Merge pull request #7174 from crocopaw/gcd

Fix issue 19514 - gcd(BigInt(2), BigInt(1)) fails
merged-on-behalf-of: Nicholas Wilson <thewilsonator@users.noreply.github.com>

std/numeric.d

@@ -2734,6 +2734,8 @@ if (!isIntegral!T &&
             shift++;
         }
 
+        if ((a & 1) == 0) swap(a, b);
+
         do
         {
             assert((a & 1) != 0);
@@ -2789,6 +2791,13 @@ if (!isIntegral!T &&
     assert(gcd(CrippledInt(2310), CrippledInt(1309)) == CrippledInt(77));
 }
 
+// Issue 19514
+@system pure unittest
+{
+    import std.bigint : BigInt;
+    assert(gcd(BigInt(2), BigInt(1)) == BigInt(1));
+}
+
 // This is to make tweaking the speed/size vs. accuracy tradeoff easy,
 // though floats seem accurate enough for all practical purposes, since
 // they pass the "approxEqual(inverseFft(fft(arr)), arr)" test even for