Fix type parameter name conflict diagnostic

src/Compilers/CSharp/Portable/Binder/LocalScopeBinder.cs

@@ -381,28 +381,23 @@ private bool ReportConflictWithLocal(Symbol local, Symbol newSymbol, string name
 
             if (newSymbolKind == SymbolKind.ErrorType) return true;
 
-            if (newSymbolKind == SymbolKind.Local)
-            {
-                if (this.Locals.Contains((LocalSymbol)newSymbol) && newLocation.SourceSpan.Start >= local.Locations[0].SourceSpan.Start)
-                {
-                    // A local variable named '{0}' is already defined in this scope
-                    diagnostics.Add(ErrorCode.ERR_LocalDuplicate, newLocation, name);
-                    return true;
-                }
-            }
+            var declaredInThisScope = false;
+
+            declaredInThisScope |= newSymbolKind == SymbolKind.Local && this.Locals.Contains((LocalSymbol)newSymbol);
+            declaredInThisScope |= newSymbolKind == SymbolKind.Method && this.LocalFunctions.Contains((LocalFunctionSymbol)newSymbol);
 
-            if (newSymbolKind == SymbolKind.Method)
+            if (declaredInThisScope && newLocation.SourceSpan.Start >= local.Locations[0].SourceSpan.Start)
             {
-                if (this.LocalFunctions.Contains((LocalFunctionSymbol)newSymbol) && newLocation.SourceSpan.Start >= local.Locations[0].SourceSpan.Start)
-                {
-                    // A local variable named '{0}' is already defined in this scope
-                    diagnostics.Add(ErrorCode.ERR_LocalDuplicate, newLocation, name);
-                    return true;
-                }
+                // TODO: Message should change to something like "A {0} named '{1}' is already defined in this scope"
+
+                // A local variable named '{0}' is already defined in this scope
+                diagnostics.Add(ErrorCode.ERR_LocalDuplicate, newLocation, name);
+                return true;
             }
 
-            if (newSymbolKind == SymbolKind.Local || newSymbolKind == SymbolKind.Parameter || newSymbolKind == SymbolKind.Method)
+            if (newSymbolKind == SymbolKind.Local || newSymbolKind == SymbolKind.Parameter || newSymbolKind == SymbolKind.Method || newSymbolKind == SymbolKind.TypeParameter)
             {
+                // TODO: Fix up the message for local functions and type parameters. Maybe like the above todo - $"A {newSymbolKind.Localize()} named '{name}' cannot ..."
                 // A local or parameter named '{0}' cannot be declared in this scope because that name is used in an enclosing local scope to define a local or parameter
                 diagnostics.Add(ErrorCode.ERR_LocalIllegallyOverrides, newLocation, name);
                 return true;
@@ -422,30 +417,24 @@ private bool ReportConflictWithLocal(Symbol local, Symbol newSymbol, string name
 
         internal virtual bool EnsureSingleDefinition(Symbol symbol, string name, Location location, DiagnosticBag diagnostics)
         {
-            LocalSymbol existingLocal;
-            var localsMap = this.LocalsMap;
-            if (localsMap != null && localsMap.TryGetValue(name, out existingLocal))
-            {
-                if (symbol == existingLocal)
-                {
-                    // reference to same symbol, by far the most common case.
-                    return false;
-                }
+            LocalSymbol existingLocal = null;
+            LocalFunctionSymbol existingLocalFunction = null;
 
-                return ReportConflictWithLocal(existingLocal, symbol, name, location, diagnostics);
-            }
-
-            LocalFunctionSymbol existingLocalFunction;
+            var localsMap = this.LocalsMap;
             var localFunctionsMap = this.LocalFunctionsMap;
-            if (localFunctionsMap != null && localFunctionsMap.TryGetValue(name, out existingLocalFunction))
+
+            // TODO: Handle case where 'name' exists in both localsMap and localFunctionsMap. Right now locals are preferred over local functions.
+            if ((localsMap != null && localsMap.TryGetValue(name, out existingLocal)) ||
+                (localFunctionsMap != null && localFunctionsMap.TryGetValue(name, out existingLocalFunction)))
             {
-                if (symbol == existingLocalFunction)
+                var existingSymbol = (Symbol)existingLocal ?? existingLocalFunction;
+                if (symbol == existingSymbol)
                 {
                     // reference to same symbol, by far the most common case.
                     return false;
                 }
 
