cmd/fix,x/tools/go/analysis/passes/modernize: reflect.TypeOf(nil) transformed into reflect.TypeFor[untyped nil]() #77636
Description
Description
The modernize pass in go fix (introduced/enhanced in Go 1.26) attempts to replace reflect.TypeOf(x) with reflect.TypeFor[T](). However, it does not correctly handle cases where the argument is an untyped nil.
When reflect.TypeOf(nil) is encountered, the tool generates reflect.TypeFor[untyped nil](), which is not valid Go syntax and results in a compilation error.
Steps to Reproduce
Create a file main.go with the following content:
package main
import (
"reflect"
)
func main() {
_ = reflect.TypeOf(nil)
}Run the modernize fix (via Go 1.26 toolchain):
go fix main.goExpected Result
The code should remain unchanged because nil does not have a static type that can be used as a generic type argument, OR it should be transformed into something valid like reflect.TypeFor[any]().
// Remain as is
_ = reflect.TypeOf(nil)Actual Result
The tool produces invalid syntax:
func main() {
_ = reflect.TypeFor[untyped nil]()
}Attempting to build this results in:
syntax error: unexpected name nil, expected comma, : or ]
Additional Context
This issue was discovered while building a database driver (SQLite) where reflect.TypeOf(nil) is commonly used to represent the type of a NULL value. The current implementation in cmd/vendor/golang.org/x/tools/go/analysis/passes/modernize/reflect.go appears to lack validation for types.UntypedNil before performing the transformation.