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Improve trait error messages.

--- a/parse/infer.c

+++ b/parse/infer.c

@@ -1560,7 +1560,7 @@

 static void specializeimpl(Inferstate *st, Node *n)

 {

-	Node *dcl, *proto, *name;

+	Node *dcl, *proto, *name, *sym;

 	Type *ty;

 	Htab *ht;

 	Trait *t;

@@ -1620,6 +1620,11 @@

 		/* and put the specialization into the global stab */

 		name = genericname(proto, ty);

+		sym = getdcl(file->file.globls, name);

+		if (sym)

+			fatal(n, "trait %s already specialized with %s on %s:%d",

+				namestr(t->name), tystr(n->impl.type),

+				fname(sym->loc), lnum(sym->loc));

 		dcl->decl.name = name;

 		putdcl(file->file.globls, dcl);

 		if (debugopt['S'])

@@ -1848,8 +1853,7 @@

 	if (t->type == Tyvar && !noerr) {

 		if (debugopt['T'])

 			dump(file, stdout);

-		fatal(

-				ctx, "underconstrained type %s near %s", tyfmt(buf, 1024, t), ctxstr(st, ctx));

+		fatal(ctx, "underconstrained type %s near %s", tyfmt(buf, 1024, t), ctxstr(st, ctx));

 	}

 	if (debugopt['u'] && !tyeq(orig, t)) {

@@ -1964,9 +1968,11 @@

 static void checkvar(Inferstate *st, Node *n)

 {

 	Node *dcl;

+	Type *ty;

 	dcl = decls[n->expr.did];

-	unify(st, n, type(st, n), tyfreshen(st, NULL, type(st, dcl)));

+	ty = tyfreshen(st, NULL, type(st, dcl));

+	unify(st, n, type(st, n), ty);

 }

 static void postcheck(Inferstate *st, Node *file)

@@ -2412,7 +2418,7 @@

 			trait = gettrait(f->file.globls, n->impl.traitname);

 			if (!trait)

 				fatal(n, "trait %s does not exist near %s",

-						namestr(n->impl.traitname), ctxstr(st, n));

+					namestr(n->impl.traitname), ctxstr(st, n));

 			ty = tf(st, n->impl.type);

 			settrait(ty, trait);

 		}

@@ -2433,7 +2439,7 @@

 			/* we merge, so we need to get it back again when error checking */

 			if (n->impl.isproto)

 				fatal(n, "missing implementation for prototype '%s %s'",

-						namestr(n->impl.traitname), tystr(n->impl.type));

+					namestr(n->impl.traitname), tystr(n->impl.type));

 		}

 	}

 }

--- a/parse/specialize.c

+++ b/parse/specialize.c

@@ -415,7 +415,7 @@

 		return d;

 	if (g->decl.trait) {

 		printf("%s\n", namestr(n));

-		fatal(g, "No trait implemented for for %s:%s", namestr(g->decl.name), tystr(to));

+		fatal(g, "no trait implemented for for %s:%s", namestr(g->decl.name), tystr(to));

 	}

 	/* namespaced names need to be looked up in their correct

 	 * context. */