Make spread overwrite error more lenient.

Fixes #46463, but I don't thik it's a good fix. See
https://github.com/microsoft/TypeScript/issues/46463#issuecomment-1033956377
for discussion.

src/compiler/checker.ts

@@ -27593,12 +27593,11 @@ namespace ts {
 
         function checkSpreadPropOverrides(type: Type, props: SymbolTable, spread: SpreadAssignment | JsxSpreadAttribute) {
             for (const right of getPropertiesOfType(type)) {
-                if (!(right.flags & SymbolFlags.Optional)) {
-                    const left = props.get(right.escapedName);
-                    if (left) {
-                        const diagnostic = error(left.valueDeclaration, Diagnostics._0_is_specified_more_than_once_so_this_usage_will_be_overwritten, unescapeLeadingUnderscores(left.escapedName));
-                        addRelatedInfo(diagnostic, createDiagnosticForNode(spread, Diagnostics.This_spread_always_overwrites_this_property));
-                    }
+                const rightType = getTypeOfSymbol(right);
+                const left = props.get(right.escapedName);
+                if (left && (exactOptionalPropertyTypes || !maybeTypeOfKind(rightType, TypeFlags.Nullable)) && !(right.flags & SymbolFlags.Optional)) {
+                    const diagnostic = error(left.valueDeclaration, Diagnostics._0_is_specified_more_than_once_so_this_usage_will_be_overwritten, unescapeLeadingUnderscores(left.escapedName));
+                    addRelatedInfo(diagnostic, createDiagnosticForNode(spread, Diagnostics.This_spread_always_overwrites_this_property));
                 }
             }
         }

tests/baselines/reference/spreadDuplicate.errors.txt

@@ -1,8 +1,7 @@
 tests/cases/conformance/types/spread/spreadDuplicate.ts(10,12): error TS2783: 'a' is specified more than once, so this usage will be overwritten.
-tests/cases/conformance/types/spread/spreadDuplicate.ts(12,12): error TS2783: 'a' is specified more than once, so this usage will be overwritten.
 
 
-==== tests/cases/conformance/types/spread/spreadDuplicate.ts (2 errors) ====
+==== tests/cases/conformance/types/spread/spreadDuplicate.ts (1 errors) ====
     // Repro from #44438
     
