chore: Improve sortables detection

lint/package.go

@@ -1,6 +1,7 @@
 package lint
 
 import (
+	"errors"
 	"go/ast"
 	"go/importer"
 	"go/token"
@@ -31,7 +32,6 @@ type Package struct {
 var (
 	trueValue  = 1
 	falseValue = 2
-	notSet     = 3
 
 	go121 = goversion.Must(goversion.NewVersion("1.21"))
 	go122 = goversion.Must(goversion.NewVersion("1.22"))
@@ -111,6 +111,11 @@ func (p *Package) TypeCheck() error {
 		astFiles = append(astFiles, f.AST)
 	}
 
+	if anyFile == nil {
+		// this is unlikely to happen, but technically guarantees anyFile to not be nil
+		return errors.New("no ast.File found")
+	}
+
 	typesPkg, err := check(config, anyFile.AST.Name.Name, p.fset, astFiles, info)
 
 	// Remember the typechecking info, even if config.Check failed,
@@ -135,7 +140,7 @@ func check(config *types.Config, n string, fset *token.FileSet, astFiles []*ast.
 	return config.Check(n, fset, astFiles, info)
 }
 
-// TypeOf returns the type of an expression.
+// TypeOf returns the type of expression.
 func (p *Package) TypeOf(expr ast.Expr) types.Type {
 	if p.typesInfo == nil {
 		return nil
@@ -148,32 +153,72 @@ type walker struct {
 	has  map[string]int
 }
 
+// bitfield for which methods exist on each type.
+const (
+	bfLen = 1 << iota
+	bfLess
+	bfSwap
+)
+
 func (w *walker) Visit(n ast.Node) ast.Visitor {
 	fn, ok := n.(*ast.FuncDecl)
 	if !ok || fn.Recv == nil || len(fn.Recv.List) == 0 {
 		return w
 	}
-	// TODO(dsymonds): We could check the signature to be more precise.
+
 	recv := typeparams.ReceiverType(fn)
-	if i, ok := w.nmap[fn.Name.Name]; ok {
-		w.has[recv] |= i
+
+	// Ensure the method signature matches expectations.
+	switch fn.Name.Name {
+	case "Len":
+		if fn.Type.Params.NumFields() == 0 && fn.Type.Results.NumFields() == 1 {
+			resultType := fn.Type.Results.List[0].Type
+			if _, ok := resultType.(*ast.Ident); ok && resultType.(*ast.Ident).Name == "int" {
+				w.has[recv] |= bfLen
+			}
+		}
+	case "Less":
+		if fn.Type.Params.NumFields() == 2 && fn.Type.Results.NumFields() == 1 {
+			param1 := fn.Type.Params.List[0].Type
+			var param2 ast.Expr
+			if len(fn.Type.Params.List) == 2 {
+				param2 = fn.Type.Params.List[1].Type
+			} else {
+				param2 = param1
+			}
+			resultType := fn.Type.Results.List[0].Type
+
+			// Ensure parameters have the same type and the result is a bool.
+			if typesEqual(param1, param2) && isBool(resultType) {
+				w.has[recv] |= bfLess
+			}
+		}
+	case "Swap":
+		if fn.Type.Params.NumFields() == 2 && fn.Type.Results.NumFields() == 0 {
+			w.has[recv] |= bfSwap
+		}
 	}
 	return w
 }
 
+func typesEqual(a, b ast.Expr) bool {
+	identA, okA := a.(*ast.Ident)
+	identB, okB := b.(*ast.Ident)
+	return okA && okB && identA.Name == identB.Name
+}
+
+func isBool(t ast.Expr) bool {
+	ident, ok := t.(*ast.Ident)
+	return ok && ident.Name == "bool"
+}
+
 func (p *Package) scanSortable() {
 	p.sortable = map[string]bool{}
 
-	// bitfield for which methods exist on each type.
-	const (
-		bfLen = 1 << iota
-		bfLess
-		bfSwap
-	)
 	nmap := map[string]int{"Len": bfLen, "Less": bfLess, "Swap": bfSwap}
 	has := map[string]int{}
 	for _, f := range p.files {
-		ast.Walk(&walker{nmap, has}, f.AST)
+		ast.Walk(&walker{nmap: nmap, has: has}, f.AST)
 	}
 	for typ, ms := range has {
 		if ms == bfLen|bfLess|bfSwap {