cmd/gc: say 'non-constant array bound' instead of 'invalid array bound'

Fixes #8196.

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/141510044

src/cmd/gc/typecheck.c

@@ -408,7 +408,10 @@ reswitch:
 				v = toint(l->val);
 				break;
 			default:
-				yyerror("invalid array bound %N", l);
+				if(l->type != T && isint[l->type->etype] && l->op != OLITERAL)
+					yyerror("non-constant array bound %N", l);
+				else
+					yyerror("invalid array bound %N", l);
 				goto error;
 			}
 			t->bound = mpgetfix(v.u.xval);

test/fixedbugs/bug255.go

@@ -6,10 +6,15 @@
 
 package main
 
-var a [10]int	// ok
-var b [1e1]int	// ok
-var c [1.5]int	// ERROR "truncated"
-var d ["abc"]int	// ERROR "invalid array bound|not numeric"
-var e [nil]int	// ERROR "invalid array bound|not numeric"
-var f [e]int	// ERROR "invalid array bound|not constant"
-var g [1<<65]int	// ERROR "array bound is too large|overflows"
+var a [10]int      // ok
+var b [1e1]int     // ok
+var c [1.5]int     // ERROR "truncated"
+var d ["abc"]int   // ERROR "invalid array bound|not numeric"
+var e [nil]int     // ERROR "invalid array bound|not numeric"
+var f [e]int       // ERROR "invalid array bound|not constant"
+var g [1 << 65]int // ERROR "array bound is too large|overflows"
+var h [len(a)]int  // ok
+
+func ff() string
+
+var i [len([1]string{ff()})]int // ERROR "non-constant array bound|not constant"