Detect if branchConfig is Mainline and has empty prerelease tag configured to update prerelease tag in semver

Author: Enrique Raso Barbero

Diff for: src/GitVersion.Core/Extensions/StringExtensions.cs

@@ -102,4 +102,6 @@ public static bool IsEquivalentTo(this string self, string? other) =>
 
     /// <inheritdoc cref="string.IsNullOrWhiteSpace"/>
     public static bool IsNullOrWhiteSpace([NotNullWhen(false)] this string? value) => string.IsNullOrWhiteSpace(value);
+
+    public static bool IsEmpty([NotNullWhen(false)] this string? value) => string.Empty.Equals(value);
 }

Diff for: src/GitVersion.Core/PublicAPI.Shipped.txt

@@ -1325,6 +1325,7 @@ static GitVersion.Extensions.StringExtensions.IsHelp(this string! singleArgument
 static GitVersion.Extensions.StringExtensions.IsInit(this string! singleArgument) -> bool
 static GitVersion.Extensions.StringExtensions.IsNullOrEmpty(this string? value) -> bool
 static GitVersion.Extensions.StringExtensions.IsNullOrWhiteSpace(this string? value) -> bool
+static GitVersion.Extensions.StringExtensions.IsEmpty(this string? value) -> bool
 static GitVersion.Extensions.StringExtensions.IsSwitch(this string? value, string! switchName) -> bool
 static GitVersion.Extensions.StringExtensions.IsSwitchArgument(this string? value) -> bool
 static GitVersion.Extensions.StringExtensions.IsTrue(this string? value) -> bool

Diff for: src/GitVersion.Core/VersionCalculation/NextVersionCalculator.cs

@@ -76,8 +76,12 @@ public NextVersion FindVersion()
         var hasPreReleaseTag = semver.PreReleaseTag?.HasTag() == true;
         var tag = configuration.Value.Tag;
         var branchConfigHasPreReleaseTagConfigured = !tag.IsNullOrEmpty();
-        var preReleaseTagDoesNotMatchConfiguration = hasPreReleaseTag && branchConfigHasPreReleaseTagConfigured && semver.PreReleaseTag?.Name != tag;
-        if (semver.PreReleaseTag?.HasTag() != true && branchConfigHasPreReleaseTagConfigured || preReleaseTagDoesNotMatchConfiguration)
+        var branchConfigIsMainlineAndHasEmptyPreReleaseTagConfigured = configuration.Value.IsMainline && tag.IsEmpty();
+        var preReleaseTagDoesNotMatchConfiguration = hasPreReleaseTag
+                                                     && (branchConfigHasPreReleaseTagConfigured || branchConfigIsMainlineAndHasEmptyPreReleaseTagConfigured)
+                                                     && semver.PreReleaseTag?.Name != tag;
+        var preReleaseTagOnlyInBranchConfig = !hasPreReleaseTag && branchConfigHasPreReleaseTagConfigured;
+        if (preReleaseTagOnlyInBranchConfig || preReleaseTagDoesNotMatchConfiguration)
         {
             UpdatePreReleaseTag(configuration.Value, semver, baseVersion.BranchNameOverride);
         }
@@ -112,6 +116,12 @@ private void UpdatePreReleaseTag(EffectiveConfiguration configuration, SemanticV
     {
         var tagToUse = configuration.GetBranchSpecificTag(this.log, Context.CurrentBranch.Name.Friendly, branchNameOverride);
 
+        if (configuration.IsMainline && tagToUse.IsEmpty())
+        {
+            semanticVersion.PreReleaseTag = new SemanticVersionPreReleaseTag(tagToUse, null);
+            return;
+        }
+
         long? number = null;
 
         var lastTag = this.repositoryStore