Gopls does not re-evaluate metadata when an import declaration goes from invalid to valid. For example, if I type the following:

import (
  _ "embed"

and then type the closing ), to get to this:

import (
  _ "embed"
)

I will get the error "could not import embed (no required module provides package "embed"). This error is misleading: of course std provides embed, but gopls assumes the validity of its pre-computed package graph for its importer. In this case, the problem is that before the import statement was parseable, go list did not return the "embed" dependency, and when it becomes parseable we do not invalidate metadata and re-run go list. Typing within an import spec resolves the problem (e.g. deleting and re-typing "d").

Clearly our heuristic for detecting import changes is wrong.

CC @adonovan