cmd/gc: Friendlier errors on oversized arrays.

Someone new to the language may not know the connection between ints and arrays, which was the only thing that the previous error told you anything about.

Fixes #4256.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6739048

src/cmd/gc/const.c

@@ -348,13 +348,9 @@ toint(Val v)
 	return v;
 }
 
-void
+int
-overflow(Val v, Type *t)
+doesoverflow(Val v, Type *t)
 {
-	// v has already been converted
-	// to appropriate form for t.
-	if(t == T || t->etype == TIDEAL)
-		return;
 	switch(v.ctype) {
 	case CTINT:
 	case CTRUNE:
@@ -362,14 +358,14 @@ overflow(Val v, Type *t)
 			fatal("overflow: %T integer constant", t);
 		if(mpcmpfixfix(v.u.xval, minintval[t->etype]) < 0 ||
 		   mpcmpfixfix(v.u.xval, maxintval[t->etype]) > 0)
-			yyerror("constant %B overflows %T", v.u.xval, t);
+			return 1;
 		break;
 	case CTFLT:
 		if(!isfloat[t->etype])
 			fatal("overflow: %T floating-point constant", t);
 		if(mpcmpfltflt(v.u.fval, minfltval[t->etype]) <= 0 ||
 		   mpcmpfltflt(v.u.fval, maxfltval[t->etype]) >= 0)
-			yyerror("constant %#F overflows %T", v.u.fval, t);
+			return 1;
 		break;
 	case CTCPLX:
 		if(!iscomplex[t->etype])
@@ -378,6 +374,32 @@ overflow(Val v, Type *t)
 		   mpcmpfltflt(&v.u.cval->real, maxfltval[t->etype]) >= 0 ||
 		   mpcmpfltflt(&v.u.cval->imag, minfltval[t->etype]) <= 0 ||
 		   mpcmpfltflt(&v.u.cval->imag, maxfltval[t->etype]) >= 0)
+			return 1;
+		break;
+	}
+	return 0;
+}
+
+void
+overflow(Val v, Type *t)
+{
+	// v has already been converted
+	// to appropriate form for t.
+	if(t == T || t->etype == TIDEAL)
+		return;
+
+	if(!doesoverflow(v, t))
+		return;
+
+	switch(v.ctype) {
+	case CTINT:
+	case CTRUNE:
+		yyerror("constant %B overflows %T", v.u.xval, t);
+		break;
+	case CTFLT:
+		yyerror("constant %#F overflows %T", v.u.fval, t);
+		break;
+	case CTCPLX:
 		yyerror("constant %#F overflows %T", v.u.fval, t);
 		break;
 	}

src/cmd/gc/go.h

@@ -178,7 +178,7 @@ struct	Type
 	Strlit*	note;		// literal string annotation
 
 	// TARRAY
-	int32	bound;		// negative is dynamic array
+	vlong	bound;		// negative is dynamic array
 
 	int32	maplineno;	// first use of TFORW as map key
 	int32	embedlineno;	// first use of TFORW as embedded type
@@ -977,6 +977,7 @@ int	isconst(Node *n, int ct);
 Node*	nodcplxlit(Val r, Val i);
 Node*	nodlit(Val v);
 long	nonnegconst(Node *n);
+int	doesoverflow(Val v, Type *t);
 void	overflow(Val v, Type *t);
 int	smallintconst(Node *n);
 Val	toint(Val v);

src/cmd/gc/typecheck.c

@@ -390,8 +390,10 @@ reswitch:
 			if(t->bound < 0) {
 				yyerror("array bound must be non-negative");
 				goto error;
-			} else
+			} else if(doesoverflow(v, types[TINT])) {
-				overflow(v, types[TINT]);
+				yyerror("array bound is too large"); 
+				goto error;
+			}
 		}
 		typecheck(&r, Etype);
 		if(r->type == T)

test/fixedbugs/bug255.go

@@ -12,4 +12,4 @@ 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 "overflows"
+var g [1<<65]int	// ERROR "array bound is too large|overflows"