go/analysis/passes/stringintconv: audit for types.Alias safety

Updates golang/go#65294

Change-Id: Idfb583f0d3ad3753b770946cb9b9360625670d0d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/559917
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Tim King <taking@google.com>

go/analysis/passes/stringintconv/string.go

@@ -15,6 +15,7 @@ import (
 	"golang.org/x/tools/go/analysis/passes/inspect"
 	"golang.org/x/tools/go/analysis/passes/internal/analysisutil"
 	"golang.org/x/tools/go/ast/inspector"
+	"golang.org/x/tools/internal/aliases"
 	"golang.org/x/tools/internal/typeparams"
 )
 
@@ -194,16 +195,15 @@ func run(pass *analysis.Pass) (interface{}, error) {
 
 func structuralTypes(t types.Type) ([]types.Type, error) {
 	var structuralTypes []types.Type
-	switch t := t.(type) {
-	case *types.TypeParam:
-		terms, err := typeparams.StructuralTerms(t)
+	if tp, ok := aliases.Unalias(t).(*types.TypeParam); ok {
+		terms, err := typeparams.StructuralTerms(tp)
 		if err != nil {
 			return nil, err
 		}
 		for _, term := range terms {
 			structuralTypes = append(structuralTypes, term.Type())
 		}
-	default:
+	} else {
 		structuralTypes = append(structuralTypes, t)
 	}
 	return structuralTypes, nil