cmd/go: fix compile -p flag for building test of main package

Fixes #23180.

Change-Id: I52404ee98dcc60b96972d4242c13db0ec4340d0d
Reviewed-on: https://go-review.googlesource.com/86235
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Alessandro Arzilli <alessandro.arzilli@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/go_test.go

@@ -1589,6 +1589,19 @@ func TestPackageMainTestImportsArchiveNotBinary(t *testing.T) {
 	tg.run("test", "main_test")
 }
 
+func TestPackageMainTestCompilerFlags(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.makeTempdir()
+	tg.setenv("GOPATH", tg.path("."))
+	tg.tempFile("src/p1/p1.go", "package main\n")
+	tg.tempFile("src/p1/p1_test.go", "package main\nimport \"testing\"\nfunc Test(t *testing.T){}\n")
+	tg.run("test", "-c", "-n", "p1")
+	tg.grepBothNot(`[\\/]compile.* -p main.*p1\.go`, "should not have run compile -p main p1.go")
+	tg.grepStderr(`[\\/]compile.* -p p1.*p1\.go`, "should have run compile -p p1 p1.go")
+}
+
 // The runtime version string takes one of two forms:
 // "go1.X[.Y]" for Go releases, and "devel +hash" at tip.
 // Determine whether we are in a released copy by

src/cmd/go/internal/work/gc.go

@@ -49,7 +49,7 @@ func (gcToolchain) gc(b *Builder, a *Action, archive string, importcfg []byte, a
 	pkgpath := p.ImportPath
 	if cfg.BuildBuildmode == "plugin" {
 		pkgpath = pluginPath(a)
-	} else if p.Name == "main" {
+	} else if p.Name == "main" && !p.Internal.ForceLibrary {
 		pkgpath = "main"
 	}
 	gcargs := []string{"-p", pkgpath}