Fix nullable analysis of lambdas within conditional expressions (#47405)

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

@@ -4229,6 +4229,7 @@ private static TypeWithState GetNullCoalescingResultType(TypeWithState rightResu
                     consequenceConversion,
                     resultTypeWithAnnotations,
                     consequenceRValue,
+                    consequenceState,
                     consequenceEndReachable);
 
                 TypeWithState convertedAlternativeResult = convertResult(
@@ -4237,6 +4238,7 @@ private static TypeWithState GetNullCoalescingResultType(TypeWithState rightResu
                     alternativeConversion,
                     resultTypeWithAnnotations,
                     alternativeRValue,
+                    alternativeState,
                     alternativeEndReachable);
 
                 resultState = convertedConsequenceResult.State.Join(convertedAlternativeResult.State);
@@ -4272,8 +4274,12 @@ private static TypeWithState GetNullCoalescingResultType(TypeWithState rightResu
                 Conversion conversion,
                 TypeWithAnnotations targetType,
                 TypeWithState operandType,
+                LocalState state,
                 bool isReachable)
             {
+                var savedState = this.State;
+                this.State = state;
+
                 bool previousDisabledDiagnostics = _disableDiagnostics;
                 // If the node is not reachable, then we're only visiting to get
                 // nullability information for the public API, and not to produce diagnostics.
@@ -4301,6 +4307,7 @@ private static TypeWithState GetNullCoalescingResultType(TypeWithState rightResu
                     resultType = default;
                     _disableDiagnostics = previousDisabledDiagnostics;
                 }
+                this.State = savedState;
 
                 return resultType;
             }

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

@@ -51615,6 +51615,75 @@ static void F()
                 Diagnostic(ErrorCode.WRN_NullabilityMismatchInParameterTypeOfTargetDelegate, "(I<object?> o) => { }").WithArguments("o", "lambda expression", "D<I<object>>").WithLocation(10, 26));
         }
 
+        [Fact, WorkItem(40561, "https://github.com/dotnet/roslyn/issues/40561")]
+        public void ReturnLambda_InsideConditionalExpr()
+        {
+            var source = @"
+using System;
+
+class C
+{
+    static void M0(string s) { }
+
+    static Action? M1(string? s)
+        => s != null
+            ? () => M0(s)
+            : (Action?)null;
+
+    static Action? M2(string? s)
+        => s != null
+            ? (Action)(() => M0(s))
+            : null;
+
+    static Action? M3(string? s)
+        => s != null
+            ? () => { M0(s); s = null; }
+            : (Action?)null;
+
+    static Action? M4(string? s)
+    {
+        return s != null
+            ? local(() => M0(s), s = null)
+            : (Action?)null;
+
+        Action local(Action a1, string? s)
+        {
+            return a1;
+        }
+    }
+
+    static Action? M5(string? s)
+    {
+        return s != null
+            ? local(s = null, () => M0(s)) // 1
+            : (Action?)null;
+
+        Action local(string? s, Action a1)
+        {
+            return a1;
+        }
+    }
+
+    static Action? M6(string? s)
+    {
+        return s != null
+            ? local(() => M0(s))
+            : (Action?)null;
+
+        Action local(Action a1)
+        {
+            s = null;
+            return a1;
+        }
+    }
+}";
+            var comp = CreateNullableCompilation(source);
+            comp.VerifyDiagnostics(
+                // (38,40): warning CS8604: Possible null reference argument for parameter 's' in 'void C.M0(string s)'.
+                //             ? local(s = null, () => M0(s)) // 1
+                Diagnostic(ErrorCode.WRN_NullReferenceArgument, "s").WithArguments("s", "void C.M0(string s)").WithLocation(38, 40));
+        }
+
         [Fact]
         [WorkItem(29617, "https://github.com/dotnet/roslyn/issues/29617")]
         public void ReturnTypeInference_01()