cmd/compile: allow := to shadow dot-imported names

Historically, gc optimistically parsed the left-hand side of
assignments as expressions. Later, if it discovered a ":=" assignment,
it rewrote the parsed expressions as declarations.

This failed in the presence of dot imports though, because we lost
information about whether an imported object was named via a bare
identifier "Foo" or a normal qualified "pkg.Foo".

This CL fixes the issue by specially noding the left-hand side of ":="
assignments.

Fixes #22076.

Change-Id: I18190ecdb863112e7d009e1687e6112eec559921
Reviewed-on: https://go-review.googlesource.com/66810
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -758,15 +758,10 @@ func (p *noder) stmtFall(stmt syntax.Stmt, fallOK bool) *Node {
 			return n
 		}
 
-		lhs := p.exprList(stmt.Lhs)
-		rhs := p.exprList(stmt.Rhs)
-
 		n := p.nod(stmt, OAS, nil, nil) // assume common case
 
-		if stmt.Op == syntax.Def {
+		rhs := p.exprList(stmt.Rhs)
-			n.SetColas(true)
+		lhs := p.assignList(stmt.Lhs, n, stmt.Op == syntax.Def)
-			colasdefn(lhs, n) // modifies lhs, call before using lhs[0] in common case
-		}
 
 		if len(lhs) == 1 && len(rhs) == 1 {
 			// common case
@@ -845,6 +840,66 @@ func (p *noder) stmtFall(stmt syntax.Stmt, fallOK bool) *Node {
 	panic("unhandled Stmt")
 }
 
+func (p *noder) assignList(expr syntax.Expr, defn *Node, colas bool) []*Node {
+	if !colas {
+		return p.exprList(expr)
+	}
+
+	defn.SetColas(true)
+
+	var exprs []syntax.Expr
+	if list, ok := expr.(*syntax.ListExpr); ok {
+		exprs = list.ElemList
+	} else {
+		exprs = []syntax.Expr{expr}
+	}
+
+	res := make([]*Node, len(exprs))
+	seen := make(map[*types.Sym]bool, len(exprs))
+
+	newOrErr := false
+	for i, expr := range exprs {
+		p.lineno(expr)
+		res[i] = nblank
+
+		name, ok := expr.(*syntax.Name)
+		if !ok {
+			yyerrorpos(expr.Pos(), "non-name %v on left side of :=", p.expr(expr))
+			newOrErr = true
+			continue
+		}
+
+		sym := p.name(name)
+		if sym.IsBlank() {
+			continue
+		}
+
+		if seen[sym] {
+			yyerrorpos(expr.Pos(), "%v repeated on left side of :=", sym)
+			newOrErr = true
+			continue
+		}
+		seen[sym] = true
+
+		if sym.Block == types.Block {
+			res[i] = oldname(sym)
+			continue
+		}
+
+		newOrErr = true
+		n := newname(sym)
+		declare(n, dclcontext)
+		n.Name.Defn = defn
+		defn.Ninit.Append(nod(ODCL, n, nil))
+		res[i] = n
+	}
+
+	if !newOrErr {
+		yyerrorl(defn.Pos, "no new variables on left side of :=")
+	}
+	return res
+}
+
 func (p *noder) blockStmt(stmt *syntax.BlockStmt) []*Node {
 	p.openScope(stmt.Pos())
 	nodes := p.stmts(stmt.List)
@@ -884,12 +939,7 @@ func (p *noder) forStmt(stmt *syntax.ForStmt) *Node {
 
 		n = p.nod(r, ORANGE, nil, p.expr(r.X))
 		if r.Lhs != nil {
-			lhs := p.exprList(r.Lhs)
+			n.List.Set(p.assignList(r.Lhs, n, r.Def))
-			n.List.Set(lhs)
-			if r.Def {
-				n.SetColas(true)
-				colasdefn(lhs, n)
-			}
 		}
 	} else {
 		n = p.nod(stmt, OFOR, nil, nil)

test/fixedbugs/bug388.go

@@ -14,7 +14,7 @@ func foo(runtime.UintType, i int) {  // ERROR "cannot declare name runtime.UintT
 }
 
 func bar(i int) {
-	runtime.UintType := i       // ERROR "cannot declare name runtime.UintType|non-name on left side|undefined identifier"
+	runtime.UintType := i       // ERROR "non-name runtime.UintType|non-name on left side|undefined identifier"
 	println(runtime.UintType)	// GCCGO_ERROR "invalid use of type|undefined identifier"
 }
 

test/fixedbugs/issue20250.go

@@ -16,8 +16,8 @@ type T struct {
 
 func f(a T) { // ERROR "live at entry to f: a"
 	var e interface{}
-	func() { // ERROR "live at entry to f.func1: &e a"
+	func() { // ERROR "live at entry to f.func1: a &e"
-		e = a.s // ERROR "live at call to convT2Estring: &e a" "live at call to writebarrierptr: a"
+		e = a.s // ERROR "live at call to convT2Estring: a &e" "live at call to writebarrierptr: a"
 	}() // ERROR "live at call to f.func1: e$"
 	// Before the fix, both a and e were live at the previous line.
 	_ = e

test/fixedbugs/issue22076.go

@@ -0,0 +1,25 @@
+// compile
+
+// Copyright 2017 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.
+
+// Issue 22076: Couldn't use ":=" to declare names that refer to
+// dot-imported symbols.
+
+package p
+
+import . "bytes"
+
+var _ Reader // use "bytes" import
+
+func _() {
+	Buffer := 0
+	_ = Buffer
+}
+
+func _() {
+	for Buffer := range []int{} {
+		_ = Buffer
+	}
+}