cmd/compile: better error when assigning to struct field in map

Identify this assignment case and instead of the more general error

    prog.go:6: cannot assign to students["sally"].age

produce

    prog.go:6: cannot directly assign to struct field students["sally"].age in map

that explains why the assignment is not possible.

Fixes #13779.

Change-Id: I90c10b445f907834fc1735aa66e44a0f447aa74f
Reviewed-on: https://go-review.googlesource.com/21462
Reviewed-by: David Chase <drchase@google.com>

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

@@ -3207,6 +3207,11 @@ func checkassign(stmt *Node, n *Node) {
 		return
 	}
 
+	if n.Op == ODOT && n.Left.Op == OINDEXMAP {
+		Yyerror("cannot directly assign to struct field %v in map", n)
+		return
+	}
+
 	Yyerror("cannot assign to %v", n)
 }
 

test/fixedbugs/issue13779.go

@@ -0,0 +1,15 @@
+// errorcheck
+
+// Copyright 2016 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 13779: provide better error message when directly assigning to struct field in map
+
+package main
+
+func main() {
+	type person struct{ age, weight, height int }
+	students := map[string]person{"sally": person{12, 50, 32}}
+	students["sally"].age = 3 // ERROR "cannot directly assign to struct field .* in map"
+}