     declare let a: { a: string };
@@ -18,13 +17,17 @@ tests/cases/conformance/types/spread/spreadDuplicate.ts(12,12): error TS2783: 'a
 !!! related TS2785 tests/cases/conformance/types/spread/spreadDuplicate.ts:10:20: This spread always overwrites this property.
     let b1 = { a: 123, ...b };  // string | number
     let c1 = { a: 123, ...c };  // string | undefined (Error)
-               ~~~~~~
-!!! error TS2783: 'a' is specified more than once, so this usage will be overwritten.
-!!! related TS2785 tests/cases/conformance/types/spread/spreadDuplicate.ts:12:20: This spread always overwrites this property.
     let d1 = { a: 123, ...d };  // string | number
     
     let a2 = { a: 123, ...(t ? a : {}) };  // string | number
     let b2 = { a: 123, ...(t ? b : {}) };  // string | number
     let c2 = { a: 123, ...(t ? c : {}) };  // string | number
     let d2 = { a: 123, ...(t ? d : {}) };  // string | number
+    
+    // from #46463
+    declare const conditional: boolean;
+    const example = {
+        color: "red",  // OK
+        ...(conditional ? { color: "green" } : { size: "large" })
+    }
     

tests/baselines/reference/spreadDuplicate.js

@@ -17,6 +17,13 @@ let a2 = { a: 123, ...(t ? a : {}) };  // string | number
 let b2 = { a: 123, ...(t ? b : {}) };  // string | number
 let c2 = { a: 123, ...(t ? c : {}) };  // string | number
 let d2 = { a: 123, ...(t ? d : {}) };  // string | number
+
+// from #46463
+declare const conditional: boolean;
+const example = {
+    color: "red",  // OK
+    ...(conditional ? { color: "green" } : { size: "large" })
+}
 
 
 //// [spreadDuplicate.js]
@@ -41,6 +48,7 @@ var a2 = __assign({ a: 123 }, (t ? a : {})); // string | number
 var b2 = __assign({ a: 123 }, (t ? b : {})); // string | number
 var c2 = __assign({ a: 123 }, (t ? c : {})); // string | number
 var d2 = __assign({ a: 123 }, (t ? d : {})); // string | number
+var example = __assign({ color: "red" }, (conditional ? { color: "green" } : { size: "large" }));
 
 
 //// [spreadDuplicate.d.ts]
@@ -81,3 +89,10 @@ declare let c2: {
 declare let d2: {
     a: string | number;
 };
+declare const conditional: boolean;
+declare const example: {
+    color: string;
+} | {
+    size: string;
+    color: string;
+};

tests/baselines/reference/spreadDuplicate.symbols

@@ -64,3 +64,19 @@ let d2 = { a: 123, ...(t ? d : {}) };  // string | number
 >t : Symbol(t, Decl(spreadDuplicate.ts, 7, 11))
 >d : Symbol(d, Decl(spreadDuplicate.ts, 5, 11))
 
+// from #46463
+declare const conditional: boolean;
+>conditional : Symbol(conditional, Decl(spreadDuplicate.ts, 20, 13))
+
+const example = {
+>example : Symbol(example, Decl(spreadDuplicate.ts, 21, 5))
+
+    color: "red",  // OK
+>color : Symbol(color, Decl(spreadDuplicate.ts, 21, 17))
+
+    ...(conditional ? { color: "green" } : { size: "large" })
+>conditional : Symbol(conditional, Decl(spreadDuplicate.ts, 20, 13))
+>color : Symbol(color, Decl(spreadDuplicate.ts, 23, 23))
+>size : Symbol(size, Decl(spreadDuplicate.ts, 23, 44))
+}
+

tests/baselines/reference/spreadDuplicate.types

@@ -92,3 +92,27 @@ let d2 = { a: 123, ...(t ? d : {}) };  // string | number
 >d : { a?: string | undefined; }
 >{} : {}
 
+// from #46463
+declare const conditional: boolean;
+>conditional : boolean
+
+const example = {
+>example : { color: string; } | { size: string; color: string; }
+>{    color: "red",  // OK    ...(conditional ? { color: "green" } : { size: "large" })} : { color: string; } | { size: string; color: string; }
+
+    color: "red",  // OK
+>color : string
+>"red" : "red"
+
+    ...(conditional ? { color: "green" } : { size: "large" })
+>(conditional ? { color: "green" } : { size: "large" }) : { color: string; } | { size: string; }
+>conditional ? { color: "green" } : { size: "large" } : { color: string; } | { size: string; }
+>conditional : boolean
+>{ color: "green" } : { color: string; }
+>color : string
+>"green" : "green"
+>{ size: "large" } : { size: string; }
+>size : string
+>"large" : "large"
+}
+

tests/baselines/reference/spreadDuplicateExact.errors.txt

@@ -1,8 +1,9 @@
 tests/cases/conformance/types/spread/spreadDuplicateExact.ts(10,12): error TS2783: 'a' is specified more than once, so this usage will be overwritten.
 tests/cases/conformance/types/spread/spreadDuplicateExact.ts(12,12): error TS2783: 'a' is specified more than once, so this usage will be overwritten.
+tests/cases/conformance/types/spread/spreadDuplicateExact.ts(23,5): error TS2783: 'color' is specified more than once, so this usage will be overwritten.
 
 
-==== tests/cases/conformance/types/spread/spreadDuplicateExact.ts (2 errors) ====
+==== tests/cases/conformance/types/spread/spreadDuplicateExact.ts (3 errors) ====
     // Repro from #44438
     
     declare let a: { a: string };
@@ -27,4 +28,14 @@ tests/cases/conformance/types/spread/spreadDuplicateExact.ts(12,12): error TS278
     let b2 = { a: 123, ...(t ? b : {}) };  // string | number
     let c2 = { a: 123, ...(t ? c : {}) };  // string | number | undefined
     let d2 = { a: 123, ...(t ? d : {}) };  // string | number | undefined
+    
+    // from #46463
+    declare const conditional: boolean;
+    const example = {
+        color: "red",  // error
+        ~~~~~~~~~~~~
+!!! error TS2783: 'color' is specified more than once, so this usage will be overwritten.
+!!! related TS2785 tests/cases/conformance/types/spread/spreadDuplicateExact.ts:24:5: This spread always overwrites this property.
+        ...(conditional ? { color: "green" } : { size: "large" })
+    }
     

tests/baselines/reference/spreadDuplicateExact.js

@@ -17,6 +17,13 @@ let a2 = { a: 123, ...(t ? a : {}) };  // string | number
 let b2 = { a: 123, ...(t ? b : {}) };  // string | number
 let c2 = { a: 123, ...(t ? c : {}) };  // string | number | undefined
 let d2 = { a: 123, ...(t ? d : {}) };  // string | number | undefined
+
+// from #46463
+declare const conditional: boolean;
+const example = {
+    color: "red",  // error
+    ...(conditional ? { color: "green" } : { size: "large" })
+}
 
 
 //// [spreadDuplicateExact.js]
@@ -41,6 +48,7 @@ var a2 = __assign({ a: 123 }, (t ? a : {})); // string | number
 var b2 = __assign({ a: 123 }, (t ? b : {})); // string | number
 var c2 = __assign({ a: 123 }, (t ? c : {})); // string | number | undefined
 var d2 = __assign({ a: 123 }, (t ? d : {})); // string | number | undefined
+var example = __assign({ color: "red" }, (conditional ? { color: "green" } : { size: "large" }));
 
 
 //// [spreadDuplicateExact.d.ts]
@@ -81,3 +89,10 @@ declare let c2: {
 declare let d2: {
     a: string | number | undefined;
 };
+declare const conditional: boolean;
+declare const example: {
+    color: string;
+} | {
+    size: string;
+    color: string;
+};

tests/baselines/reference/spreadDuplicateExact.symbols

@@ -64,3 +64,19 @@ let d2 = { a: 123, ...(t ? d : {}) };  // string | number | undefined
 >t : Symbol(t, Decl(spreadDuplicateExact.ts, 7, 11))
 >d : Symbol(d, Decl(spreadDuplicateExact.ts, 5, 11))
 
