diff --git a/src/cmd/go/testdata/mod/example.net_ambiguous_nested_v0.1.0.txt b/src/cmd/go/testdata/mod/example.net_ambiguous_nested_v0.1.0.txt
new file mode 100644
index 00000000000..8c9de7a5f47
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.net_ambiguous_nested_v0.1.0.txt
@@ -0,0 +1,19 @@
+Written by hand.
+
+Test module containing a package that is also provided by a nested module tagged
+with the same version.
+
+-- .mod --
+module example.net/ambiguous/nested
+
+go 1.16
+-- .info --
+{"Version": "v0.1.0"}
+-- go.mod --
+module example.net/ambiguous/nested
+
+go 1.16
+-- pkg/pkg.go --
+// Package pkg exists in both example.net/ambiguous v0.1.0
+// and example.net/ambiguous/nested v0.1.0
+package pkg
diff --git a/src/cmd/go/testdata/mod/example.net_ambiguous_v0.1.0.txt b/src/cmd/go/testdata/mod/example.net_ambiguous_v0.1.0.txt
new file mode 100644
index 00000000000..8fa6d83346d
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.net_ambiguous_v0.1.0.txt
@@ -0,0 +1,19 @@
+Written by hand.
+
+Test module containing a package that is also provided by a nested module tagged
+with the same version.
+
+-- .mod --
+module example.net/ambiguous
+
+go 1.16
+-- .info --
+{"Version": "v0.1.0"}
+-- go.mod --
+module example.net/ambiguous
+
+go 1.16
+-- nested/pkg/pkg.go --
+// Package pkg exists in both example.net/ambiguous v0.1.0
+// and example.net/ambiguous/nested v0.1.0
+package pkg
diff --git a/src/cmd/go/testdata/mod/example.net_ambiguous_v0.2.0.txt b/src/cmd/go/testdata/mod/example.net_ambiguous_v0.2.0.txt
new file mode 100644
index 00000000000..7589ad76a31
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.net_ambiguous_v0.2.0.txt
@@ -0,0 +1,18 @@
+Written by hand.
+
+Test module containing a package that is also provided by a nested module tagged
+with the same version.
+
+-- .mod --
+module example.net/ambiguous
+
+go 1.16
+-- .info --
+{"Version": "v0.2.0"}
+-- go.mod --
+module example.net/ambiguous
+
+go 1.16
+-- nested/pkg/README.txt --
+// Package pkg no longer exists in this module at v0.2.0.
+// Find it in module example.net/ambiguous/nested instead.
diff --git a/src/cmd/go/testdata/script/mod_get_ambiguous_arg.txt b/src/cmd/go/testdata/script/mod_get_ambiguous_arg.txt
index 7729f29ced7..f64da3a3fd0 100644
--- a/src/cmd/go/testdata/script/mod_get_ambiguous_arg.txt
+++ b/src/cmd/go/testdata/script/mod_get_ambiguous_arg.txt
@@ -70,7 +70,7 @@ module m
 
 go 1.16
 -- m01/README.txt --
-Module m at v0.3.0 does not yet contain package p.
+Module m at v0.1.0 does not yet contain package p.
 
 -- m02/go.mod --
 module m
@@ -107,18 +107,18 @@ module m/p
 go 1.16
 -- mp01/README.txt --
 This module is m/p.
-Package m/p no longer exists.
+Package m/p does not exist in this module.
 -- mp02/go.mod --
 module m/p
 
 go 1.16
 -- mp02/README.txt --
 This module is m/p.
-Package m/p no longer exists.
+Package m/p does not exist in this module.
 -- mp03/go.mod --
 module m/p
 
