Fix panic in getRestTypeAtPosition for contextually typed function with optional and rest params (#3924)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

Author: Copilot

internal/checker/relater.go

@@ -1801,8 +1801,12 @@ func (c *Checker) getRestTypeAtPosition(source *Signature, pos int, readonly boo
 			return c.createArrayType(c.getIndexedAccessType(restType, c.numberType))
 		}
 	}
-	types := make([]*Type, parameterCount-pos)
-	infos := make([]TupleElementInfo, parameterCount-pos)
+	length := parameterCount - pos
+	if length <= 0 {
+		return c.createTupleTypeEx(nil, nil, readonly)
+	}
+	types := make([]*Type, length)
+	infos := make([]TupleElementInfo, length)
 	for i := range types {
 		var flags ElementFlags
 		if restType == nil || i < len(types)-1 {

testdata/baselines/reference/compiler/contextuallyTypedFunctionOptionalAndRest.errors.txt

@@ -0,0 +1,8 @@
+contextuallyTypedFunctionOptionalAndRest.ts(1,24): error TS7006: Parameter 'a' implicitly has an 'any' type.
+
+
+==== contextuallyTypedFunctionOptionalAndRest.ts (1 errors) ====
+    const f: () => void = (a?, ...b) => {};
+                           ~~
+!!! error TS7006: Parameter 'a' implicitly has an 'any' type.
+    

testdata/baselines/reference/compiler/contextuallyTypedFunctionOptionalAndRest.js

@@ -0,0 +1,9 @@
+//// [tests/cases/compiler/contextuallyTypedFunctionOptionalAndRest.ts] ////
+
+//// [contextuallyTypedFunctionOptionalAndRest.ts]
+const f: () => void = (a?, ...b) => {};
+
+
+//// [contextuallyTypedFunctionOptionalAndRest.js]
+"use strict";
+const f = (a, ...b) => { };

testdata/baselines/reference/compiler/contextuallyTypedFunctionOptionalAndRest.symbols

@@ -0,0 +1,8 @@
+//// [tests/cases/compiler/contextuallyTypedFunctionOptionalAndRest.ts] ////
+
+=== contextuallyTypedFunctionOptionalAndRest.ts ===
+const f: () => void = (a?, ...b) => {};
+>f : Symbol(f, Decl(contextuallyTypedFunctionOptionalAndRest.ts, 0, 5))
+>a : Symbol(a, Decl(contextuallyTypedFunctionOptionalAndRest.ts, 0, 23))
+>b : Symbol(b, Decl(contextuallyTypedFunctionOptionalAndRest.ts, 0, 26))
+

testdata/baselines/reference/compiler/contextuallyTypedFunctionOptionalAndRest.types

@@ -0,0 +1,9 @@
+//// [tests/cases/compiler/contextuallyTypedFunctionOptionalAndRest.ts] ////
+
+=== contextuallyTypedFunctionOptionalAndRest.ts ===
+const f: () => void = (a?, ...b) => {};
+>f : () => void
+>(a?, ...b) => {} : (a?: any) => void
+>a : any
+>b : []
+

testdata/tests/cases/compiler/contextuallyTypedFunctionOptionalAndRest.ts

@@ -0,0 +1,3 @@
+// @strict: true
+
+const f: () => void = (a?, ...b) => {};