go/types: make gotype continue after syntax errors

This avoids odd behavior where sometimes a lot of useful
errors are not reported simply because of a small syntax
error.

Tested manually with non-existing files. (We cannot easily
add an automatic test because this is a stand-alone binary
in this directory that must be built manually.)

Fixes #23593.

Change-Id: Iff90f95413bed7d1023fa0a5c9eb0414144428a9
Reviewed-on: https://go-review.googlesource.com/93815Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/gotype.go

@@ -209,14 +209,30 @@ func parseFiles(dir string, filenames []string) ([]*ast.File, error) {
 	}
 	wg.Wait()
 
-	// if there are errors, return the first one for deterministic results
+	// If there are errors, return the first one for deterministic results.
+	var first error
 	for _, err := range errors {
 		if err != nil {
-			return nil, err
+			first = err
+			// If we have an error, some files may be nil.
+			// Remove them. (The go/parser always returns
+			// a possibly partial AST even in the presence
+			// of errors, except if the file doesn't exist
+			// in the first place, in which case it cannot
+			// matter.)
+			i := 0
+			for _, f := range files {
+				if f != nil {
+					files[i] = f
+					i++
+				}
+			}
+			files = files[:i]
+			break
 		}
 	}
 
-	return files, nil
+	return files, first
 }
 
 func parseDir(dir string) ([]*ast.File, error) {
@@ -318,7 +334,7 @@ func main() {
 	files, err := getPkgFiles(flag.Args())
 	if err != nil {
 		report(err)
-		os.Exit(2)
+		// ok to continue (files may be empty, but not nil)
 	}
 
 	checkPkgFiles(files)