cmd/compile: don't bother to declare closure inside redeclared func

Fixes #17758

Change-Id: I75f5dc5be85fd8a6791ac89dfc0681be759cca36
Reviewed-on: https://go-review.googlesource.com/c/go/+/248517
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -108,7 +108,17 @@ func typecheckclosure(clo *Node, top int) {
 
 	xfunc.Func.Nname.Sym = closurename(Curfn)
 	disableExport(xfunc.Func.Nname.Sym)
-	declare(xfunc.Func.Nname, PFUNC)
+	if xfunc.Func.Nname.Sym.Def != nil {
+		// The only case we can reach here is when the outer function was redeclared.
+		// In that case, don't bother to redeclare the closure. Otherwise, we will get
+		// a spurious error message, see #17758. While we are here, double check that
+		// we already reported other error.
+		if nsavederrors+nerrors == 0 {
+			Fatalf("unexpected symbol collision %v", xfunc.Func.Nname.Sym)
+		}
+	} else {
+		declare(xfunc.Func.Nname, PFUNC)
+	}
 	xfunc = typecheck(xfunc, ctxStmt)
 
 	// Type check the body now, but only if we're inside a function.

test/fixedbugs/issue17758.go

@@ -0,0 +1,17 @@
+// errorcheck
+
+// Copyright 2020 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 foo() {
+	_ = func() {}
+}
+
+func foo() { // ERROR "foo redeclared in this block"
+	_ = func() {}
+}
+
+func main() {}