Set-only accessors spread to undefined (#28213)

* Set-only accessors spread to undefined

Previously they were skipped. The runtime behaviour is to create a
property of type undefined, unlike (for example) spreading numbers or
other primitives. So now spreading a set-only accessor creates a
property of type undefined:

```ts
const o: { foo: undefined } = { ...{ set foo(v: number) { } } }
```

Notably, `o.foo: undefined` not `number`.

Fixes #26337

* Fix isSpreadableProperty oversimplification

src/compiler/checker.ts

@@ -4608,7 +4608,7 @@ namespace ts {
                 if (!names.has(prop.escapedName)
                     && !(getDeclarationModifierFlagsFromSymbol(prop) & (ModifierFlags.Private | ModifierFlags.Protected))
                     && isSpreadableProperty(prop)) {
-                    members.set(prop.escapedName, getNonReadonlySymbol(prop));
+                    members.set(prop.escapedName, getSpreadSymbol(prop));
                 }
             }
             const stringIndexInfo = getIndexInfoOfType(source, IndexKind.String);
@@ -9887,7 +9887,7 @@ namespace ts {
                     skippedPrivateMembers.set(rightProp.escapedName, true);
                 }
                 else if (isSpreadableProperty(rightProp)) {
-                    members.set(rightProp.escapedName, getNonReadonlySymbol(rightProp));
+                    members.set(rightProp.escapedName, getSpreadSymbol(rightProp));
                 }
             }
 
@@ -9911,7 +9911,7 @@ namespace ts {
                     }
                 }
                 else {
-                    members.set(leftProp.escapedName, getNonReadonlySymbol(leftProp));
+                    members.set(leftProp.escapedName, getSpreadSymbol(leftProp));
                 }
             }
 
@@ -9929,18 +9929,19 @@ namespace ts {
 
         /** We approximate own properties as non-methods plus methods that are inside the object literal */
         function isSpreadableProperty(prop: Symbol): boolean {
-            return prop.flags & (SymbolFlags.Method | SymbolFlags.GetAccessor)
-                ? !prop.declarations.some(decl => isClassLike(decl.parent))
-                : !(prop.flags & SymbolFlags.SetAccessor); // Setter without getter is not spreadable
+            return !(prop.flags & (SymbolFlags.Method | SymbolFlags.GetAccessor | SymbolFlags.SetAccessor)) ||
+                !prop.declarations.some(decl => isClassLike(decl.parent));
         }
 
-        function getNonReadonlySymbol(prop: Symbol) {
-            if (!isReadonlySymbol(prop)) {
+        function getSpreadSymbol(prop: Symbol) {
+            const isReadonly = isReadonlySymbol(prop);
+            const isSetonlyAccessor = prop.flags & SymbolFlags.SetAccessor && !(prop.flags & SymbolFlags.GetAccessor);
+            if (!isReadonly && !isSetonlyAccessor) {
                 return prop;
             }
             const flags = SymbolFlags.Property | (prop.flags & SymbolFlags.Optional);
             const result = createSymbol(flags, prop.escapedName);
-            result.type = getTypeOfSymbol(prop);
+            result.type = isSetonlyAccessor ? undefinedType : getTypeOfSymbol(prop);
             result.declarations = prop.declarations;
             result.nameType = prop.nameType;
             result.syntheticOrigin = prop;

