go/parser: better error (recovery) for Allman/BSD-style func decls

This matches the behavior and error of cmd/compile.

Fixes #34946.

Change-Id: I329ef358deea63d8425f76f1d54c95749b96c365
Reviewed-on: https://go-review.googlesource.com/c/go/+/202484Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/go/parser/parser.go

@@ -2439,8 +2439,18 @@ func (p *parser) parseFuncDecl() *ast.FuncDecl {
 	var body *ast.BlockStmt
 	if p.tok == token.LBRACE {
 		body = p.parseBody(scope)
+		p.expectSemi()
+	} else if p.tok == token.SEMICOLON {
+		p.next()
+		if p.tok == token.LBRACE {
+			// opening { of function declaration on next line
+			p.error(p.pos, "unexpected semicolon or newline before {")
+			body = p.parseBody(scope)
+			p.expectSemi()
+		}
+	} else {
+		p.expectSemi()
 	}
-	p.expectSemi()
 
 	decl := &ast.FuncDecl{
 		Doc:  doc,

src/go/parser/testdata/issue34946.src

+// Copyright 2019 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 case for issue 34946: Better synchronization of
+// parser for function declarations that start their
+// body's opening { on a new line.
+
+package p
+
+// accept Allman/BSD-style declaration but complain
+// (implicit semicolon between signature and body)
+func _() int
+{ /* ERROR "unexpected semicolon or newline before {" */
+	{ return 0 }
+}
+
+func _() {}
+
+func _(); { /* ERROR "unexpected semicolon or newline before {" */ }
+
+func _() {}