cmd/go: fix missing case checking for empty slice

When we were comparing the first element of import stacks when sorting
depserrors we checked if the first stack was non empty, but not the
second one. Do the check for both stacks.

Fixes #61816
For #59905

Change-Id: Id5c11c2b1104eec93196a08c53372ee2ba97c701
Reviewed-on: https://go-review.googlesource.com/c/go/+/516739
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>

src/cmd/go/internal/list/list.go

@@ -957,7 +957,10 @@ func collectDepsErrors(p *load.Package) {
 			if len(stkj) != 0 {
 				return true
 			}
+
 			return p.DepsErrors[i].Err.Error() < p.DepsErrors[j].Err.Error()
+		} else if len(stkj) == 0 {
+			return false
 		}
 		pathi, pathj := stki[len(stki)-1], stkj[len(stkj)-1]
 		return pathi < pathj

src/cmd/go/testdata/script/list_issue_59905.txt

@@ -1,8 +1,13 @@
 # Expect no panic
 go list -f '{{if .DepsErrors}}{{.DepsErrors}}{{end}}' -export -e -deps
-cmpenv stdout wanterr
+cmpenv stdout wanterr_59905
 
--- wanterr --
+# Expect no panic (Issue 61816)
+cp level1b_61816.txt level1b/pkg.go
+go list -f '{{if .DepsErrors}}{{.DepsErrors}}{{end}}' -export -e -deps
+cmpenv stdout wanterr_61816
+
+-- wanterr_59905 --
 [# test/main/level1a
 level1a${/}pkg.go:5:2: level2x redeclared in this block
 	level1a${/}pkg.go:4:2: other declaration of level2x
@@ -14,6 +19,23 @@ level1b${/}pkg.go:5:2: level2x redeclared in this block
 level1b${/}pkg.go:5:2: "test/main/level1b/level2y" imported as level2x and not used
 level1b${/}pkg.go:8:39: undefined: level2y
 ]
+-- wanterr_61816 --
+[level1b${/}pkg.go:4:2: package foo is not in std ($GOROOT${/}src${/}foo)]
+[# test/main/level1a
+level1a${/}pkg.go:5:2: level2x redeclared in this block
+	level1a${/}pkg.go:4:2: other declaration of level2x
+level1a${/}pkg.go:5:2: "test/main/level1a/level2y" imported as level2x and not used
+level1a${/}pkg.go:8:39: undefined: level2y
+ level1b${/}pkg.go:4:2: package foo is not in std ($GOROOT${/}src${/}foo)]
+-- level1b_61816.txt --
+package level1b
+
+import (
+	"foo"
+)
+
+func Print() { println(level2x.Value, level2y.Value) }
+
 -- go.mod --
 module test/main