Fix #7878 for interfaces

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

@@ -10,6 +10,7 @@
 using Microsoft.CodeAnalysis.Text;
 using Roslyn.Utilities;
 using System.Collections.Generic;
+using Microsoft.CodeAnalysis.Collections;
 
 namespace Microsoft.CodeAnalysis.CSharp.Symbols
 {
@@ -226,6 +227,7 @@ internal override ImmutableArray<NamedTypeSymbol> GetDeclaredInterfaces(ConsList
 
             NamedTypeSymbol baseType = null;
             SourceLocation baseTypeLocation = null;
+            var interfaceLocations = PooledDictionary<NamedTypeSymbol, SourceLocation>.GetInstance();
 
             foreach (var decl in this.declaration.Declarations)
             {
@@ -258,34 +260,32 @@ internal override ImmutableArray<NamedTypeSymbol> GetDeclaredInterfaces(ConsList
                     }
                 }
 
-                int n = baseInterfaces.Count;
-                foreach (var t in partInterfaces) // this could probably be done more efficiently with a side hash table if it proves necessary
+                foreach (var t in partInterfaces)
                 {
-                    for (int i = 0; i < n; i++)
+                    if (!interfaceLocations.ContainsKey(t))
                     {
-                        if (t == baseInterfaces[i])
-                        {
-                            goto alreadyInInterfaceList;
-                        }
+                        baseInterfaces.Add(t);
+                        interfaceLocations.Add(t, decl.NameLocation);
                     }
-
-                    baseInterfaces.Add(t);
-                alreadyInInterfaceList:;
                 }
             }
 
-            if ((object)baseType != null && baseType.IsStatic)
-            {
-                // '{1}': cannot derive from static class '{0}'
-                diagnostics.Add(ErrorCode.ERR_StaticBaseClass, baseTypeLocation ?? Locations[0], baseType, this);
-            }
-
             HashSet<DiagnosticInfo> useSiteDiagnostics = null;
 
-            if ((object)baseType != null && !this.IsNoMoreVisibleThan(baseType, ref useSiteDiagnostics))
+            if ((object)baseType != null)
             {
-                // Inconsistent accessibility: base class '{1}' is less accessible than class '{0}'
-                diagnostics.Add(ErrorCode.ERR_BadVisBaseClass, baseTypeLocation ?? Locations[0], this, baseType);
+                Debug.Assert(baseTypeLocation != null);
+                if (baseType.IsStatic)
+                {
+                    // '{1}': cannot derive from static class '{0}'
+                    diagnostics.Add(ErrorCode.ERR_StaticBaseClass, baseTypeLocation, baseType, this);
+                }
+
+                if (!this.IsNoMoreVisibleThan(baseType, ref useSiteDiagnostics))
+                {
+                    // Inconsistent accessibility: base class '{1}' is less accessible than class '{0}'
+                    diagnostics.Add(ErrorCode.ERR_BadVisBaseClass, baseTypeLocation, this, baseType);
+                }
             }
 
             var baseInterfacesRO = baseInterfaces.ToImmutableAndFree();
@@ -296,11 +296,13 @@ internal override ImmutableArray<NamedTypeSymbol> GetDeclaredInterfaces(ConsList
                     if (!i.IsAtLeastAsVisibleAs(this, ref useSiteDiagnostics))
                     {
                         // Inconsistent accessibility: base interface '{1}' is less accessible than interface '{0}'
-                        diagnostics.Add(ErrorCode.ERR_BadVisBaseInterface, Locations[0], this, i);
+                        diagnostics.Add(ErrorCode.ERR_BadVisBaseInterface, interfaceLocations[i], this, i);
                     }
                 }
             }
 
+            interfaceLocations.Free();
+
             diagnostics.Add(Locations[0], useSiteDiagnostics);
 
             return new Tuple<NamedTypeSymbol, ImmutableArray<NamedTypeSymbol>>(baseType, baseInterfacesRO);

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

@@ -395,6 +395,54 @@ public partial class C1
                 Diagnostic(ErrorCode.ERR_BadVisBaseClass, "C1").WithArguments("C1", "NV").WithLocation(10, 15));
         }
 
+
+        [Fact, WorkItem(7878, "https://github.com/dotnet/roslyn/issues/7878")]
+        public void BadVisInterfacePartial()
+        {
+            var text = @"
+interface IFoo
+{
+    void Moo();
+}
+
+interface IBaz
+{
+    void Noo();
+}
+
+interface IBam
+{
+    void Zoo();
+}
+
+public partial interface IBar
+{
+}
+
+partial interface IBar : IFoo, IBam
+{
+}
+
+partial interface IBar : IBaz, IBaz
+{
+}
+";
+            var comp = CreateCompilationWithMscorlib(text);
+            comp.VerifyDiagnostics(
+                // (25,32): error CS0528: 'IBaz' is already listed in interface list
+                // partial interface IBar : IBaz, IBaz
+                Diagnostic(ErrorCode.ERR_DuplicateInterfaceInBaseList, "IBaz").WithArguments("IBaz").WithLocation(25, 32),
+                // (21,19): error CS0061: Inconsistent accessibility: base interface 'IFoo' is less accessible than interface 'IBar'
+                // partial interface IBar : IFoo, IBam
+                Diagnostic(ErrorCode.ERR_BadVisBaseInterface, "IBar").WithArguments("IBar", "IFoo").WithLocation(21, 19),
+                // (21,19): error CS0061: Inconsistent accessibility: base interface 'IBam' is less accessible than interface 'IBar'
+                // partial interface IBar : IFoo, IBam
+                Diagnostic(ErrorCode.ERR_BadVisBaseInterface, "IBar").WithArguments("IBar", "IBam").WithLocation(21, 19),
+                // (25,19): error CS0061: Inconsistent accessibility: base interface 'IBaz' is less accessible than interface 'IBar'
+                // partial interface IBar : IBaz, IBaz
+                Diagnostic(ErrorCode.ERR_BadVisBaseInterface, "IBar").WithArguments("IBar", "IBaz").WithLocation(25, 19));
+        }
+
         [Fact]
         public void EricLiCase1()
         {