feat(#1170): add lint for package members without void

.github/styles/config/vocabularies/Custom/accept.txt

@@ -18,3 +18,4 @@ inlined
 [Ii]nlining
 [Ss]ubgraph
 [Aa]pplication
+[Ff]orma(e)?

src/main/resources/org/eolang/lints/design/package-member-without-void.xsl

@@ -0,0 +1,35 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+* SPDX-FileCopyrightText: Copyright (c) 2016-2026 Objectionary.com
+* SPDX-License-Identifier: MIT
+-->
+<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:eo="https://www.eolang.org" version="2.0" id="package-member-without-void">
+  <xsl:import href="/org/eolang/parser/_funcs.xsl"/>
+  <xsl:import href="/org/eolang/funcs/lineno.xsl"/>
+  <xsl:import href="/org/eolang/funcs/escape.xsl"/>
+  <xsl:import href="/org/eolang/funcs/defect-context.xsl"/>
+  <xsl:output encoding="UTF-8" method="xml"/>
+  <xsl:template match="/">
+    <defects>
+      <xsl:if test="/object/metas/meta[head='package']">
+        <xsl:for-each select="/object/o[eo:abstract(.) and @name and not(o[eo:void(.)])]">
+          <defect>
+            <xsl:variable name="line" select="eo:lineno(@line)"/>
+            <xsl:attribute name="line">
+              <xsl:value-of select="$line"/>
+            </xsl:attribute>
+            <xsl:if test="$line = '0'">
+              <xsl:attribute name="context">
+                <xsl:value-of select="eo:defect-context(.)"/>
+              </xsl:attribute>
+            </xsl:if>
+            <xsl:attribute name="severity">warning</xsl:attribute>
+            <xsl:text>The package member </xsl:text>
+            <xsl:value-of select="eo:escape(@name)"/>
+            <xsl:text> does not declare any void attributes, so it can never be reached in the implicit (receiver) form, only through the package namespace</xsl:text>
+          </defect>
+        </xsl:for-each>
+      </xsl:if>
+    </defects>
+  </xsl:template>
+</xsl:stylesheet>

src/main/resources/org/eolang/motives/design/package-member-without-void.md

@@ -0,0 +1,36 @@
+# Package member without void
+
+A member of a package is also implicitly a method of any object whose forma
+matches the package name. `bytes.as-number x` and `x.as-number` reach the
+same `Φ.bytes.as-number` object, because the runtime binds the receiver into
+the member's first void attribute when the implicit, dotted form is used.
+
+A member that declares no void attributes has no slot for that receiver.
+The implicit form fails at runtime then, even though the explicit,
+package-qualified form works fine.
+
+Incorrect:
+
+```eo
++package bytes
+
+# As number.
+[] > as-number
+```
+
+Correct:
+
+```eo
++package bytes
+
+# As number.
+[b] > as-number
+```
+
+Only top-level objects of a file that declares `+package` are checked. A
+void-less formation is fine everywhere else: a ready-made constant such as
+`[] > stdin`, or a local helper nested inside another formation's body.
+
+See [objectionary/lints#1170] for the discussion that motivated this lint.
+
+[objectionary/lints#1170]: https://github.com/objectionary/lints/issues/1170

src/test/resources/org/eolang/lints/packs/single/package-member-without-void/allows-nested-void-less-formation.yaml

@@ -0,0 +1,13 @@
+# SPDX-FileCopyrightText: Copyright (c) 2016-2026 Objectionary.com
+# SPDX-License-Identifier: MIT
+---
+sheets:
+  - /org/eolang/lints/design/package-member-without-void.xsl
+defects: 0
+input: |
+  +package foo
+
+  # Bar.
+  [x] > bar
+    # Baz.
+    [] > baz

src/test/resources/org/eolang/lints/packs/single/package-member-without-void/allows-package-member-with-void.yaml

@@ -0,0 +1,11 @@
+# SPDX-FileCopyrightText: Copyright (c) 2016-2026 Objectionary.com
+# SPDX-License-Identifier: MIT
+---
+sheets:
+  - /org/eolang/lints/design/package-member-without-void.xsl
+defects: 0
+input: |
+  +package foo
+
+  # Bar.
+  [x] > bar

src/test/resources/org/eolang/lints/packs/single/package-member-without-void/allows-void-less-formation-without-package.yaml

@@ -0,0 +1,9 @@
+# SPDX-FileCopyrightText: Copyright (c) 2016-2026 Objectionary.com
+# SPDX-License-Identifier: MIT
+---
+sheets:
+  - /org/eolang/lints/design/package-member-without-void.xsl
+defects: 0
+input: |
+  # Stdin.
+  [] > stdin

src/test/resources/org/eolang/lints/packs/single/package-member-without-void/catches-atom-package-member-without-void.yaml

@@ -0,0 +1,13 @@
+# SPDX-FileCopyrightText: Copyright (c) 2016-2026 Objectionary.com
+# SPDX-License-Identifier: MIT
+---
+sheets:
+  - /org/eolang/lints/design/package-member-without-void.xsl
+asserts:
+  - /defects[count(defect[@severity='warning'])=1]
+  - /defects/defect[@line='4']
+input: |
+  +package foo
+
+  # Bar.
+  [] > bar /number

src/test/resources/org/eolang/lints/packs/single/package-member-without-void/catches-only-top-level-member.yaml

@@ -0,0 +1,15 @@
+# SPDX-FileCopyrightText: Copyright (c) 2016-2026 Objectionary.com
+# SPDX-License-Identifier: MIT
+---
+sheets:
+  - /org/eolang/lints/design/package-member-without-void.xsl
+asserts:
+  - /defects[count(defect[@severity='warning'])=1]
+  - /defects/defect[@line='4']
+input: |
+  +package foo
+
+  # Bar.
+  [] > bar
+    # Baz.
+    [] > baz

src/test/resources/org/eolang/lints/packs/single/package-member-without-void/catches-package-member-without-void.yaml

@@ -0,0 +1,13 @@
+# SPDX-FileCopyrightText: Copyright (c) 2016-2026 Objectionary.com
+# SPDX-License-Identifier: MIT
+---
+sheets:
+  - /org/eolang/lints/design/package-member-without-void.xsl
+asserts:
+  - /defects[count(defect[@severity='warning'])=1]
+  - /defects/defect[@line='4']
+input: |
+  +package foo
+
+  # Bar.
+  [] > bar