cmd/compile: fix lookup package of redeclared dot import symbol

The compiler is relying on Sym.Def field to lookup symbol package in
DotImportRefs map. But the Sym.Def field is clear whenever the compiler
finish processing a file. If the dot import happen in file A, then the
redeclaration happen in file B, then the symbol lookup in file B will
see a nil Sym.Def, that cause the compiler crashes.

To fix this, we can interate over DotImportRefs and check for matching
symbol name and return the corresponding package. Though this operation
can be slow, but it only happens in invalid program, when printing error
message, so it's not worth to optimize it further.

Fixes #47201

Change-Id: I4ca1cb0a8e7432b19cf71434592a4cbb58d54adf
Reviewed-on: https://go-review.googlesource.com/c/go/+/334589
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/typecheck/dcl.go

@@ -106,7 +106,17 @@ func Export(n *ir.Name) {
 // Redeclared emits a diagnostic about symbol s being redeclared at pos.
 func Redeclared(pos src.XPos, s *types.Sym, where string) {
 	if !s.Lastlineno.IsKnown() {
-		pkgName := DotImportRefs[s.Def.(*ir.Ident)]
+		var pkgName *ir.PkgName
+		if s.Def == nil {
+			for id, pkg := range DotImportRefs {
+				if id.Sym().Name == s.Name {
+					pkgName = pkg
+					break
+				}
+			}
+		} else {
+			pkgName = DotImportRefs[s.Def.(*ir.Ident)]
+		}
 		base.ErrorfAt(pos, "%v redeclared %s\n"+
 			"\t%v: previous declaration during import %q", s, where, base.FmtPos(pkgName.Pos()), pkgName.Pkg.Path)
 	} else {

test/fixedbugs/issue47201.dir/a.go

@@ -0,0 +1,13 @@
+// Copyright 2021 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 main
+
+import (
+	. "fmt"
+)
+
+func test() {
+	Println("foo")
+}

test/fixedbugs/issue47201.dir/b.go

@@ -0,0 +1,9 @@
+// Copyright 2021 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 main
+
+func Println() {} // ERROR "Println redeclared in this block"
+
+func main() {}

test/fixedbugs/issue47201.go

@@ -0,0 +1,7 @@
+// errorcheckdir
+
+// Copyright 2021 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 ignored