compiler: implement blank package name semantics

cmd/wcc/check.c

@@ -62,6 +62,8 @@ static const char *use_path(Node *file, const char *curmod, int source,
     const char *alias);
 static const char *find_use_path(Node *file, const char *curmod, int source,
     const char *alias, int mark);
+static Node *find_use_binding(Node *file, const char *curmod, int source,
+    const char *alias, int mark);
 static int src_imports(Node *file, const char *modtag, int source,
     const char *name);
 static Sym *lookup_visible(Checker *c, const char *name);
@@ -96,6 +98,11 @@ resolve_typename(Checker *c, Node *n)
 			char *head = astrndup(c->a, nm, hl);
 			Sym *m = scope_lookup(c->cur, head);
 			if (m && (m->kind == SK_USE || m->use_alias)) {
+				Node *u = find_use_binding(c->file, c->cur_mod,
+				    c->cur_source, head, 1);
+				if (u != NULL && u->usepkgname != NULL
+				    && strcmp(u->usepkgname, "_") == 0)
+					return ty_err;
 				/* M1 #22: map the qualifier alias to its dotted
 				 * import path (symbols are path-keyed). */
 				const char *mk = use_path(c->file, c->cur_mod,
@@ -1521,6 +1528,11 @@ cexpr(Checker *c, Node *n)
 		if (n->lhs && n->lhs->kind == N_IDENT) {
 			Sym *ms = lookup_visible(c, n->lhs->str);
 			if (ms && (ms->kind == SK_USE || ms->use_alias)) {
+				Node *u = find_use_binding(c->file, c->cur_mod,
+				    c->cur_source, n->lhs->str, 1);
+				if (u != NULL && u->usepkgname != NULL
+				    && strcmp(u->usepkgname, "_") == 0)
+					return n->type = ty_err;
 				/* Module-qualified ref. `use_alias` covers
 				 * the self-import case where the module's
 				 * type name shadowed the SK_USE; the leaf
@@ -3016,6 +3028,18 @@ import_binding_pos(Node *u)
 	return p;
 }
 
+static Pos
+import_path_pos(Node *u)
+{
+	Pos p = u->pos;
+	if (u->usepathfile != NULL) {
+		p.file = u->usepathfile;
+		p.line = u->usepathline;
+		p.col = u->usepathcol;
+	}
+	return p;
+}
+
 /*
  * use_path — map a source-file default qualifier (the imported package's
  * declared name) to the full canonical import path it binds, for
@@ -3056,6 +3080,33 @@ find_use_path(Node *file, const char *curmod, int source, const char *alias,
 	return NULL;
 }
 
+static Node *
+find_use_binding(Node *file, const char *curmod, int source,
+    const char *alias, int mark)
+{
+	if (file == NULL || alias == NULL) return NULL;
+	/* Source-zero self-qualification wins before import lookup in
+	 * find_use_path and therefore cannot denote an imported package object. */
+	if (source == 0 && curmod != NULL) {
+		const char *dot = strrchr(curmod, '.');
+		const char *leaf = dot ? dot + 1 : curmod;
+		if (strcmp(alias, leaf) == 0) return NULL;
+	}
+	for (Node *u = file->list; u; u = u->next) {
+		if (u->kind != N_USE || u->str == NULL || invalid_init_import(u)
+		    || u->sourceid != source || strcmp(u->str, alias) != 0)
+			continue;
+		const char *um = decl_mod(file, u);
+		int same = (um == NULL) ? (curmod == NULL)
+		    : (curmod != NULL && strcmp(um, curmod) == 0);
+		if (same) {
+			if (mark) u->used = 1;
+			return u;
+		}
+	}
+	return NULL;
+}
+
 static const char *
 use_path(Node *file, const char *curmod, int source, const char *alias)
 {
@@ -4379,11 +4430,48 @@ check_test_target(const Checker *c, const char *path)
 	return 0;
 }
 
+static void
+reject_blank_package_names(Checker *c, Node *file)
+{
+	for (Node *p = file->body; p; p = p->next)
+		if (p->kind == N_FILE && p->pkgname != NULL
+		    && strcmp(p->pkgname, "_") == 0)
+			err(c, p->pos, "invalid package name _");
+}
+
+static void
+reject_invalid_imports(Checker *c, Node *file)
+{
+	for (Node *u = file->list; u; u = u->next) {
+		if (u->kind != N_USE || u->usepath == NULL
+		    || u->usepkgname == NULL
+		    || strcmp(u->usepkgname, "_") != 0)
+			continue;
+		u->used = 1;
+		int seen = 0;
+		for (Node *v = file->list; v != u; v = v->next) {
+			if (v->kind == N_USE && v->usepath != NULL
+			    && v->usepkgname != NULL
+			    && strcmp(v->usepkgname, "_") == 0
+			    && strcmp(v->usepath, u->usepath) == 0) {
+				seen = 1;
+				break;
+			}
+		}
+		if (!seen)
+			err(c, import_path_pos(u),
+			    "could not import %s (invalid package name: \"_\")",
+			    u->usepath);
+	}
+}
+
 void
 check_file(Checker *c, Node *file)
 {
 	if (file == NULL || file->kind != N_FILE) return;
 	c->file = file;
+	reject_blank_package_names(c, file);
+	reject_invalid_imports(c, file);
 
 	/* Under -T, prepend the dispatcher support import before pass 1 so
 	 * decl_mod keys the runner under the selected support module. A pure