x/tools/gopls/internal/analysis/modernize: nil deref in minmax

[adonovan]

This input _ = f() causes minmax to panic because TypeOf(_) is nil. (There's another edge-case to document in the big checklist.)

We need to make it easier to do routine large-scale testing of analyzers.

[gopherbot]

Change https://go.dev/cl/662195 mentions this issue: internal/typeparams/copretype: check nil before getting coretype