+// from #46463
+declare const conditional: boolean;
+>conditional : Symbol(conditional, Decl(spreadDuplicateExact.ts, 20, 13))
+
+const example = {
+>example : Symbol(example, Decl(spreadDuplicateExact.ts, 21, 5))
+
+    color: "red",  // error
+>color : Symbol(color, Decl(spreadDuplicateExact.ts, 21, 17))
+
+    ...(conditional ? { color: "green" } : { size: "large" })
+>conditional : Symbol(conditional, Decl(spreadDuplicateExact.ts, 20, 13))
+>color : Symbol(color, Decl(spreadDuplicateExact.ts, 23, 23))
+>size : Symbol(size, Decl(spreadDuplicateExact.ts, 23, 44))
+}
+

tests/baselines/reference/spreadDuplicateExact.types

@@ -92,3 +92,27 @@ let d2 = { a: 123, ...(t ? d : {}) };  // string | number | undefined
 >d : { a?: string | undefined; }
 >{} : {}
 
+// from #46463
+declare const conditional: boolean;
+>conditional : boolean
+
+const example = {
+>example : { color: string; } | { size: string; color: string; }
+>{    color: "red",  // error    ...(conditional ? { color: "green" } : { size: "large" })} : { color: string; } | { size: string; color: string; }
+
+    color: "red",  // error
+>color : string
+>"red" : "red"
+
+    ...(conditional ? { color: "green" } : { size: "large" })
+>(conditional ? { color: "green" } : { size: "large" }) : { color: string; } | { size: string; }
+>conditional ? { color: "green" } : { size: "large" } : { color: string; } | { size: string; }
+>conditional : boolean
+>{ color: "green" } : { color: string; }
+>color : string
+>"green" : "green"
+>{ size: "large" } : { size: string; }
+>size : string
+>"large" : "large"
+}
+

tests/baselines/reference/spreadOverwritesPropertyStrict.errors.txt

@@ -1,11 +1,10 @@
 tests/cases/conformance/types/spread/spreadOverwritesPropertyStrict.ts(3,17): error TS2783: 'b' is specified more than once, so this usage will be overwritten.
-tests/cases/conformance/types/spread/spreadOverwritesPropertyStrict.ts(9,14): error TS2783: 'x' is specified more than once, so this usage will be overwritten.
 tests/cases/conformance/types/spread/spreadOverwritesPropertyStrict.ts(15,14): error TS2783: 'x' is specified more than once, so this usage will be overwritten.
 tests/cases/conformance/types/spread/spreadOverwritesPropertyStrict.ts(24,14): error TS2783: 'command' is specified more than once, so this usage will be overwritten.
 tests/cases/conformance/types/spread/spreadOverwritesPropertyStrict.ts(28,14): error TS2783: 'a' is specified more than once, so this usage will be overwritten.
 
 
-==== tests/cases/conformance/types/spread/spreadOverwritesPropertyStrict.ts (5 errors) ====
+==== tests/cases/conformance/types/spread/spreadOverwritesPropertyStrict.ts (4 errors) ====
     declare var ab: { a: number, b: number };
     declare var abq: { a: number, b?: number };
     var unused1 = { b: 1, ...ab } // error
@@ -18,9 +17,6 @@ tests/cases/conformance/types/spread/spreadOverwritesPropertyStrict.ts(28,14): e
     var unused5 = { ...abq, b: 1 } // ok
     function g(obj: { x: number | undefined }) {
         return { x: 1, ...obj }; // ok, obj might have x: undefined
-                 ~~~~
-!!! error TS2783: 'x' is specified more than once, so this usage will be overwritten.
-!!! related TS2785 tests/cases/conformance/types/spread/spreadOverwritesPropertyStrict.ts:9:20: This spread always overwrites this property.
     }
     function f(obj: { x: number } | undefined) {
         return { x: 1, ...obj }; // ok, obj might be undefined

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

@@ -19,3 +19,10 @@ let a2 = { a: 123, ...(t ? a : {}) };  // string | number
 let b2 = { a: 123, ...(t ? b : {}) };  // string | number
 let c2 = { a: 123, ...(t ? c : {}) };  // string | number
 let d2 = { a: 123, ...(t ? d : {}) };  // string | number
+
+// from #46463
+declare const conditional: boolean;
+const example = {
+    color: "red",  // OK
+    ...(conditional ? { color: "green" } : { size: "large" })
+}

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

@@ -20,3 +20,10 @@ let a2 = { a: 123, ...(t ? a : {}) };  // string | number
 let b2 = { a: 123, ...(t ? b : {}) };  // string | number
 let c2 = { a: 123, ...(t ? c : {}) };  // string | number | undefined
 let d2 = { a: 123, ...(t ? d : {}) };  // string | number | undefined
+
+// from #46463
+declare const conditional: boolean;
+const example = {
+    color: "red",  // error
+    ...(conditional ? { color: "green" } : { size: "large" })
+}