Use a temp for pattern input when a local or parameter is ref/in/out (#40479)

src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_Patterns.cs

@@ -397,7 +397,8 @@ private BoundExpression MakeEqual(BoundExpression loweredLiteral, BoundExpressio
                 out BoundExpression savedInputExpression)
             {
                 var inputDagTemp = BoundDagTemp.ForOriginalInput(loweredInput);
-                if (loweredInput.Kind == BoundKind.Local || loweredInput.Kind == BoundKind.Parameter)
+                if ((loweredInput.Kind == BoundKind.Local || loweredInput.Kind == BoundKind.Parameter)
+                    && loweredInput.GetRefKind() == RefKind.None)
                 {
                     // If we're switching on a local variable and there is no when clause (checked by the caller),
                     // we assume the value of the local variable does not change during the execution of the

src/Compilers/CSharp/Test/Emit/CodeGen/PatternTests.cs

@@ -773,6 +773,195 @@ .maxstack  2
 }");
         }
 
+        [Fact, WorkItem(40403, "https://github.com/dotnet/roslyn/issues/40403")]
+        public void RefParameter_StoreToTemp()
+        {
+            var source =
+@"public class C
+{
+    static bool M1(ref object x)
+    {
+        return x is 42;
+    }
+}";
+            var compilation = CreateCompilation(source, options: TestOptions.ReleaseDll);
+            compilation.VerifyDiagnostics();
+            var compVerifier = CompileAndVerify(compilation);
+            compVerifier.VerifyIL("C.M1",
+@"{
+  // Code size       24 (0x18)
+  .maxstack  2
+  .locals init (object V_0)
+  IL_0000:  ldarg.0
+  IL_0001:  ldind.ref
+  IL_0002:  stloc.0
+  IL_0003:  ldloc.0
+  IL_0004:  isinst     ""int""
+  IL_0009:  brfalse.s  IL_0016
+  IL_000b:  ldloc.0
+  IL_000c:  unbox.any  ""int""
+  IL_0011:  ldc.i4.s   42
+  IL_0013:  ceq
+  IL_0015:  ret
+  IL_0016:  ldc.i4.0
+  IL_0017:  ret
+}");
+        }
+
+        [Fact, WorkItem(40403, "https://github.com/dotnet/roslyn/issues/40403")]
+        public void RefLocal_StoreToTemp()
+        {
+            var source =
+@"public class C
+{
+    static bool M1(bool b, ref object x, ref object y)
+    {
+        ref object z = ref b ? ref x : ref y;
+        return z is 42;
+    }
+}";
+            var compilation = CreateCompilation(source, options: TestOptions.ReleaseDll);
+            compilation.VerifyDiagnostics();
+            var compVerifier = CompileAndVerify(compilation);
+            compVerifier.VerifyIL("C.M1",
+@"{
+  // Code size       30 (0x1e)
+  .maxstack  2
+  .locals init (object V_0)
+  IL_0000:  ldarg.0
+  IL_0001:  brtrue.s   IL_0006
+  IL_0003:  ldarg.2
+  IL_0004:  br.s       IL_0007
+  IL_0006:  ldarg.1
+  IL_0007:  ldind.ref
+  IL_0008:  stloc.0
+  IL_0009:  ldloc.0
+  IL_000a:  isinst     ""int""
+  IL_000f:  brfalse.s  IL_001c
+  IL_0011:  ldloc.0
+  IL_0012:  unbox.any  ""int""
+  IL_0017:  ldc.i4.s   42
+  IL_0019:  ceq
+  IL_001b:  ret
+  IL_001c:  ldc.i4.0
+  IL_001d:  ret
+}");
+        }
+
+        [Fact, WorkItem(40403, "https://github.com/dotnet/roslyn/issues/40403")]
