cmd/compile: better recovery after := (rather than =) in declarations

Before this fix, a mistaken := in a (const/type/var) declaration
ended that declaration with an error from which the parser didn't
recover well. This low-cost change will provide a better error
message and lets the parser recover perfectly.

Fixes #31092.

Change-Id: Ic4f94dc5e29dd00b7ef6d53a80dded638e3cea80
Reviewed-on: https://go-review.googlesource.com/c/go/+/169958
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

diff --git a/src/cmd/compile/internal/syntax/parser.go b/src/cmd/compile/internal/syntax/parser.go
index e1cd8f9f5a1bb1de66c0d0f4db4d82efbe6b5967..d4e9bf2f9651540c4eed125c7ce69876209b6242 100644 (file)
--- a/src/cmd/compile/internal/syntax/parser.go
+++ b/src/cmd/compile/internal/syntax/parser.go
@@ -170,6 +170,20 @@ func (p *parser) want(tok token) {
        }
 }
 
+// gotAssign is like got(_Assign) but it also accepts ":="
+// (and reports an error) for better parser error recovery.
+func (p *parser) gotAssign() bool {
+       switch p.tok {
+       case _Define:
+               p.syntaxError("expecting =")
+               fallthrough
+       case _Assign:
+               p.next()
+               return true
+       }
+       return false
+}
+
 // ----------------------------------------------------------------------------
 // Error handling
 
@@ -514,7 +528,7 @@ func (p *parser) constDecl(group *Group) Decl {
        d.NameList = p.nameList(p.name())
        if p.tok != _EOF && p.tok != _Semi && p.tok != _Rparen {
                d.Type = p.typeOrNil()
-               if p.got(_Assign) {
+               if p.gotAssign() {
                        d.Values = p.exprList()
                }
        }
@@ -533,7 +547,7 @@ func (p *parser) typeDecl(group *Group) Decl {
        d.pos = p.pos()
 
        d.Name = p.name()
-       d.Alias = p.got(_Assign)
+       d.Alias = p.gotAssign()
        d.Type = p.typeOrNil()
        if d.Type == nil {
                d.Type = p.bad()
@@ -556,11 +570,11 @@ func (p *parser) varDecl(group *Group) Decl {
        d.pos = p.pos()
 
        d.NameList = p.nameList(p.name())
-       if p.got(_Assign) {
+       if p.gotAssign() {
                d.Values = p.exprList()
        } else {
                d.Type = p.type_()
-               if p.got(_Assign) {
+               if p.gotAssign() {
                        d.Values = p.exprList()
                }
        }

diff --git a/src/cmd/compile/internal/syntax/testdata/issue31092.src b/src/cmd/compile/internal/syntax/testdata/issue31092.src
new file mode 100644 (file)
index 0000000..b1839b8
--- /dev/null
+++ b/src/cmd/compile/internal/syntax/testdata/issue31092.src
@@ -0,0 +1,16 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test cases for issue 31092: Better synchronization of
+// parser after seeing an := rather than an = in a const,
+// type, or variable declaration.
+
+package p
+
+const _ /* ERROR unexpected := */ := 0
+type _ /* ERROR unexpected := */ := int
+var _ /* ERROR unexpected := */ := 0
+
+const _ int /* ERROR unexpected := */ := 0
+var _ int /* ERROR unexpected := */ := 0