cmd/compile: fix position for "missing type in composite literal" error

Fixes #18231.

Change-Id: If1615da4db0e6f0516369a1dc37340d80c78f237
Reviewed-on: https://go-review.googlesource.com/37018
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -2882,10 +2882,7 @@ func typecheckcomplit(n *Node) *Node {
 	}()
 
 	if n.Right == nil {
-		if n.List.Len() != 0 {
+		yyerrorl(n.Pos, "missing type in composite literal")
-			setlineno(n.List.First())
-		}
-		yyerror("missing type in composite literal")
 		n.Type = nil
 		return n
 	}

test/fixedbugs/issue18231.go

@@ -0,0 +1,20 @@
+// 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.
+
+// Test that error message for composite literals with
+// missing type is at the right place.
+
+package p
+
+type T struct {
+	f map[string]string
+}
+
+var _ = T{
+	f: {                // ERROR "missing type in composite literal"
+		"a": "b",
+	},
+}