gc: check parameter declarations in interface fields

Fixes #1663.
Fixes #1871.

R=rsc, lstoakes
CC=golang-dev
https://golang.org/cl/4530084

src/cmd/gc/dcl.c

@@ -523,6 +523,30 @@ colas(NodeList *left, NodeList *right)
 	return as;
 }
 
+/*
+ * declare the arguments in an
+ * interface field declaration.
+ */
+void
+ifacedcl(Node *n)
+{
+	if(n->op != ODCLFIELD || n->right == N)
+		fatal("ifacedcl");
+
+	dclcontext = PAUTO;
+	markdcl();
+	funcdepth++;
+	n->outer = curfn;
+	curfn = n;
+	funcargs(n->right);
+
+	// funcbody is normally called after the parser has
+	// seen the body of a function but since an interface
+	// field declaration does not have a body, we must
+	// call it now to pop the current declaration context.
+	funcbody(n);
+}
+
 /*
  * declare the function proper
  * and declare the arguments.

src/cmd/gc/go.h

@@ -870,6 +870,7 @@ void	funcbody(Node *n);
 void	funccompile(Node *n, int isclosure);
 void	funchdr(Node *n);
 Type*	functype(Node *this, NodeList *in, NodeList *out);
+void	ifacedcl(Node *n);
 int	isifacemethod(Type *f);
 void	markdcl(void);
 Node*	methodname(Node *n, Type *t);

src/cmd/gc/go.y

@@ -1380,6 +1380,7 @@ interfacedcl:
 	new_name indcl
 	{
 		$$ = nod(ODCLFIELD, $1, $2);
+		ifacedcl($$);
 	}
 |	packname
 	{

test/fixedbugs/bug342.go

+// errchk $G $D/$F.go
+
+// Copyright 2011 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 1871.
+
+package p
+
+type a interface {
+	foo(x int) (x int) // ERROR "redeclared|redefinition"
+}
+
+var b interface {
+	bar(y int) (y int) // ERROR "redeclared|redefinition"
+}
+
+/*
+Previously:
+
+bug.go:1 x redclared in this block
+    previous declaration at bug.go:1
+*/