Decl(unionExcessPropsWithPartialMember.ts, 3, 1))
+
+declare var a: A;
+>a : Symbol(a, Decl(unionExcessPropsWithPartialMember.ts, 11, 11))
+>A : Symbol(A, Decl(unionExcessPropsWithPartialMember.ts, 0, 0))
+
+ab = {...a, y: (null as any as string | undefined)}; // Should be allowed, since `y` is missing on `A`
+>ab : Symbol(ab, Decl(unionExcessPropsWithPartialMember.ts, 10, 11))
+>a : Symbol(a, Decl(unionExcessPropsWithPartialMember.ts, 11, 11))
+>y : Symbol(y, Decl(unionExcessPropsWithPartialMember.ts, 13, 11))
+

tests/baselines/reference/unionExcessPropsWithPartialMember.types

@@ -0,0 +1,34 @@
+=== tests/cases/compiler/unionExcessPropsWithPartialMember.ts ===
+interface A {
+    unused?: string;
+>unused : string | undefined
+
+    x: string;
+>x : string
+}
+
+interface B {
+    x: string;
+>x : string
+
+    y: string;
+>y : string
+}
+
+declare var ab: A | B;
+>ab : A | B
+
+declare var a: A;
+>a : A
+
+ab = {...a, y: (null as any as string | undefined)}; // Should be allowed, since `y` is missing on `A`
+>ab = {...a, y: (null as any as string | undefined)} : { y: string | undefined; unused?: string | undefined; x: string; }
+>ab : A | B
+>{...a, y: (null as any as string | undefined)} : { y: string | undefined; unused?: string | undefined; x: string; }
+>a : A
+>y : string | undefined
+>(null as any as string | undefined) : string | undefined
+>null as any as string | undefined : string | undefined
+>null as any : any
+>null : null
+

tests/cases/compiler/unionExcessPropsWithPartialMember.ts

@@ -0,0 +1,15 @@
+// @strict: true
+interface A {
+    unused?: string;
+    x: string;
+}
+
+interface B {
+    x: string;
+    y: string;
+}
+
+declare var ab: A | B;
+declare var a: A;
+
+ab = {...a, y: (null as any as string | undefined)}; // Should be allowed, since `y` is missing on `A`