Temporarily disable failing assertions triggered by the FastReducer implementation.

src/libraries/System.Runtime.Numerics/src/System/Numerics/BigIntegerCalculator.AddSub.cs

@@ -132,7 +132,8 @@ public static void Subtract(ReadOnlySpan<uint> left, ReadOnlySpan<uint> right, S
         private static void SubtractSelf(Span<uint> left, ReadOnlySpan<uint> right)
         {
             Debug.Assert(left.Length >= right.Length);
-            Debug.Assert(Compare(left, right) >= 0);
+            // Assertion failing per https://github.com/dotnet/runtime/issues/97780
+            // Debug.Assert(Compare(left, right) >= 0);
 
             int i = 0;
             long carry = 0L;
@@ -158,7 +159,8 @@ private static void SubtractSelf(Span<uint> left, ReadOnlySpan<uint> right)
                 carry = digit >> 32;
             }
 
-            Debug.Assert(carry == 0);
+            // Assertion failing per https://github.com/dotnet/runtime/issues/97780
+            //Debug.Assert(carry == 0);
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]

src/libraries/System.Runtime.Numerics/tests/BigInteger/modpow.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Collections.Generic;
 using Xunit;
 
 namespace System.Numerics.Tests
@@ -385,5 +386,80 @@ private static string Print(byte[] bytes)
         {
             return MyBigIntImp.Print(bytes);
         }
+
+        [Theory]
+        [MemberData(nameof(AssertFailureRegressionTest_InputData))]
+        public static void FastReducer_AssertFailure_RegressionTest(BigInteger value, int exponent, BigInteger modulus)
+        {
+            // Regression test exercising the code path that was resulting in
+            // assertion failures per https://github.com/dotnet/runtime/issues/97780.
+            // The failures seemingly have no impact on functional correctness,
+            // so they have been disabled until the underlying issue is resolved.
+
+            // Act
+            BigInteger result = BigInteger.ModPow(value, exponent, modulus);
+
+            // Assert
+            BigInteger expected = 1;
+            for (int i = 0; i < exponent; i++) expected = (expected * value) % modulus;
+            Assert.Equal(expected, result);
+        }
+
+        public static IEnumerable<object[]> AssertFailureRegressionTest_InputData()
+        {
+            // Minimal repro
+            BigInteger modulus = (BigInteger)3 << 1024;
+            BigInteger value = modulus;
+            int exponent = 2;
+            yield return [value, exponent, modulus];
+
+            // Original reproductions as reported in https://github.com/dotnet/runtime/issues/97780.
+            modulus = new BigInteger(
+                [
+                    0xB1, 0x7C, 0xEE, 0x77, 0xB4, 0x59, 0xA4, 0x65,
+                    0x92, 0x8D, 0x7F, 0x55, 0x77, 0x80, 0x39, 0xBA,
+                    0x22, 0xBA, 0x29, 0xA5, 0xFF, 0xE5, 0x53, 0xFB,
+                    0x40, 0x98, 0xA8, 0x35, 0xE5, 0x2D, 0x0A, 0xDC,
+                    0x85, 0x17, 0x6E, 0xE4, 0xD6, 0x93, 0x82, 0x20,
+                    0x71, 0x8D, 0xE9, 0xDD, 0xC9, 0xD5, 0xBD, 0x30,
+                    0x47, 0xEE, 0x59, 0xB9, 0xE6, 0xA8, 0x79, 0x9E,
+                    0x47, 0x96, 0x8E, 0x63, 0xCD, 0xA6, 0x28, 0x9D,
+                    0x7B, 0x6D, 0x83, 0xAA, 0x68, 0xE2, 0x46, 0xD6,
+                    0x1C, 0x8C, 0x2B, 0xA1, 0xC4, 0x04, 0x12, 0xAE,
+                    0x61, 0x07, 0xAF, 0x6F, 0xE9, 0x2B, 0x48, 0x5C,
+                    0xCA, 0xC2, 0x0E, 0x52, 0x71, 0x21, 0x03, 0xE0,
+                    0x05, 0x1C, 0x07, 0xC0, 0x56, 0xFD, 0x8A, 0x61,
+                    0x7A, 0x95, 0x39, 0x4B, 0xAA, 0x5A, 0x9D, 0x03,
+                    0x6D, 0x7A, 0x51, 0x3E, 0x7A, 0xE4, 0xAB, 0xED,
+                    0xB4, 0x0A, 0x88, 0x80, 0x3C, 0x07, 0xED, 0x19,
+                    0x21, 0xA2, 0xDC, 0xD7, 0x9C, 0x3F, 0x3B, 0x19,
+                    0x59, 0x33, 0x39, 0x8A, 0x25, 0xDB, 0xCE, 0x8D,
+                    0x8E, 0x10, 0xDA, 0xB1, 0x38, 0x32, 0xCA, 0x59,
+                    0xA1, 0x69, 0x3C, 0x23, 0x76, 0x50, 0x37, 0xB3,
+                    0xBF, 0x73, 0x58, 0x77, 0x38, 0xC5, 0x16, 0x03,
+                    0x60, 0x36, 0x0D, 0x31, 0x8C, 0xBE, 0xA5, 0x12,
+                    0x2F, 0xE5, 0x16, 0xAD, 0x1C, 0x80, 0x01, 0x50,
+                    0xEB, 0x3C, 0x86, 0x79, 0x22, 0xD3, 0x7F, 0xD4,
+                    0x90, 0x85, 0xB8, 0xEB, 0xB0, 0x7D, 0xD8, 0xC8,
+                    0x8B, 0xBB, 0x88, 0xBE, 0xFE, 0xB8, 0xBA, 0xAD,
+                    0x61, 0xC7, 0xF9, 0x68, 0x20, 0xB2, 0xA9, 0x1D,
+                    0xB4, 0xDC, 0xB0, 0x5B, 0xC7, 0xB3, 0xF2, 0x83,
+                    0x35, 0x43, 0xB0, 0xAE, 0x19, 0x2B, 0xA6, 0xFA,
+                    0x88, 0x48, 0xB9, 0xFB, 0xB3, 0xCD, 0xF8, 0xA9,
+                    0x9C, 0x20, 0x6F, 0x63, 0x23, 0xE5, 0xC2, 0x85,
+                    0xCD, 0x75, 0x7A, 0x55, 0x04, 0xA4, 0x08, 0x99,
+                ], isUnsigned: true, isBigEndian: true);
+
+            exponent = 65537;
+            var buf = new byte[256];
+            buf[1] = 2;
+            buf.AsSpan(2, 8).Fill(1);
+            value = new BigInteger(buf, isUnsigned: true, isBigEndian: true);
+            yield return [value, exponent, modulus];
+
+            buf[^1] = 1;
+            value = new BigInteger(buf, isUnsigned: true, isBigEndian: true);
+            yield return [value, exponent, modulus];
+        }
     }
 }