cmd/compile: only print one error for bad-type literal in assignment

Fixes #8438

Change-Id: Ib43cdcdc962a8d9e14faf984bc859a92ba1eb517
Reviewed-on: https://go-review.googlesource.com/40531
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -982,7 +982,9 @@ func assignconvfn(n *Node, t *types.Type, context func() string) *Node {
 	var why string
 	op := assignop(n.Type, t, &why)
 	if op == 0 {
+		if !old.Diag() {
 			yyerror("cannot use %L as type %v in %s%s", n, t, context(), why)
+		}
 		op = OCONV
 	}
 

test/fixedbugs/issue8438.go

@@ -0,0 +1,17 @@
+// errorcheck
+
+// 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.
+
+// Check that we don't print duplicate errors for string ->
+// array-literal conversion
+
+package main
+
+func main() {
+	_ = []byte{"foo"}   // ERROR "cannot convert"
+	_ = []int{"foo"}    // ERROR "cannot convert"
+	_ = []rune{"foo"}   // ERROR "cannot convert"
+	_ = []string{"foo"} // OK
+}

test/interface/explicit.go

@@ -53,7 +53,7 @@ func main() {
 	i2 = I2(i) // ERROR "invalid|missing N method"
 
 	e = E(t) // ok
-	t = T(e) // ERROR "need explicit|need type assertion|incompatible" "as type [*]T"
+	t = T(e) // ERROR "need explicit|need type assertion|incompatible"
 }
 
 type M interface {
@@ -81,7 +81,6 @@ var m2 M = jj // ERROR "incompatible|wrong type for M method"
 var m3 = M(ii) // ERROR "invalid|missing"
 var m4 = M(jj) // ERROR "invalid|wrong type for M method"
 
-
 type B1 interface {
 	_() // ERROR "methods must have a unique non-blank name"
 }