cmd/go/internal/module: fix inverted condition in MatchPathMajor

This was spotted in CL 200767. This change just ensures internal
packages match their equivalents in x/mod.

Also pulled in test added in CL 201517.

Change-Id: I51d23d62697c256548f411930fcb6bccce51bf34
Reviewed-on: https://go-review.googlesource.com/c/go/+/201497
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/module/module.go

@@ -503,14 +503,17 @@ func splitGopkgIn(path string) (prefix, pathMajor string, ok bool) {
 // MatchPathMajor reports whether the semantic version v
 // matches the path major version pathMajor.
 //
-// MatchPathMajor returns true if and only if CheckPathMajor returns non-nil.
+// MatchPathMajor returns true if and only if CheckPathMajor returns nil.
 func MatchPathMajor(v, pathMajor string) bool {
-	return CheckPathMajor(v, pathMajor) != nil
+	return CheckPathMajor(v, pathMajor) == nil
 }
 
 // CheckPathMajor returns a non-nil error if the semantic version v
 // does not match the path major version pathMajor.
 func CheckPathMajor(v, pathMajor string) error {
+	// TODO(jayconrod): return errors or panic for invalid inputs. This function
+	// (and others) was covered by integration tests for cmd/go, and surrounding
+	// code protected against invalid inputs like non-canonical versions.
 	if strings.HasPrefix(pathMajor, ".v") && strings.HasSuffix(pathMajor, "-unstable") {
 		pathMajor = strings.TrimSuffix(pathMajor, "-unstable")
 	}

src/cmd/go/internal/module/module_test.go

@@ -318,3 +318,26 @@ func TestUnescapePath(t *testing.T) {
 		}
 	}
 }
+
+func TestMatchPathMajor(t *testing.T) {
+	for _, test := range []struct {
+		v, pathMajor string
+		want         bool
+	}{
+		{"v0.0.0", "", true},
+		{"v0.0.0", "/v2", false},
+		{"v0.0.0", ".v0", true},
+		{"v0.0.0-20190510104115-cbcb75029529", ".v1", true},
+		{"v1.0.0", "/v2", false},
+		{"v1.0.0", ".v1", true},
+		{"v1.0.0", ".v1-unstable", true},
+		{"v2.0.0+incompatible", "", true},
+		{"v2.0.0", "", false},
+		{"v2.0.0", "/v2", true},
+		{"v2.0.0", ".v2", true},
+	} {
+		if got := MatchPathMajor(test.v, test.pathMajor); got != test.want {
+			t.Errorf("MatchPathMajor(%q, %q) = %v, want %v", test.v, test.pathMajor, got, test.want)
+		}
+	}
+}