Fix rest tuple arity check

src/compiler/checker.ts

@@ -20132,10 +20132,7 @@ namespace ts {
                     if (pos - paramCount < getLengthOfTupleType(restType)) {
                         return restType.typeArguments![pos - paramCount];
                     }
-                    const tupleRestType = getRestTypeOfTupleType(restType);
-                    if (tupleRestType) {
-                        return tupleRestType;
-                    }
+                    return getRestTypeOfTupleType(restType);
                 }
                 return getIndexTypeOfType(restType, IndexKind.Number);
             }

tests/baselines/reference/genericRestParameters1.errors.txt

@@ -1,8 +1,11 @@
 tests/cases/conformance/types/rest/genericRestParameters1.ts(22,1): error TS2556: Expected 3 arguments, but got 1 or more.
 tests/cases/conformance/types/rest/genericRestParameters1.ts(31,1): error TS2556: Expected 3 arguments, but got 1 or more.
+tests/cases/conformance/types/rest/genericRestParameters1.ts(166,1): error TS2322: Type '(a: never) => void' is not assignable to type '(...args: any[]) => void'.
+  Types of parameters 'a' and 'args' are incompatible.
+    Type 'any' is not assignable to type 'never'.
 
 
-==== tests/cases/conformance/types/rest/genericRestParameters1.ts (2 errors) ====
+==== tests/cases/conformance/types/rest/genericRestParameters1.ts (3 errors) ====
     declare let f1: (...x: [number, string, boolean]) => void;
     declare let f2: (x0: number, x1: string, x2: boolean) => void;
 
@@ -161,4 +164,20 @@ tests/cases/conformance/types/rest/genericRestParameters1.ts(31,1): error TS2556
     events.emit('jump', 20, 'up');
     events.emit('stop', 'Bye!');
     events.emit('done');
+
+    // Repro from #25871
+
+    declare var ff1: (... args: any[]) => void;
+
+    declare var ff2: () => void;
+    declare var ff3: (...args: []) => void;
+    declare var ff4: (a: never) => void;
+
+    ff1 = ff2;
+    ff1 = ff3;
+    ff1 = ff4;  // Error
+    ~~~
+!!! error TS2322: Type '(a: never) => void' is not assignable to type '(...args: any[]) => void'.
+!!! error TS2322:   Types of parameters 'a' and 'args' are incompatible.
+!!! error TS2322:     Type 'any' is not assignable to type 'never'.
 

tests/baselines/reference/genericRestParameters1.js

@@ -153,6 +153,18 @@ events.emit('move', 10, 'left');
 events.emit('jump', 20, 'up');
 events.emit('stop', 'Bye!');
 events.emit('done');
+
+// Repro from #25871
+
+declare var ff1: (... args: any[]) => void;
+
+declare var ff2: () => void;
+declare var ff3: (...args: []) => void;
+declare var ff4: (a: never) => void;
+
+ff1 = ff2;
+ff1 = ff3;
+ff1 = ff4;  // Error
 
 
 //// [genericRestParameters1.js]
@@ -256,6 +268,9 @@ events.emit('move', 10, 'left');
 events.emit('jump', 20, 'up');
 events.emit('stop', 'Bye!');
 events.emit('done');
+ff1 = ff2;
+ff1 = ff3;
+ff1 = ff4; // Error
 
 
 //// [genericRestParameters1.d.ts]
@@ -332,3 +347,7 @@ declare type EventType<T> = {
     emit<K extends keyof T = keyof T>(e: K, ...payload: T[K] extends any[] ? T[K] : [T[K]]): void;
 };
 declare var events: EventType<Record1>;
+declare var ff1: (...args: any[]) => void;
+declare var ff2: () => void;
+declare var ff3: (...args: []) => void;
+declare var ff4: (a: never) => void;

tests/baselines/reference/genericRestParameters1.symbols

@@ -622,3 +622,32 @@ events.emit('done');
 >events : Symbol(events, Decl(genericRestParameters1.ts, 149, 11))
 >emit : Symbol(emit, Decl(genericRestParameters1.ts, 145, 21))
 
+// Repro from #25871
+
+declare var ff1: (... args: any[]) => void;
+>ff1 : Symbol(ff1, Decl(genericRestParameters1.ts, 157, 11))
+>args : Symbol(args, Decl(genericRestParameters1.ts, 157, 18))
+
+declare var ff2: () => void;
+>ff2 : Symbol(ff2, Decl(genericRestParameters1.ts, 159, 11))
+
+declare var ff3: (...args: []) => void;
+>ff3 : Symbol(ff3, Decl(genericRestParameters1.ts, 160, 11))
+>args : Symbol(args, Decl(genericRestParameters1.ts, 160, 18))
+
+declare var ff4: (a: never) => void;
+>ff4 : Symbol(ff4, Decl(genericRestParameters1.ts, 161, 11))
+>a : Symbol(a, Decl(genericRestParameters1.ts, 161, 18))
+
+ff1 = ff2;
+>ff1 : Symbol(ff1, Decl(genericRestParameters1.ts, 157, 11))
+>ff2 : Symbol(ff2, Decl(genericRestParameters1.ts, 159, 11))
+
+ff1 = ff3;
+>ff1 : Symbol(ff1, Decl(genericRestParameters1.ts, 157, 11))
+>ff3 : Symbol(ff3, Decl(genericRestParameters1.ts, 160, 11))
+
+ff1 = ff4;  // Error
+>ff1 : Symbol(ff1, Decl(genericRestParameters1.ts, 157, 11))
+>ff4 : Symbol(ff4, Decl(genericRestParameters1.ts, 161, 11))
+

tests/baselines/reference/genericRestParameters1.types

@@ -849,3 +849,35 @@ events.emit('done');
 >emit : <K extends "move" | "jump" | "stop" | "done" = "move" | "jump" | "stop" | "done">(e: K, ...payload: Record1[K] extends any[] ? Record1[K] : [Record1[K]]) => void
 >'done' : "done"
 
+// Repro from #25871
+
+declare var ff1: (... args: any[]) => void;
+>ff1 : (...args: any[]) => void
+>args : any[]
+
+declare var ff2: () => void;
+>ff2 : () => void
+
+declare var ff3: (...args: []) => void;
+>ff3 : () => void
+>args : []
+
+declare var ff4: (a: never) => void;
+>ff4 : (a: never) => void
+>a : never
+
+ff1 = ff2;
+>ff1 = ff2 : () => void
+>ff1 : (...args: any[]) => void
+>ff2 : () => void
+
+ff1 = ff3;
+>ff1 = ff3 : () => void
+>ff1 : (...args: any[]) => void
+>ff3 : () => void
+
+ff1 = ff4;  // Error
+>ff1 = ff4 : (a: never) => void
+>ff1 : (...args: any[]) => void
+>ff4 : (a: never) => void
+

tests/cases/conformance/types/rest/genericRestParameters1.ts

@@ -155,3 +155,15 @@ events.emit('move', 10, 'left');
 events.emit('jump', 20, 'up');
 events.emit('stop', 'Bye!');
 events.emit('done');
+
+// Repro from #25871
+
+declare var ff1: (... args: any[]) => void;
+
+declare var ff2: () => void;
+declare var ff3: (...args: []) => void;
+declare var ff4: (a: never) => void;
+
+ff1 = ff2;
+ff1 = ff3;
+ff1 = ff4;  // Error