Enforce DoesNotReturn on return; (#46659)

Author: jcouv

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

@@ -1957,6 +1957,7 @@ private static TypeWithState GetParameterState(TypeWithAnnotations parameterType
             var expr = node.ExpressionOpt;
             if (expr == null)
             {
+                EnforceDoesNotReturn(node.Syntax);
                 PendingBranches.Add(new PendingBranch(node, this.State, label: null));
                 SetUnreachable();
                 return null;

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

@@ -30461,6 +30461,29 @@ void boom()
             comp.VerifyDiagnostics();
         }
 
+        [Fact, WorkItem(45791, "https://github.com/dotnet/roslyn/issues/45791")]
+        public void DoesNotReturn_VoidReturningMethod()
+        {
+            string source = @"
+using System.Diagnostics.CodeAnalysis;
+
+public class C
+{
+    [DoesNotReturn]
+    public void M()
+    {
+        return;
+    }
+}
+";
+            var comp = CreateNullableCompilation(new[] { source, DoesNotReturnAttributeDefinition });
+            comp.VerifyDiagnostics(
+                // (9,9): warning CS8763: A method marked [DoesNotReturn] should not return.
+                //         return;
+                Diagnostic(ErrorCode.WRN_ShouldNotReturn, "return;").WithLocation(9, 9)
+                );
+        }
+
         [Fact]
         public void DoesNotReturn_Operator()
         {