cmd/compile: use "N variables but M values" error for OAS

Makes the error message more consistent between OAS and OAS2.

Fixes #26616.

Change-Id: I07ab46c5ef8a37efb2cb557632697f5d1bf789f7
Reviewed-on: https://go-review.googlesource.com/131280
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -3333,10 +3333,17 @@ func typecheckas(n *Node) {
 		n.Left = typecheck(n.Left, Erv|Easgn)
 	}
 
-	n.Right = typecheck(n.Right, Erv)
+	// Use Efnstruct so we can emit an "N variables but M values" error
+	// to be consistent with typecheckas2 (#26616).
+	n.Right = typecheck(n.Right, Erv|Efnstruct)
 	checkassign(n, n.Left)
 	if n.Right != nil && n.Right.Type != nil {
-		if n.Left.Type != nil {
+		if n.Right.Type.IsFuncArgStruct() {
+			yyerror("assignment mismatch: 1 variable but %d values", n.Right.Type.NumFields())
+			// Multi-value RHS isn't actually valid for OAS; nil out
+			// to indicate failed typechecking.
+			n.Right.Type = nil
+		} else if n.Left.Type != nil {
 			n.Right = assignconv(n.Right, n.Left.Type, "assignment")
 		}
 	}

test/fixedbugs/issue26616.go

+// errorcheck
+
+// Copyright 2018 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 p
+
+var x int = three() // ERROR "1 variable but 3 values"
+
+func f() {
+	var _ int = three() // ERROR "1 variable but 3 values"
+	var a int = three() // ERROR "1 variable but 3 values"
+	a = three()         // ERROR "1 variable but 3 values"
+	b := three()        // ERROR "1 variable but 3 values"
+
+	_, _ = a, b
+}
+
+func three() (int, int, int)