cmd/go: don't try to add replaced versions that won't be selected

In Go 1.12, we added a heuristic to 'go mod tidy' to resolve packages
by adding replaced-but-not-required modules before falling back to
searching for modules from the network. Unfortunately, that heuristic
fails when the replaced version is already lower than the selected
version: adding such a module to the build list doesn't change the
selected version of that module, and so it doesn't make progress
toward resolving the missing package.

Fixes #46659

Change-Id: I75e2387d5290e769f6b0fa1231dcc4605db68597
Reviewed-on: https://go-review.googlesource.com/c/go/+/330432
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/src/cmd/go/internal/modload/import.go b/src/cmd/go/internal/modload/import.go
index 60bd26fb22..d2bbe5cbe0 100644
--- a/src/cmd/go/internal/modload/import.go
+++ b/src/cmd/go/internal/modload/import.go
@@ -428,6 +428,15 @@ func queryImport(ctx context.Context, path string, rs *Requirements) (module.Ver
 					mv = module.ZeroPseudoVersion("v0")
 				}
 			}
+			mg, err := rs.Graph(ctx)
+			if err != nil {
+				return module.Version{}, err
+			}
+			if cmpVersion(mg.Selected(mp), mv) >= 0 {
+				// We can't resolve the import by adding mp@mv to the module graph,
+				// because the selected version of mp is already at least mv.
+				continue
+			}
 			mods = append(mods, module.Version{Path: mp, Version: mv})
 		}