Fixes #7878

src/Compilers/CSharp/Portable/Symbols/Source/SourceNamedTypeSymbol_Bases.cs

@@ -225,6 +225,7 @@ internal override ImmutableArray<NamedTypeSymbol> GetDeclaredInterfaces(ConsList
             var baseInterfaces = ArrayBuilder<NamedTypeSymbol>.GetInstance();
 
             NamedTypeSymbol baseType = null;
+            SourceLocation baseTypeLocation = null;
 
             foreach (var decl in this.declaration.Declarations)
             {
@@ -238,18 +239,21 @@ internal override ImmutableArray<NamedTypeSymbol> GetDeclaredInterfaces(ConsList
                     if ((object)baseType == null)
                     {
                         baseType = partBase;
+                        baseTypeLocation = decl.NameLocation;
                     }
                     else if (baseType.TypeKind == TypeKind.Error && (object)partBase != null)
                     {
                         // if the old base was an error symbol, copy it to the interfaces list so it doesn't get lost
                         partInterfaces = partInterfaces.Add(baseType);
                         baseType = partBase;
+                        baseTypeLocation = decl.NameLocation;
                     }
                     else if ((object)partBase != null && partBase != baseType && partBase.TypeKind != TypeKind.Error)
                     {
                         // the parts do not agree
                         var info = diagnostics.Add(ErrorCode.ERR_PartialMultipleBases, Locations[0], this);
                         baseType = new ExtendedErrorTypeSymbol(baseType, LookupResultKind.Ambiguous, info);
+                        baseTypeLocation = decl.NameLocation;
                         reportedPartialConflict = true;
                     }
                 }
@@ -273,7 +277,7 @@ internal override ImmutableArray<NamedTypeSymbol> GetDeclaredInterfaces(ConsList
             if ((object)baseType != null && baseType.IsStatic)
             {
                 // '{1}': cannot derive from static class '{0}'
-                diagnostics.Add(ErrorCode.ERR_StaticBaseClass, Locations[0], baseType, this);
+                diagnostics.Add(ErrorCode.ERR_StaticBaseClass, baseTypeLocation ?? Locations[0], baseType, this);
             }
 
             HashSet<DiagnosticInfo> useSiteDiagnostics = null;
@@ -281,7 +285,7 @@ internal override ImmutableArray<NamedTypeSymbol> GetDeclaredInterfaces(ConsList
             if ((object)baseType != null && !this.IsNoMoreVisibleThan(baseType, ref useSiteDiagnostics))
             {
                 // Inconsistent accessibility: base class '{1}' is less accessible than class '{0}'
-                diagnostics.Add(ErrorCode.ERR_BadVisBaseClass, Locations[0], this, baseType);
+                diagnostics.Add(ErrorCode.ERR_BadVisBaseClass, baseTypeLocation ?? Locations[0], this, baseType);
             }
 
             var baseInterfacesRO = baseInterfaces.ToImmutableAndFree();

src/Compilers/CSharp/Test/Symbol/Symbols/Source/BaseClassTests.cs

@@ -338,6 +338,63 @@ public class E : C.X { }
                 );
         }
 
+        [Fact, WorkItem(7878, "https://github.com/dotnet/roslyn/issues/7878")]
+        public void BadVisibilityPartial()
+        {
+            var text = @"
+internal class NV
+{
+}
+
+public partial class C1
+{
+}
+
+partial class C1 : NV
+{
+}
+
+public partial class C1
+{
+}
+";
+            var comp = CreateCompilationWithMscorlib(text);
+            comp.VerifyDiagnostics(
+                // (10,15): error CS0060: Inconsistent accessibility: base class 'NV' is less accessible than class 'C1'
+                // partial class C1 : NV
+                Diagnostic(ErrorCode.ERR_BadVisBaseClass, "C1").WithArguments("C1", "NV").WithLocation(10, 15));
+        }
+
+        [Fact, WorkItem(7878, "https://github.com/dotnet/roslyn/issues/7878")]
+        public void StaticBasePartial()
+        {
+            var text = @"
+static class NV
+{
+}
+
+public partial class C1
+{
+}
+
+partial class C1 : NV
+{
+}
+
+public partial class C1
+{
+}
+";
+            var comp = CreateCompilationWithMscorlib(text);
+            comp.VerifyDiagnostics(
+                // (10,15): error CS0709: 'C1': cannot derive from static class 'NV'
+                // partial class C1 : NV
+                Diagnostic(ErrorCode.ERR_StaticBaseClass, "C1").WithArguments("NV", "C1").WithLocation(10, 15),
+                // (10,15): error CS0060: Inconsistent accessibility: base class 'NV' is less accessible than class 'C1'
+                // partial class C1 : NV
+                Diagnostic(ErrorCode.ERR_BadVisBaseClass, "C1").WithArguments("C1", "NV").WithLocation(10, 15));
+        }
+
         [Fact]
         public void EricLiCase1()
         {