cmd/compile: fix contrived line number errors

If a general comment contains multiple newline characters, we can't
simply unread one and then re-lex it via the general whitespace lexing
phase, because then we'll reset lineno to the line before the "*/"
marker, rather than keeping it where we found the "/*" marker.

Also, for processing imports, call importfile before advancing the
lexer with p.next(), so that lineno reflects the line where we found
the import path, and not the token afterwards.

Fixes #14520.

Change-Id: I785a2d83d632280113d4b757de0d57c88ba2caf4
Reviewed-on: https://go-review.googlesource.com/19934Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/lex.go

@@ -1000,31 +1000,31 @@ l0:
 	case '/':
 		c1 = l.getr()
 		if c1 == '*' {
-			nl := false
-			for {
 			c = l.getr()
-				if c == '\n' {
-					nl = true
-				}
-				for c == '*' {
+			for {
+				if c == '*' {
 					c = l.getr()
 					if c == '/' {
-						if nl {
-							l.ungetr('\n')
-						}
-						goto l0
-					}
-
-					if c == '\n' {
-						nl = true
+						break
 					}
+					continue
 				}
-
 				if c == EOF {
 					Yyerror("eof in comment")
 					errorexit()
 				}
+				c = l.getr()
 			}
+
+			// A comment containing newlines acts like a newline.
+			if lexlineno > lineno && nlsemi {
+				if Debug['x'] != 0 {
+					fmt.Printf("lex: implicit semi\n")
+				}
+				l.tok = ';'
+				return
+			}
+			goto l0
 		}
 
 		if c1 == '/' {

src/cmd/compile/internal/gc/parser.go

@@ -333,20 +333,22 @@ func (p *parser) importdcl() {
 	}
 
 	line := int32(parserline())
-	path := p.val
-	p.next()
 
-	importfile(&path, p.indent)
-	if importpkg == nil {
+	// We need to clear importpkg before calling p.next(),
+	// otherwise it will affect lexlineno.
+	// TODO(mdempsky): Fix this clumsy API.
+	importfile(&p.val, p.indent)
+	ipkg := importpkg
+	importpkg = nil
+
+	p.next()
+	if ipkg == nil {
 		if nerrors == 0 {
 			Fatalf("phase error in import")
 		}
 		return
 	}
 
-	ipkg := importpkg
-	importpkg = nil
-
 	ipkg.Direct = true
 
 	if my == nil {

test/fixedbugs/issue14520.go

+// errorcheck
+
+// Copyright 2016 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.
+
+package f
+
+import /* // ERROR "import path" */ `
+bogus`
+
+func f(x int /* // ERROR "unexpected semicolon"
+
+*/)