tests/baselines/reference/objectSpreadNegative.errors.txt

@@ -13,7 +13,7 @@ tests/cases/conformance/types/spread/objectSpreadNegative.ts(34,20): error TS269
 tests/cases/conformance/types/spread/objectSpreadNegative.ts(36,20): error TS2698: Spread types may only be created from object types.
 tests/cases/conformance/types/spread/objectSpreadNegative.ts(38,19): error TS2698: Spread types may only be created from object types.
 tests/cases/conformance/types/spread/objectSpreadNegative.ts(43,1): error TS2349: Cannot invoke an expression whose type lacks a call signature. Type '{}' has no compatible call signatures.
-tests/cases/conformance/types/spread/objectSpreadNegative.ts(47,12): error TS2339: Property 'b' does not exist on type '{}'.
+tests/cases/conformance/types/spread/objectSpreadNegative.ts(47,1): error TS2322: Type '12' is not assignable to type 'undefined'.
 tests/cases/conformance/types/spread/objectSpreadNegative.ts(53,9): error TS2339: Property 'm' does not exist on type '{ p: number; }'.
 tests/cases/conformance/types/spread/objectSpreadNegative.ts(58,11): error TS2339: Property 'a' does not exist on type '{}'.
 tests/cases/conformance/types/spread/objectSpreadNegative.ts(62,14): error TS2698: Spread types may only be created from object types.
@@ -95,8 +95,8 @@ tests/cases/conformance/types/spread/objectSpreadNegative.ts(65,14): error TS269
     // write-only properties get skipped
     let setterOnly = { ...{ set b (bad: number) { } } };
     setterOnly.b = 12; // error, 'b' does not exist
-               ~
-!!! error TS2339: Property 'b' does not exist on type '{}'.
+    ~~~~~~~~~~~~
+!!! error TS2322: Type '12' is not assignable to type 'undefined'.
     
     // methods are skipped because they aren't enumerable
     class C { p = 1; m() { } }

tests/baselines/reference/objectSpreadNegative.symbols

@@ -132,7 +132,9 @@ let setterOnly = { ...{ set b (bad: number) { } } };
 >bad : Symbol(bad, Decl(objectSpreadNegative.ts, 45, 31))
 
 setterOnly.b = 12; // error, 'b' does not exist
+>setterOnly.b : Symbol(b, Decl(objectSpreadNegative.ts, 45, 23))
 >setterOnly : Symbol(setterOnly, Decl(objectSpreadNegative.ts, 45, 3))
+>b : Symbol(b, Decl(objectSpreadNegative.ts, 45, 23))
 
 // methods are skipped because they aren't enumerable
 class C { p = 1; m() { } }

tests/baselines/reference/objectSpreadNegative.types

@@ -173,17 +173,17 @@ spreadFunc(); // error, no call signature
 
 // write-only properties get skipped
 let setterOnly = { ...{ set b (bad: number) { } } };
->setterOnly : {}
->{ ...{ set b (bad: number) { } } } : {}
+>setterOnly : { b: undefined; }
+>{ ...{ set b (bad: number) { } } } : { b: undefined; }
 >{ set b (bad: number) { } } : { b: number; }
 >b : number
 >bad : number
 
 setterOnly.b = 12; // error, 'b' does not exist
 >setterOnly.b = 12 : 12
->setterOnly.b : any
->setterOnly : {}
->b : any
+>setterOnly.b : undefined
+>setterOnly : { b: undefined; }
+>b : undefined
 >12 : 12
 
 // methods are skipped because they aren't enumerable

tests/baselines/reference/objectSpreadSetonlyAccessor.js

@@ -0,0 +1,9 @@
+//// [objectSpreadSetonlyAccessor.ts]
+const o1: { foo: number, bar: undefined } = { foo: 1, ... { set bar(_v: number) { } } }
+const o2: { foo: undefined } = { foo: 1, ... { set foo(_v: number) { } } }
+
+
+//// [objectSpreadSetonlyAccessor.js]
+"use strict";
+const o1 = { foo: 1, ...{ set bar(_v) { } } };
+const o2 = { foo: 1, ...{ set foo(_v) { } } };

tests/baselines/reference/objectSpreadSetonlyAccessor.symbols

@@ -0,0 +1,16 @@
+=== tests/cases/conformance/types/spread/objectSpreadSetonlyAccessor.ts ===
+const o1: { foo: number, bar: undefined } = { foo: 1, ... { set bar(_v: number) { } } }
+>o1 : Symbol(o1, Decl(objectSpreadSetonlyAccessor.ts, 0, 5))
+>foo : Symbol(foo, Decl(objectSpreadSetonlyAccessor.ts, 0, 11))
+>bar : Symbol(bar, Decl(objectSpreadSetonlyAccessor.ts, 0, 24))
+>foo : Symbol(foo, Decl(objectSpreadSetonlyAccessor.ts, 0, 45))
+>bar : Symbol(bar, Decl(objectSpreadSetonlyAccessor.ts, 0, 59))
+>_v : Symbol(_v, Decl(objectSpreadSetonlyAccessor.ts, 0, 68))
+
+const o2: { foo: undefined } = { foo: 1, ... { set foo(_v: number) { } } }
+>o2 : Symbol(o2, Decl(objectSpreadSetonlyAccessor.ts, 1, 5))
+>foo : Symbol(foo, Decl(objectSpreadSetonlyAccessor.ts, 1, 11))
+>foo : Symbol(foo, Decl(objectSpreadSetonlyAccessor.ts, 1, 32))
+>foo : Symbol(foo, Decl(objectSpreadSetonlyAccessor.ts, 1, 46))
+>_v : Symbol(_v, Decl(objectSpreadSetonlyAccessor.ts, 1, 55))
+

tests/baselines/reference/objectSpreadSetonlyAccessor.types

@@ -0,0 +1,22 @@
+=== tests/cases/conformance/types/spread/objectSpreadSetonlyAccessor.ts ===
+const o1: { foo: number, bar: undefined } = { foo: 1, ... { set bar(_v: number) { } } }
+>o1 : { foo: number; bar: undefined; }
+>foo : number
+>bar : undefined
+>{ foo: 1, ... { set bar(_v: number) { } } } : { bar: undefined; foo: number; }
+>foo : number
+>1 : 1
+>{ set bar(_v: number) { } } : { bar: number; }
+>bar : number
+>_v : number
+
+const o2: { foo: undefined } = { foo: 1, ... { set foo(_v: number) { } } }
+>o2 : { foo: undefined; }
+>foo : undefined
+>{ foo: 1, ... { set foo(_v: number) { } } } : { foo: undefined; }
+>foo : number
+>1 : 1
+>{ set foo(_v: number) { } } : { foo: number; }
+>foo : number
+>_v : number
+

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

@@ -0,0 +1,4 @@
+// @strict: true
+// @target: esnext
+const o1: { foo: number, bar: undefined } = { foo: 1, ... { set bar(_v: number) { } } }
+const o2: { foo: undefined } = { foo: 1, ... { set foo(_v: number) { } } }