From 0e62377dd2f9917d090bb508577e7b9f27a1fc8a Mon Sep 17 00:00:00 2001
From: Calum Grant <calum@semmle.com>
Date: Thu, 4 Jul 2019 16:29:36 +0100
Subject: [PATCH] C#: Treat _ as an ordinary variable in a foreach.

---
 .../Entities/Statements/ForEach.cs            |  5 +---
 .../Entities/TypeMention.cs                   | 30 +++++++++++++------
 .../test/library-tests/regressions/Program.cs | 15 ++++++++--
 .../regressions/TypeMentions.expected         |  8 +++--
 4 files changed, 40 insertions(+), 18 deletions(-)

diff --git a/csharp/extractor/Semmle.Extraction.CSharp/Entities/Statements/ForEach.cs b/csharp/extractor/Semmle.Extraction.CSharp/Entities/Statements/ForEach.cs
index dbc6f5694eb..3404203ace0 100644
--- a/csharp/extractor/Semmle.Extraction.CSharp/Entities/Statements/ForEach.cs
+++ b/csharp/extractor/Semmle.Extraction.CSharp/Entities/Statements/ForEach.cs
@@ -26,10 +26,7 @@ namespace Semmle.Extraction.CSharp.Entities.Statements
 
             var location = cx.Create(Stmt.Identifier.GetLocation());
 
-            if (typeSymbol.Name != "_")
-                Expressions.VariableDeclaration.Create(cx, typeSymbol, type, Stmt.Type, location, location, Stmt.Type.IsVar, this, 0);
-            else
-                TypeMention.Create(cx, Stmt.Type, this, type);
+            Expressions.VariableDeclaration.Create(cx, typeSymbol, type, Stmt.Type, location, location, Stmt.Type.IsVar, this, 0);
 
             Statement.Create(cx, Stmt.Statement, this, 2);
         }
diff --git a/csharp/extractor/Semmle.Extraction.CSharp/Entities/TypeMention.cs b/csharp/extractor/Semmle.Extraction.CSharp/Entities/TypeMention.cs
index f65307706c5..f57401984e2 100644
--- a/csharp/extractor/Semmle.Extraction.CSharp/Entities/TypeMention.cs
+++ b/csharp/extractor/Semmle.Extraction.CSharp/Entities/TypeMention.cs
@@ -22,16 +22,30 @@ namespace Semmle.Extraction.CSharp.Entities
             Loc = loc;
         }
 
-        static TypeSyntax GetArrayElementType(ArrayTypeSyntax array)
+        static TypeSyntax GetElementType(TypeSyntax type)
         {
-            return array.ElementType is ArrayTypeSyntax a ? 
-                GetArrayElementType(a) :
-                array.ElementType;
+            switch(type)
+            {
+                case ArrayTypeSyntax ats:
+                    return GetElementType(ats.ElementType);
+                case NullableTypeSyntax nts:
+                    return GetElementType(nts.ElementType);
+                default:
+                    return type;
+            }
         }
 
-        static Type GetArrayElementType(Type t)
+        static Type GetElementType(Type t)
         {
-            return t is ArrayType a ? GetArrayElementType(a.ElementType.Type) : t;
+            switch(t)
+            {
+                case ArrayType at:
+                    return GetElementType(at.ElementType.Type);
+                case NamedType nt when nt.symbol.IsBoundNullable():
+                    return nt.TypeArguments.Single();
+                default:
+                    return t;
+            }
         }
 
         void Populate()
@@ -39,10 +53,8 @@ namespace Semmle.Extraction.CSharp.Entities
             switch (Syntax.Kind())
             {
                 case SyntaxKind.ArrayType:
-                    var ats = (ArrayTypeSyntax)Syntax;
-                    var at = (ArrayType)Type;
                     Emit(Loc ?? Syntax.GetLocation(), Parent, Type);
-                    Create(cx, GetArrayElementType(ats), this, GetArrayElementType(at));
+                    Create(cx, GetElementType(Syntax), this, GetElementType(Type));
                     return;
                 case SyntaxKind.NullableType:
                     var nts = (NullableTypeSyntax)Syntax;
diff --git a/csharp/ql/test/library-tests/regressions/Program.cs b/csharp/ql/test/library-tests/regressions/Program.cs
index a5b24b7385e..047c24164ad 100644
--- a/csharp/ql/test/library-tests/regressions/Program.cs
+++ b/csharp/ql/test/library-tests/regressions/Program.cs
@@ -161,7 +161,7 @@ namespace NoPia
 
 unsafe class ArrayTypesTest
 {
-  int*[][] field;
+    int*[][] field;
 }
 
 class NameofNamespace
@@ -169,4 +169,15 @@ class NameofNamespace
     string s = nameof(System);
 }
 
-// semmle-extractor-options: /r:System.Dynamic.Runtime.dll
+class UsingDiscard
+{
+    void F()
+    {
+        foreach(var _ in new IDisposable[] { })
+            using(_)
+            {
+            }
+    }
+}
+
+// semmle-extractor-options: /r:System.Dynamic.Runtime.dll /langversion:8.0
diff --git a/csharp/ql/test/library-tests/regressions/TypeMentions.expected b/csharp/ql/test/library-tests/regressions/TypeMentions.expected
index 9bdeafa2d87..69a57a1059d 100644
--- a/csharp/ql/test/library-tests/regressions/TypeMentions.expected
+++ b/csharp/ql/test/library-tests/regressions/TypeMentions.expected
@@ -76,7 +76,9 @@
 | Program.cs:157:31:157:50 | EmbeddedTypesManager<,> |
 | Program.cs:157:52:157:72 | TEmbeddedTypesManager |
 | Program.cs:157:75:157:87 | TEmbeddedType |
-| Program.cs:164:3:164:5 | Int32 |
-| Program.cs:164:3:164:6 | Int32* |
-| Program.cs:164:3:164:10 | Int32*[][] |
+| Program.cs:164:5:164:7 | Int32 |
+| Program.cs:164:5:164:8 | Int32* |
+| Program.cs:164:5:164:12 | Int32*[][] |
 | Program.cs:169:5:169:10 | String |
+| Program.cs:174:5:174:8 | Void |
+| Program.cs:176:17:176:19 | IDisposable |