compiling this code:

///
template foo()
if (__traits(compiles,undefined))
{
	enum foo = false;
}
template foo()
if (true)
{
	enum foo = true;
}

pragma(msg,__traits(compiles,foo!()));

const bool bar = foo!();

/////
compiles, but outputs:
false

This is caused by __traits(compiles) not restoring the global error count in recursive calls (more specifically: when the error output is gagged before evaluating __traits).
Here's the patch:

Index: traits.c
===================================================================
--- traits.c	(revision 532)
+++ traits.c	(working copy)
@@ -397,8 +397,8 @@
 
             global.gag--;
             if (errors != global.errors)
-            {   if (global.gag == 0)
-                    global.errors = errors;
+            {
+                global.errors = errors;
                 goto Lfalse;
             }
         }

Issue 3448 seems similar, but this patch does not help there.