From b76446aa88179575e2ae3f60f34bb1a564091611 Mon Sep 17 00:00:00 2001
From: Julien Couvreur <jcouv@microsoft.com>
Date: Thu, 8 May 2025 17:00:39 -0700
Subject: [PATCH] Extensions: only count extensions for determining identifier

---
 .../Source/SourceNamedTypeSymbol_Extension.cs | 17 +++++++++++++++--
 .../Test/Emit3/Semantics/ExtensionTests.cs    | 19 ++++++++++---------
 2 files changed, 25 insertions(+), 11 deletions(-)

diff --git a/src/Compilers/CSharp/Portable/Symbols/Source/SourceNamedTypeSymbol_Extension.cs b/src/Compilers/CSharp/Portable/Symbols/Source/SourceNamedTypeSymbol_Extension.cs
index 241e3a99b7502..c99a475b07b2d 100644
--- a/src/Compilers/CSharp/Portable/Symbols/Source/SourceNamedTypeSymbol_Extension.cs
+++ b/src/Compilers/CSharp/Portable/Symbols/Source/SourceNamedTypeSymbol_Extension.cs
@@ -45,8 +45,21 @@ internal override string ExtensionName
                     declaration = ((SourceNamespaceSymbol)this.ContainingSymbol).MergedDeclaration;
                 }
 
-                var index = declaration.Children.IndexOf(this.declaration);
-                return GeneratedNames.MakeExtensionName(index);
+                int index = 0;
+                foreach (Declaration child in declaration.Children)
+                {
+                    if (child == this.declaration)
+                    {
+                        return GeneratedNames.MakeExtensionName(index);
+                    }
+
+                    if (child.Kind == DeclarationKind.Extension)
+                    {
+                        index++;
+                    }
+                }
+
+                throw ExceptionUtilities.Unreachable();
             }
         }
 
diff --git a/src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests.cs b/src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests.cs
index ee1c3924b647f..4df892712c474 100644
--- a/src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests.cs
+++ b/src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests.cs
@@ -919,7 +919,7 @@ public static partial class Extensions
     }
 
     [Fact]
-    public void ExtensionIndex_TwoExtensions_SameSignatures_01()
+    public void ExtensionIndex_TwoExtensions_01()
     {
         var src = """
 public static class Extensions
@@ -1560,7 +1560,7 @@ [0] int32
     }
 
     [Fact]
-    public void ExtensionIndex_TwoExtensions_SameSignatures_02()
+    public void ExtensionIndex_TwoExtensions_02()
     {
         var src = """
 public static class Extensions
@@ -1585,12 +1585,12 @@ class C { }
         var extension2 = tree.GetRoot().DescendantNodes().OfType<ExtensionDeclarationSyntax>().Last();
         var symbol2 = model.GetDeclaredSymbol(extension2);
         var sourceExtension2 = symbol2.GetSymbol<SourceNamedTypeSymbol>();
-        Assert.Equal("<>E__2`1", symbol2.MetadataName);
-        Assert.Equal("Extensions.<>E__2<T>", symbol2.ToTestDisplayString());
+        Assert.Equal("<>E__1`1", symbol2.MetadataName);
+        Assert.Equal("Extensions.<>E__1<T>", symbol2.ToTestDisplayString());
     }
 
     [Fact]
-    public void ExtensionIndex_TwoExtensions_SameSignatures_03()
+    public void ExtensionIndex_TwoExtensions_03()
     {
         var src = """
 extension<T>(T) { }
@@ -1618,12 +1618,12 @@ class C { }
         var extension2 = tree.GetRoot().DescendantNodes().OfType<ExtensionDeclarationSyntax>().Last();
         var symbol2 = model.GetDeclaredSymbol(extension2);
         var sourceExtension2 = symbol2.GetSymbol<SourceNamedTypeSymbol>();
-        Assert.Equal("<>E__2`1", symbol2.MetadataName);
-        Assert.Equal("<>E__2<T>", symbol2.ToTestDisplayString());
+        Assert.Equal("<>E__1`1", symbol2.MetadataName);
+        Assert.Equal("<>E__1<T>", symbol2.ToTestDisplayString());
     }
 
     [Fact]
-    public void ExtensionIndex_TwoExtensions_DifferentSignatures_01()
+    public void ExtensionIndex_TwoExtensions_05()
     {
         var src = """
 public static class Extensions
@@ -1652,7 +1652,7 @@ public static class Extensions
     }
 
     [Fact]
-    public void ExtensionIndex_TwoExtensions_DifferentSignatures_02()
+    public void ExtensionIndex_TwoExtensions_06()
     {
         var src = """
 public static class Extensions
@@ -1695,6 +1695,7 @@ public static class Extensions
     extension<T8>(T8 o8) { }
     extension<T9>(T9 o9) { }
     extension<T10>(T10 o10) { }
+    class C { }
     extension<T11>(T11 o11) { }
 }
 """;