[mcs] Missing type forwarded types should not cause type collision. F…

…ixes #16196

mcs/errors/CS1070-lib.il

@@ -14,7 +14,7 @@
 
 .module 'CS1070-lib.dll'
 
-.class extern forwarder E
+.class extern forwarder N.E
 {
   .assembly extern 'CS1070-lib-missing'
 }

mcs/errors/cs1070-2.cs

@@ -0,0 +1,12 @@
+// CS1070: The type `N.E' has been forwarded to an assembly that is not referenced. Consider adding a reference to assembly `CS1070-lib-missing, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'
+// Line: 9
+// Compiler options: -r:CS1070-lib.dll
+
+using N;
+
+public class D
+{
+	public void Foo (E e)
+	{
+	}
+}

mcs/mcs/namespace.cs

@@ -353,11 +353,20 @@ public TypeSpec LookupType (IMemberContext ctx, string name, int arity, LookupMo
 					}
 
 					if (best.MemberDefinition.IsImported && ts.MemberDefinition.IsImported) {
+						if (ts.Kind == MemberKind.MissingType)
+							continue;
+
+						if (best.Kind == MemberKind.MissingType) {
+							best = ts;
+							continue;
+						}
+
 						if (mode == LookupMode.Normal) {
 							ctx.Module.Compiler.Report.SymbolRelatedToPreviousError (best);
 							ctx.Module.Compiler.Report.SymbolRelatedToPreviousError (ts);
 							ctx.Module.Compiler.Report.Error (433, loc, "The imported type `{0}' is defined multiple times", ts.GetSignatureForError ());
 						}
+
 						break;
 					}
 

mcs/tests/test-875-2-lib.il

@@ -0,0 +1,21 @@
+.assembly extern mscorlib
+{
+}
+
+.assembly extern 'test-875-lib-missing'
+{
+}
+
+.assembly 'test-875-2-lib'
+{
+  .hash algorithm 0x00008004
+  .ver 0:0:0:0
+}
+
+.module 'test-875-2-lib'
+
+.class extern forwarder N.Lib
+{
+  .assembly extern 'test-875-lib-missing'
+}
+

mcs/tests/test-875-lib.cs

@@ -0,0 +1,8 @@
+// Compiler options: -t:library
+
+namespace N
+{
+	public class Lib
+	{
+	}
+}

mcs/tests/test-875.cs

@@ -0,0 +1,11 @@
+// Compiler options: -r:test-875-lib.dll -r:test-875-2-lib.dll
+
+using N;
+
+public class Test: Lib
+{
+	public static void Main ()
+	{
+		new Test ();
+	}
+}

mcs/tests/ver-il-net_4_5.xml

@@ -48339,6 +48339,16 @@
       </method>
     </type>
   </test>
+  <test name="test-875.cs">
+    <type name="Test">
+      <method name="Void Main()" attrs="150">
+        <size>8</size>
+      </method>
+      <method name="Void .ctor()" attrs="6278">
+        <size>7</size>
+      </method>
+    </type>
+  </test>
   <test name="test-88.cs">
     <type name="X">
       <method name="Void f(System.String)" attrs="145">