+        public void InParameter_StoreToTemp()
+        {
+            var source =
+@"public class C
+{
+    static bool M1(in object x)
+    {
+        return x is 42;
+    }
+}";
+            var compilation = CreateCompilation(source, options: TestOptions.ReleaseDll);
+            compilation.VerifyDiagnostics();
+            var compVerifier = CompileAndVerify(compilation);
+            compVerifier.VerifyIL("C.M1",
+@"{
+  // Code size       24 (0x18)
+  .maxstack  2
+  .locals init (object V_0)
+  IL_0000:  ldarg.0
+  IL_0001:  ldind.ref
+  IL_0002:  stloc.0
+  IL_0003:  ldloc.0
+  IL_0004:  isinst     ""int""
+  IL_0009:  brfalse.s  IL_0016
+  IL_000b:  ldloc.0
+  IL_000c:  unbox.any  ""int""
+  IL_0011:  ldc.i4.s   42
+  IL_0013:  ceq
+  IL_0015:  ret
+  IL_0016:  ldc.i4.0
+  IL_0017:  ret
+}");
+        }
+
+        [Fact, WorkItem(40403, "https://github.com/dotnet/roslyn/issues/40403")]
+        public void RefReadonlyLocal_StoreToTemp()
+        {
+            var source =
+@"public class C
+{
+    static bool M1(bool b, in object x, in object y)
+    {
+        ref readonly object z = ref b ? ref x : ref y;
+        return z is 42;
+    }
+}";
+            var compilation = CreateCompilation(source, options: TestOptions.ReleaseDll);
+            compilation.VerifyDiagnostics();
+            var compVerifier = CompileAndVerify(compilation);
+            compVerifier.VerifyIL("C.M1",
+@"{
+  // Code size       30 (0x1e)
+  .maxstack  2
+  .locals init (object V_0)
+  IL_0000:  ldarg.0
+  IL_0001:  brtrue.s   IL_0006
+  IL_0003:  ldarg.2
+  IL_0004:  br.s       IL_0007
+  IL_0006:  ldarg.1
+  IL_0007:  ldind.ref
+  IL_0008:  stloc.0
+  IL_0009:  ldloc.0
+  IL_000a:  isinst     ""int""
+  IL_000f:  brfalse.s  IL_001c
+  IL_0011:  ldloc.0
+  IL_0012:  unbox.any  ""int""
+  IL_0017:  ldc.i4.s   42
+  IL_0019:  ceq
+  IL_001b:  ret
+  IL_001c:  ldc.i4.0
+  IL_001d:  ret
+}");
+        }
+
+        [Fact, WorkItem(40403, "https://github.com/dotnet/roslyn/issues/40403")]
+        public void OutParameter_StoreToTemp()
+        {
+            var source =
+@"public class C
+{
+    static bool M1(out object x)
+    {
+        x = null;
+        return x is 42;
+    }
+}";
+            var compilation = CreateCompilation(source, options: TestOptions.ReleaseDll);
+            compilation.VerifyDiagnostics();
+            var compVerifier = CompileAndVerify(compilation);
+            compVerifier.VerifyIL("C.M1",
+@"{
+  // Code size       27 (0x1b)
+  .maxstack  2
+  .locals init (object V_0)
+  IL_0000:  ldarg.0
+  IL_0001:  ldnull
+  IL_0002:  stind.ref
+  IL_0003:  ldarg.0
+  IL_0004:  ldind.ref
+  IL_0005:  stloc.0
+  IL_0006:  ldloc.0
+  IL_0007:  isinst     ""int""
+  IL_000c:  brfalse.s  IL_0019
+  IL_000e:  ldloc.0
+  IL_000f:  unbox.any  ""int""
+  IL_0014:  ldc.i4.s   42
+  IL_0016:  ceq
+  IL_0018:  ret
+  IL_0019:  ldc.i4.0
+  IL_001a:  ret
+}");
+        }
+
         [Fact, WorkItem(22654, "https://github.com/dotnet/roslyn/issues/22654")]
         public void NoRedundantTypeCheck()
         {