-                return ReportConflictWithLocal(existingLocalFunction, symbol, name, location, diagnostics);
+                return ReportConflictWithLocal(existingSymbol, symbol, name, location, diagnostics);
             }
 
             return false;

src/Compilers/CSharp/Test/Semantic/Semantics/LocalFunctionTests.cs

@@ -418,12 +418,15 @@ void M2()
                 int c = b;
                 void M3()
                 {
+                    Print(c);
                     c = 2;
                 }
+                Print(b);
                 M3();
                 Print(c);
                 b = 2;
             }
+            Print(a);
             M2();
             Print(b);
             a = 2;
@@ -442,11 +445,13 @@ void M2()
             {
                 void M3()
                 {
+                    Print(b);
                     b = 2;
                 }
                 M3();
                 Print(b);
             }
+            Print(a);
             M2();
             Print(b);
             a = 2;
@@ -465,8 +470,10 @@ void M2()
                 int c = a;
                 void M3()
                 {
+                    Print(c);
                     c = 2;
                 }
+                Print(a);
                 M3();
                 Print(c);
                 a = 2;
@@ -488,8 +495,10 @@ void M2()
                 int c = b;
                 void M3()
                 {
+                    Print(c);
                     c = 2;
                 }
+                Print(b);
                 M3();
                 Print(c);
                 b = 2;
@@ -509,6 +518,7 @@ void M2()
             {
                 void M3()
                 {
+                    Print(a);
                     a = 2;
                 }
                 M3();
@@ -530,6 +540,7 @@ void M2()
             {
                 void M3()
                 {
+                    Print(b);
                     b = 2;
                 }
                 M3();
@@ -550,6 +561,7 @@ void M2()
                 int c = 0;
                 void M3()
                 {
+                    Print(c);
                     c = 2;
                 }
                 M3();
@@ -581,13 +593,13 @@ static void Main(string[] args)
 ";
             var comp = CreateCompilationWithMscorlib(source, options: TestOptions.ReleaseExe, parseOptions: _parseOptions);
             var verify = CompileAndVerify(comp, expectedOutput: @"
- 2 2 2
- 2 2 2
- 2 2 2
- 2 2
- 2 2 2
- 2 2
- 2
+ 0 0 0 2 2 2
+ 0 0 2 2 2
+ 0 0 2 2 2
+ 0 0 2 2
+ 0 2 2 2
+ 0 2 2
+ 0 2
 ");
         }
 
@@ -1743,6 +1755,45 @@ void Local2()
                 );
         }
 
+        [Fact]
+        public void NameConflict()
+        {
+            var source = @"
+class Program
+{
+    static void Main(string[] args)
+    {
+        void Duplicate() { }
+        void Duplicate() { }
+        int T;
+        void Param(int T) { }
+        void Generic<T>() { }
+        int Conflict;
+        void Conflict() { }
+        void Conflict2() { }
+        int Conflict2;
+    }
+}
+";
+            CreateCompilationWithMscorlib(source, options: TestOptions.ReleaseExe.WithWarningLevel(0), parseOptions: _parseOptions).VerifyDiagnostics(
+    // (7,14): error CS0128: A local variable named 'Duplicate' is already defined in this scope
+    //         void Duplicate() { }
+    Diagnostic(ErrorCode.ERR_LocalDuplicate, "Duplicate").WithArguments("Duplicate").WithLocation(7, 14),
+    // (9,24): error CS0136: A local or parameter named 'T' cannot be declared in this scope because that name is used in an enclosing local scope to define a local or parameter
+    //         void Param(int T) { }
+    Diagnostic(ErrorCode.ERR_LocalIllegallyOverrides, "T").WithArguments("T").WithLocation(9, 24),
+    // (10,22): error CS0136: A local or parameter named 'T' cannot be declared in this scope because that name is used in an enclosing local scope to define a local or parameter
+    //         void Generic<T>() { }
+    Diagnostic(ErrorCode.ERR_LocalIllegallyOverrides, "T").WithArguments("T").WithLocation(10, 22),
+    // (12,14): error CS0128: A local variable named 'Conflict' is already defined in this scope
+    //         void Conflict() { }
+    Diagnostic(ErrorCode.ERR_LocalDuplicate, "Conflict").WithArguments("Conflict").WithLocation(12, 14),
+    // (13,14): error CS0136: A local or parameter named 'Conflict2' cannot be declared in this scope because that name is used in an enclosing local scope to define a local or parameter
+    //         void Conflict2() { }
+    Diagnostic(ErrorCode.ERR_LocalIllegallyOverrides, "Conflict2").WithArguments("Conflict2").WithLocation(13, 14)
+                );
+        }
+
         [Fact]
         public void Unsafe()
         {