go/analysis/passes/modernize: fix panic in stringscut

stringscut modernizer assumes that any CallExpr that occurs in the first
argument is a []byte(string) conversion, causing panics when given a
function call with no arguments at all.

Fixes golang/go#77208.

Change-Id: I17401c74c4681b24fb3a508cafafffee6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/tools/+/737000
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Madeline Kalil <mkalil@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>

Author: DanielMorsing

go/analysis/passes/modernize/stringscut.go

@@ -345,6 +345,7 @@ func indexArgValid(info *types.Info, index *typeindex.Index, expr ast.Expr, afte
 	switch expr := expr.(type) {
 	case *ast.CallExpr:
 		return types.Identical(tv.Type, byteSliceType) &&
+			info.Types[expr.Fun].IsType() && // make sure this isn't a function that returns a byte slice
 			indexArgValid(info, index, expr.Args[0], afterPos) // check s in []byte(s)
 	case *ast.Ident:
 		sObj := info.Uses[expr]

go/analysis/passes/modernize/testdata/src/stringscut/stringscut.go

@@ -289,6 +289,16 @@ func idx_gtr_negone(s string, sub string) string {
 	return ""
 }
 
+// Regression test for a crash (https://go.dev/issue/77208)
+func idx_call() {
+	i := bytes.Index(b(), []byte(""))
+	_ = i
+}
+
+func b() []byte {
+	return nil
+}
+
 func function(s string) {}
 
 func reference_str(s *string) {}

go/analysis/passes/modernize/testdata/src/stringscut/stringscut.go.golden

@@ -289,6 +289,16 @@ func idx_gtr_negone(s string, sub string) string {
 	return ""
 }
 
+// Regression test for a crash (https://go.dev/issue/77208)
+func idx_call() {
+	i := bytes.Index(b(), []byte(""))
+	_ = i
+}
+
+func b() []byte {
+	return nil
+}
+
 func function(s string) {}
 
 func reference_str(s *string) {}