[ApiCompat] Warn if abstract keyword is removed from member

src/Compatibility/ApiCompat/Microsoft.DotNet.ApiCompatibility/Resources.resx

@@ -186,8 +186,8 @@
   <data name="CannotAddVirtualToMember" xml:space="preserve">
     <value>Cannot add virtual keyword to member '{0}'.</value>
   </data>
-  <data name="CannotRemoveVirtualFromMember" xml:space="preserve">
-    <value>Cannot remove virtual keyword from member '{0}'.</value>
+  <data name="CannotRemoveVirtualOrAbstractFromMember" xml:space="preserve">
+    <value>Cannot remove '{0}' keyword from member '{1}'.</value>
   </data>
   <data name="ApiCompatibilityHeader" xml:space="preserve">
     <value>API compatibility errors between '{0}' ({2}) and '{1}' ({3}):</value>

src/Compatibility/ApiCompat/Microsoft.DotNet.ApiCompatibility/Rules/CannotAddOrRemoveVirtualKeyword.cs

@@ -63,7 +63,7 @@ private void RunOnMemberSymbol(ISymbol? left, ISymbol? right, ITypeSymbol leftCo
                         leftMetadata,
                         rightMetadata,
                         DiagnosticIds.CannotRemoveVirtualFromMember,
-                        string.Format(Resources.CannotRemoveVirtualFromMember, left),
+                        string.Format(Resources.CannotRemoveVirtualOrAbstractFromMember, "virtual", left),
                         DifferenceType.Removed,
                         right));
                 }
@@ -86,6 +86,21 @@ private void RunOnMemberSymbol(ISymbol? left, ISymbol? right, ITypeSymbol leftCo
                         right));
                 }
             }
+
+            if (left.IsAbstract)
+            {
+                if (!right.IsAbstract && !right.IsVirtual)
+                {
+                    // abstract can be made virtual but cannot remove abstract.
+                    differences.Add(new CompatDifference(
+                        leftMetadata,
+                        rightMetadata,
+                        DiagnosticIds.CannotRemoveVirtualFromMember,
+                        string.Format(Resources.CannotRemoveVirtualOrAbstractFromMember, "abstract", left),
+                        DifferenceType.Removed,
+                        right));
+                }
+            }
         }
     }
 }

test/Microsoft.DotNet.ApiCompatibility.Tests/Rules/CannotAddOrRemoveVirtualKeywordTests.cs

@@ -43,6 +43,20 @@ public static IEnumerable<object[]> RemovedCases()
                 false,
                 CreateDifferences((DifferenceType.Removed, "M:CompatTests.First.F")),
             };
+            // remove abstract from member
+            yield return new object[] {
+                CreateType(" abstract class", " public abstract void F();"),
+                CreateType(" abstract class", " public void F() {}"),
+                false,
+                CreateDifferences((DifferenceType.Removed, "M:CompatTests.First.F")),
+            };
+            // replace abstract with virtual (no diffs expected)
+            yield return new object[] {
+                CreateType(" abstract class", " public abstract void F();"),
+                CreateType(" abstract class", " public virtual void F() {}"),
+                false,
+                new CompatDifference[] { }
+            };
             // properties
             yield return new object[] {
                 CreateType(" class", " public virtual int F { get; }"),