Adjust type of out var based on parameter state (#36284)

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

@@ -3201,23 +3201,29 @@ private void CheckDisallowedNullAssignment(TypeWithState state, TypeWithAnnotati
                     break;
                 case RefKind.Out:
                     {
-                        var lValueType = result.LValueType;
+                        // compute the fictional parameter state
                         var parameterWithAnnotations = ApplyRValueAnnotations(parameterType, parameterAnnotations);
+                        var parameterWithState = TypeWithState.Create(parameterWithAnnotations, parameterAnnotations);
+
+                        // Adjust parameter state if MaybeNull or MaybeNullWhen are present (for `var` type and for assignment warnings)
+                        var worstCaseParameterWithState = applyPostConditionsUnconditionally(parameterWithState, parameterAnnotations);
+
+                        var lValueType = result.LValueType;
                         if (argument is BoundLocal local && local.DeclarationKind == BoundLocalDeclarationKind.WithInferredType)
                         {
-                            _variableTypes[local.LocalSymbol] = parameterWithAnnotations;
-                            lValueType = parameterWithAnnotations;
+                            var varType = worstCaseParameterWithState.ToTypeWithAnnotations();
+                            _variableTypes[local.LocalSymbol] = varType;
+                            lValueType = varType;
                         }
 
-                        // assign from a fictional value from the parameter to the argument.
-                        var parameterWithState = TypeWithState.Create(parameterWithAnnotations, parameterAnnotations);
+                        // track state by assigning from a fictional value from the parameter to the argument.
                         var parameterValue = new BoundParameter(argument.Syntax, parameter);
                         trackNullableStateForAssignment(parameterValue, lValueType, MakeSlot(argument), parameterWithState, argument.IsSuppressed, parameterAnnotations);
 
-                        // check whether parameter would unsafely let a null out in the worse case
+                        // report warnings if parameter would unsafely let a null out in the worst case
                         if (!argument.IsSuppressed)
                         {
-                            ReportNullableAssignmentIfNecessary(parameterValue, lValueType, applyPostConditionsUnconditionally(parameterWithState, parameterAnnotations), UseLegacyWarnings(argument, result.LValueType));
+                            ReportNullableAssignmentIfNecessary(parameterValue, lValueType, worstCaseParameterWithState, UseLegacyWarnings(argument, result.LValueType));
 
                             HashSet<DiagnosticInfo> useSiteDiagnostics = null;
                             if (!_conversions.HasIdentityOrImplicitReferenceConversion(parameterType.Type, lValueType.Type, ref useSiteDiagnostics))