Fix ITypeSymbol.BaseType documentation for type parameters

src/Compilers/CSharp/Test/Symbol/Symbols/GenericConstraintTests.cs

@@ -7460,5 +7460,34 @@ interface Base<N> : Base, ISetup<N> where N : Base<N>.Nest { }
             Assert.Null(model.GetAliasInfo(nest));
             Assert.Equal("Base.Nest", model.GetTypeInfo(nest).Type.ToDisplayString());
         }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/78605")]
+        public void TypeParameter_BaseType_ReturnsNull()
+        {
+            // This test verifies that ITypeSymbol.BaseType returns null for type parameters,
+            // not the effective base class.
+            var source = @"
+abstract class Base
+{
+    public abstract void Method();
+}
+
+class Derived<T> where T : Base
+{
+}";
+            var comp = CreateCompilation(source);
+            comp.VerifyDiagnostics();
+
+            var derivedType = comp.GetTypeByMetadataName("Derived`1");
+            Assert.NotNull(derivedType);
+
+            var typeParameter = derivedType.TypeParameters[0];
+            Assert.NotNull(typeParameter);
+
+            // Test the public API: ITypeSymbol.BaseType should return null for type parameters,
+            // even though there is an effective base class (Base in this case).
+            var publicTypeParameter = (ITypeParameterSymbol)typeParameter.GetPublicSymbol();
+            Assert.Null(publicTypeParameter.BaseType);
+        }
     }
 }

src/Compilers/Core/Portable/Symbols/ITypeSymbol.cs

@@ -26,8 +26,7 @@ public interface ITypeSymbol : INamespaceOrTypeSymbol
 
         /// <summary>
         /// The declared base type of this type, or null. The object type, interface types,
-        /// and pointer types do not have a base type. The base type of a type parameter
-        /// is its effective base class.
+        /// pointer types, and type parameters do not have a base type.
         /// </summary>
         INamedTypeSymbol? BaseType { get; }