Convert operand expressions in an error case (#40093)

Fixes #39975

src/Compilers/CSharp/Portable/Binder/Binder_Operators.cs

@@ -78,6 +78,8 @@ private BoundExpression BindCompoundAssignment(AssignmentExpressionSyntax node,
                     Error(diagnostics, ErrorCode.ERR_BadBinaryOps, node, node.OperatorToken.Text, left.Display, right.Display);
 
                     // error: operator can't be applied on dynamic and a type that is not convertible to dynamic:
+                    left = BindToTypeForErrorRecovery(left);
+                    right = BindToTypeForErrorRecovery(right);
                     return new BoundCompoundAssignmentOperator(node, BinaryOperatorSignature.Error, left, right,
                         Conversion.NoConversion, Conversion.NoConversion, LookupResultKind.Empty, CreateErrorType(), hasErrors: true);
                 }
@@ -90,6 +92,8 @@ private BoundExpression BindCompoundAssignment(AssignmentExpressionSyntax node,
                 // be used here.
 
                 // NOTE: no overload resolution candidates.
+                left = BindToTypeForErrorRecovery(left);
+                right = BindToTypeForErrorRecovery(right);
                 return new BoundCompoundAssignmentOperator(node, BinaryOperatorSignature.Error, left, right,
                     Conversion.NoConversion, Conversion.NoConversion, LookupResultKind.NotAVariable, CreateErrorType(), hasErrors: true);
             }

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

@@ -10904,5 +10904,24 @@ static void Main()
                 Diagnostic(ErrorCode.ERR_ConstantStringTooLong, "C1").WithLocation(28, 68)
                 );
         }
+
+        [Fact, WorkItem(39975, "https://github.com/dotnet/roslyn/issues/39975")]
+        public void EnsureOperandsConvertedInErrorExpression_01()
+        {
+            string source =
+@"class C
+{
+    static unsafe void M(dynamic d, int* p)
+    {
+        d += p;
+    }
+}
+";
+            CreateCompilation(source, options: TestOptions.ReleaseDll.WithAllowUnsafe(true)).VerifyDiagnostics(
+                // (5,9): error CS0019: Operator '+=' cannot be applied to operands of type 'dynamic' and 'int*'
+                //         d += p;
+                Diagnostic(ErrorCode.ERR_BadBinaryOps, "d += p").WithArguments("+=", "dynamic", "int*").WithLocation(5, 9)
+                );
+        }
     }
 }