Preventing crash when a local function with iterators is using an out/ref parameter (#18836)

Change data flow analysis to pass a location for error reporting if a branch is compiler generated
or if it is null (which is effectively compiler generated).

Fixes #18813

src/Compilers/CSharp/Portable/BoundTree/BoundNode.cs

@@ -88,8 +88,7 @@ public SyntaxTree SyntaxTree
         {
             get
             {
-                var syntax = Syntax;
-                return syntax == null ? null : syntax.SyntaxTree;
+                return Syntax?.SyntaxTree;
             }
         }
 

src/Compilers/CSharp/Portable/FlowAnalysis/DataFlowPass.LocalFunctions.cs

@@ -163,9 +163,13 @@ public override BoundNode VisitLocalFunctionStatement(BoundLocalFunctionStatemen
             {
                 this.State = pending.State;
                 BoundNode branch = pending.Branch;
-                LeaveParameters(localFuncSymbol.Parameters, branch?.Syntax,
-                                branch?.WasCompilerGenerated == true
-                                    ? location : null);
+
+                // Pass the local function identifier as a location if the branch
+                // is null or compiler generated.
+                LeaveParameters(localFuncSymbol.Parameters,
+                  branch?.Syntax,
+                  branch?.WasCompilerGenerated == false ? null : location);
+
                 IntersectWith(ref stateAtReturn, ref this.State);
             }
 

src/Compilers/CSharp/Portable/FlowAnalysis/DataFlowPass.cs

@@ -277,7 +277,14 @@ protected override ImmutableArray<PendingBranch> RemoveReturns()
 
         protected virtual void ReportUnassignedOutParameter(ParameterSymbol parameter, SyntaxNode node, Location location)
         {
-            if (!_requireOutParamsAssigned && topLevelMethod == currentMethodOrLambda) return;
+            if (!_requireOutParamsAssigned && topLevelMethod == currentMethodOrLambda)
+            {
+                return;
+            }
+
+            // If node and location are null "new SourceLocation(node);" will throw a NullReferenceException
+            Debug.Assert(node != null || location != null);
+
             if (Diagnostics != null && this.State.Reachable)
             {
                 if (location == null)

src/Compilers/CSharp/Test/Semantic/FlowAnalysis/LocalFunctions.cs

@@ -1318,5 +1318,66 @@ void Local()
                 //         Local();
                 Diagnostic(ErrorCode.ERR_UseDefViolationField, "Local()").WithArguments("_x").WithLocation(14, 9));
         }
+
+        [Fact]
+        [WorkItem(18813, "https://github.com/dotnet/roslyn/issues/18813")]
+        public void LocalIEnumerableFunctionWithOutParameter1()
+        {
+            var comp = CreateStandardCompilation(@"
+class c
+{
+    static void Main(string[] args)
+    {
+        System.Collections.Generic.IEnumerable<string> getFoo(int count, out bool output)
+        {
+            output = true;
+            yield return ""foo"";
+        }
+
+        getFoo(1, out bool myBool);
     }
-}
+}");
+
+            comp.VerifyDiagnostics(
+                // (6,83): error CS1623: Iterators cannot have ref or out parameters
+                //         System.Collections.Generic.IEnumerable<string> getFoo(int count, out bool foobar)
+                Diagnostic(ErrorCode.ERR_BadIteratorArgType, "output"),
+                // (6,56): error CS0177: The out parameter 'foobar' must be assigned to before control leaves the current method
+                //         System.Collections.Generic.IEnumerable<string> getFoo(int count, out bool foobar)
+                Diagnostic(ErrorCode.ERR_ParamUnassigned, "getFoo").WithArguments("output").WithLocation(6, 56));
+        }
+
+        [Fact]
+        [WorkItem(18813, "https://github.com/dotnet/roslyn/issues/18813")]
+        public void LocalIEnumerableFunctionWithOutParameter2()
+        {
+            var comp = CreateStandardCompilation(@"
+class c
+{
+    static void Main(string[] args)
+    {
+        System.Collections.Generic.IEnumerable<string> getFoo(int count, out bool output)
+        {
+            output = false;
+            for (int i = 0; i < count; i++)
+            {
+                foreach (var val in getFoo(3, out var bar))
+                yield return ""foo"";
+            }
+            yield return ""foo"";
+        }
+
+        getFoo(1, out bool myBool);
+    }
+}");
+
+            comp.VerifyDiagnostics(
+                // (6,83): error CS1623: Iterators cannot have ref or out parameters
+                //         System.Collections.Generic.IEnumerable<string> getFoo(int count, out bool foobar)
+                Diagnostic(ErrorCode.ERR_BadIteratorArgType, "output"),
+                // (6,56): error CS0177: The out parameter 'foobar' must be assigned to before control leaves the current method
+                //         System.Collections.Generic.IEnumerable<string> getFoo(int count, out bool foobar)
+                Diagnostic(ErrorCode.ERR_ParamUnassigned, "getFoo").WithArguments("output").WithLocation(6, 56));
+        }
+    }
+}
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