cmd/compile/internal/parser: improved a couple of error messages

The new syntax tree introduced with 1.8 represents send statements
(ch <- x) as statements; the old syntax tree represented them as
expressions (and parsed them as such) but complained if they were
used in expression context. As a consequence, some of the errors
that in the past were of the form "ch <- x used as value" now look
like "unexpected <- ..." because a "<-" is not valid according to
Go syntax in those situations. Accept the new error message.

Also: Fine-tune handling of misformed for loop headers.

Also: Minor cleanups/better comments.

Fixes #17590.

Change-Id: Ia541dea1f2f015c1b21f5b3ae44aacdec60a8aba
Reviewed-on: https://go-review.googlesource.com/37386
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

2 files changed, 4 insertions, 9 deletions

diff --git a/test/syntax/chan1.go b/test/syntax/chan1.go
index a33a0d4cea..4eb63796ac 100644
--- a/test/syntax/chan1.go
+++ b/test/syntax/chan1.go
@@ -10,8 +10,8 @@ var c chan int
 var v int
 
 func main() {
-	if c <- v { // ERROR "used as value|missing condition|invalid condition"
+	if c <- v { // ERROR "used as value"
 	}
 }
 
-var _ = c <- v // ERROR "used as value|unexpected <-"
+var _ = c <- v // ERROR "unexpected <-"
diff --git a/test/syntax/semi4.go b/test/syntax/semi4.go
index 0b5e677680..f21431b3f5 100644
--- a/test/syntax/semi4.go
+++ b/test/syntax/semi4.go
@@ -4,14 +4,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// TODO(mdempsky): Update error expectations for new parser.
-// The new parser emits an extra "missing { after for clause" error.
-// The old parser is supposed to emit this too, but it panics first
-// due to a nil pointer dereference.
-
 package main
 
 func main() {
 	for x		// GCCGO_ERROR "undefined"
-	{		// ERROR "expecting .*{.* after for clause|missing operand"
-		z	// ERROR "undefined|expecting { after for clause"
+	{		// ERROR "unexpected {, expecting for loop condition"
+		z