code review 4548073: gc: disallow ... in type conversions

src/cmd/gc/typecheck.c

 // Copyright 2009 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.
 
 /*
  * type check the whole tree of an expression.
  * calculates expression types.
  * evaluates compile time constants.
  * marks variables that escape the local frame.
  * rewrites n->op to be more specific in some cases.
@@ skipping 776 matching lines @@
                                 yyerror("invalid use of ... with builtin %#N", l);
                         // builtin: OLEN, OCAP, etc.
                         n->op = l->etype;
                         n->left = n->right;
                         n->right = N;
                         goto reswitch;
                 }
                 defaultlit(&n->left, T);
                 l = n->left;
                 if(l->op == OTYPE) {
-                        if(n->isddd)
+                        if(n->isddd || l->type->bound == -100)
                                 yyerror("invalid use of ... in type conversion", l);
                         // pick off before type-checking arguments
                         ok |= Erv;
                         // turn CALL(type, arg) into CONV(arg) w/ type
                         n->left = N;
                         n->op = OCONV;
                         n->type = l->type;
                         if(onearg(n, "conversion to %T", l->type) < 0)
                                 goto error;
                         goto doconv;
@@ skipping 1699 matching lines @@
                         return n->type;
 
                 // Check for ntype cycle.
                 if((f2 = resolve(f2)) != N && (f1 = resolve(f2->ntype)) != N) {
                         f2 = resolve(f1->ntype);
                         if(f1 == n || f2 == n)
                                 return T;
                 }
         }
 }