 go 1.16
 -- mp03/README.txt --
 This module is m/p.
-Package m/p no longer exists.
+Package m/p does not exist in this module.
diff --git a/src/cmd/go/testdata/script/mod_get_ambiguous_pkg.txt b/src/cmd/go/testdata/script/mod_get_ambiguous_pkg.txt
new file mode 100644
index 00000000000..f00f99ee8c8
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_ambiguous_pkg.txt
@@ -0,0 +1,101 @@
+# Both example.net/ambiguous v0.1.0 and example.net/ambiguous/pkg v0.1.0 exist.
+# 'go mod tidy' would arbitrarily choose the one with the longer path,
+# but 'go mod tidy' also arbitrarily chooses the latest version.
+
+cp go.mod go.mod.orig
+
+
+# From a clean slate, 'go get' currently does the same thing as 'go mod tidy':
+# it resolves the package from the module with the longest matching prefix.
+
+go get -d example.net/ambiguous/nested/pkg@v0.1.0
+go list -m all
+stdout '^example.net/ambiguous/nested v0.1.0$'
+! stdout '^example.net/ambiguous '
+
+
+# From an initial state that already depends on the shorter path,
+# the same 'go get' command attempts to add the longer path and fails.
+#
+# TODO(bcmills): What should really happen here?
+# Should we match the versioned package path against the existing package
+# (reducing unexpected errors), or give it the same meaning regardless of the
+# initial state?
+
+cp go.mod.orig go.mod
+go mod edit -require=example.net/ambiguous@v0.1.0
+
+! go get -d example.net/ambiguous/nested/pkg@v0.1.0
+stderr '^go get example.net/ambiguous/nested/pkg@v0.1.0: ambiguous import: found package example.net/ambiguous/nested/pkg in multiple modules:\n\texample.net/ambiguous v0.1.0 \(.*\)\n\texample.net/ambiguous/nested v0.1.0 \(.*\)\n\z'
+
+
+# The user should be able to fix the aforementioned failure by explicitly
+# upgrading the conflicting module.
+
+go get -d example.net/ambiguous@v0.2.0 example.net/ambiguous/nested/pkg@v0.1.0
+go list -m all
+stdout '^example.net/ambiguous/nested v0.1.0$'
+stdout '^example.net/ambiguous v0.2.0$'
+
+
+# ...or by explicitly NOT adding the conflicting module.
+#
+# BUG(#37438): Today, this does not work: explicit module version constraints do
+# not affect the package-to-module mapping during package upgrades, so the
+# arguments are interpreted as specifying conflicting versions of the longer
+# module path.
+
+cp go.mod.orig go.mod
+go mod edit -require=example.net/ambiguous@v0.1.0
+
+! go get -d example.net/ambiguous/nested/pkg@v0.1.0 example.net/ambiguous/nested@none
+stderr '^go get: conflicting versions for module example.net/ambiguous/nested: v0.1.0 and none$'
+
+ # go list -m all
+ # ! stdout '^example.net/ambiguous/nested '
+ # stdout '^example.net/ambiguous v0.1.0$'
+
+
+# The user should also be able to fix it by *downgrading* the conflicting module
+# away.
+#
+# BUG(#37438): Today, this does not work: the "ambiguous import" error causes
+# 'go get' to fail before applying the requested downgrade.
+
+cp go.mod.orig go.mod
+go mod edit -require=example.net/ambiguous@v0.1.0
+
+! go get -d example.net/ambiguous@none example.net/ambiguous/nested/pkg@v0.1.0
+stderr '^go get example.net/ambiguous/nested/pkg@v0.1.0: ambiguous import: found package example.net/ambiguous/nested/pkg in multiple modules:\n\texample.net/ambiguous v0.1.0 \(.*\)\n\texample.net/ambiguous/nested v0.1.0 \(.*\)\n\z'
+
+ # go list -m all
+ # stdout '^example.net/ambiguous/nested v0.1.0$'
+ # !stdout '^example.net/ambiguous '
+
+
+# In contrast, if we do the same thing tacking a wildcard pattern ('/...') on
+# the end of the package path, we get different behaviors depending on the
+# initial state, and no error. (This seems to contradict the “same meaning
+# regardless of the initial state” point above, but maybe that's ok?)
+
+cp go.mod.orig go.mod
+
+go get -d example.net/ambiguous/nested/pkg/...@v0.1.0
+go list -m all
+stdout '^example.net/ambiguous/nested v0.1.0$'
+! stdout '^example.net/ambiguous '
+
+
+cp go.mod.orig go.mod
+go mod edit -require=example.net/ambiguous@v0.1.0
+
+go get -d example.net/ambiguous/nested/pkg/...@v0.1.0
+go list -m all
+! stdout '^example.net/ambiguous/nested '
+stdout '^example.net/ambiguous v0.1.0$'
+
+
+-- go.mod --
+module test
+
+go 1.16