cmd/compile/internal/syntax: always construct a correct syntax tree

- parser creates sensible nodes in case of syntax errors instead of nil
- a new BadExpr node is used in places where we can't do better
- fixed error message for incorrect type switch guard
- minor cleanups

Fixes #19663.

Change-Id: I028394c6db9cba7371f0e417ebf93f594659786a
Reviewed-on: https://go-review.googlesource.com/38653
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/cmd/compile/internal/syntax/syntax.go b/src/cmd/compile/internal/syntax/syntax.go
index 25fafcf077..587a435e85 100644
--- a/src/cmd/compile/internal/syntax/syntax.go
+++ b/src/cmd/compile/internal/syntax/syntax.go
@@ -65,7 +65,7 @@ func Parse(base *src.PosBase, src io.Reader, errh ErrorHandler, pragh PragmaHand
 	var p parser
 	p.init(base, src, errh, pragh)
 	p.next()
-	return p.file(), p.first
+	return p.fileOrNil(), p.first
 }
 
 // ParseBytes behaves like Parse but it reads the source from the []byte slice provided.