gopls/internal/golang/completion: fix crash with extra results

Fix an incorrect condition when extracting expected result types: we
must check the length of signature, not number of returns, before
accessing the expected result type.

Fixes golang/go#70636

Change-Id: I59d84283073c99117c40de584db6f576f2484206
Reviewed-on: https://go-review.googlesource.com/c/tools/+/632936
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

gopls/internal/golang/completion/completion.go

@@ -2624,7 +2624,7 @@ func expectedAssignStmtTypes(pkg *cache.Package, node *ast.AssignStmt, pos token
 // Returns nil if enclosingSig is nil.
 func expectedReturnStmtType(enclosingSig *types.Signature, node *ast.ReturnStmt, pos token.Pos) types.Type {
 	if enclosingSig != nil {
-		if resultIdx := exprAtPos(pos, node.Results); resultIdx < len(node.Results) {
+		if resultIdx := exprAtPos(pos, node.Results); resultIdx < enclosingSig.Results().Len() {
 			return enclosingSig.Results().At(resultIdx).Type()
 		}
 	}

gopls/internal/test/marker/testdata/completion/issue70636.txt

@@ -0,0 +1,23 @@
+This test reproduces the crash of golang/go#70636, an out of bounds error when
+analyzing a return statement with more results than the signature expects.
+
+-- flags --
+-ignore_extra_diags
+
+-- go.mod --
+module example.com
+
+go 1.21
+
+-- p.go --
+package p
+
+var xx int
+var xy string
+
+
+func _() {
+	return Foo(x) //@ rank(re"x()", "xx", "xy")
+}
+
+func Foo[T any](t T) T {}