gopls/internal/lsp/cache: allow versions of form "go1.2.3"

Plus a unit test.

Fixes golang/go#63472

Change-Id: I69466024eb876457d0d3f262cef3467b25879d93
Reviewed-on: https://go-review.googlesource.com/c/tools/+/557215
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

gopls/internal/lsp/cache/check.go

@@ -1565,8 +1565,8 @@ func (b *typeCheckBatch) checkPackage(ctx context.Context, ph *packageHandle) (*
 	return &Package{ph.mp, ph.loadDiagnostics, pkg}, nil
 }
 
-// TODO(golang/go#63472): this looks wrong with the new Go version syntax.
-var goVersionRx = regexp.MustCompile(`^go([1-9][0-9]*)\.(0|[1-9][0-9]*)$`)
+// e.g. "go1" or "go1.2" or "go1.2.3"
+var goVersionRx = regexp.MustCompile(`^go[1-9][0-9]*(?:\.(0|[1-9][0-9]*)){0,2}$`)
 
 func (b *typeCheckBatch) typesConfig(ctx context.Context, inputs typeCheckInputs, onError func(e error)) *types.Config {
 	cfg := &types.Config{

gopls/internal/lsp/cache/constraints_test.go

@@ -94,3 +94,33 @@ func TestIsStandaloneFile(t *testing.T) {
 		})
 	}
 }
+
+func TestVersionRegexp(t *testing.T) {
+	// good
+	for _, s := range []string{
+		"go1",
+		"go1.2",
+		"go1.2.3",
+		"go1.0.33",
+	} {
+		if !goVersionRx.MatchString(s) {
+			t.Errorf("Valid Go version %q does not match the regexp", s)
+		}
+	}
+
+	// bad
+	for _, s := range []string{
+		"go",          // missing numbers
+		"go0",         // Go starts at 1
+		"go01",        // leading zero
+		"go1.π",       // non-decimal
+		"go1.-1",      // negative
+		"go1.02.3",    // leading zero
+		"go1.2.3.4",   // too many segments
+		"go1.2.3-pre", // textual suffix
+	} {
+		if goVersionRx.MatchString(s) {
+			t.Errorf("Invalid Go version %q unexpectedly matches the regexp", s)
+		}
+	}
+}