Clarify usage of version and requires it in releases, closes #9686

Author: José Valim

lib/elixir/lib/version.ex

@@ -5,8 +5,8 @@ defmodule Version do
   A version is a string in a specific format or a `Version`
   generated after parsing via `Version.parse/1`.
 
-  `Version` parsing and requirements follow
-  [SemVer 2.0 schema](https://semver.org/).
+  Although Elixir projects are not required to follow SemVer,
+  they must follow the format outlined on [SemVer 2.0 schema](https://semver.org/).
 
   ## Versions
 
@@ -29,7 +29,7 @@ defmodule Version do
   ## Struct
 
   The version is represented by the `Version` struct and fields
-  are named according to SemVer: `:major`, `:minor`, `:patch`,
+  are named according to SemVer 2.0: `:major`, `:minor`, `:patch`,
   `:pre`, and `:build`.
 
   ## Requirements

lib/mix/lib/mix/release.ex

@@ -133,6 +133,9 @@ defmodule Mix.Release do
 
           to_string(version)
 
+        "" ->
+          Mix.raise("The release :version cannot be an empty string")
+
         _ ->
           version
       end

lib/mix/lib/mix/tasks/compile.app.ex

@@ -200,7 +200,9 @@ defmodule Mix.Tasks.Compile.App do
     ensure_present(:version, version)
 
     unless is_binary(version) and match?({:ok, _}, Version.parse(version)) do
-      Mix.raise("Expected :version to be a SemVer version, got: #{inspect(version)}")
+      Mix.raise(
+        "Expected :version to be a valid Version, got: #{inspect(version)} (see the Version module for more information)"
+      )
     end
   end
 

lib/mix/test/mix/release_test.exs

@@ -170,6 +170,12 @@ defmodule Mix.ReleaseTest do
       end
     end
 
+    test "raises for blank version" do
+      assert_raise Mix.Error, ~r"The release :version cannot be an empty string", fn ->
+        from_config!(nil, config(version: ""), [])
+      end
+    end
+
     test "raises on invalid release names" do
       assert_raise Mix.Error, ~r"Invalid release name", fn ->
         from_config!(nil, config(releases: ["invalid name": []]), [])

lib/mix/test/mix/tasks/compile.app_test.exs

@@ -245,7 +245,7 @@ defmodule Mix.Tasks.Compile.AppTest do
     Mix.Project.push(InvalidVsnProject)
 
     in_fixture("no_mixfile", fn ->
-      message = "Expected :version to be a SemVer version, got: \"0.3\""
+      message = ~r"Expected :version to be a valid Version, got: \"0.3\""
 
       assert_raise Mix.Error, message, fn ->
         Mix.Tasks.Compile.App.run([])