Fix #20601.

hasIndirections doesn't handle static arrays of length 0 correctly (they
don't actually have any indirections), because operator precedence
resulted in the check for 0 length static arrays having no actual
effect.

I also copied the test for
https://issues.dlang.org/show_bug.cgi?id=12000 from std.traits to here,
because it's really a compiler bug that's being tested rather than
hasIndirections itself (so being in the compiler repo arguably makes
more sense), and it also arguably makes more sense to put it next to the
actual implementation than in std.traits where it's been.

druntime/src/core/internal/traits.d

@@ -543,13 +543,114 @@ template hasIndirections(T)
     else static if (__traits(isAssociativeArray, T) || is(T == class) || is(T == interface))
         enum hasIndirections = true;
     else static if (is(T == E[N], E, size_t N))
-        enum hasIndirections = T.sizeof && is(E == void) ? true : hasIndirections!(BaseElemOf!E);
+        enum hasIndirections = T.sizeof && (is(E == void) || hasIndirections!(BaseElemOf!E));
     else static if (isFunctionPointer!T)
         enum hasIndirections = false;
     else
         enum hasIndirections = isPointer!T || isDelegate!T || isDynamicArray!T;
 }
 
+@safe unittest
+{
+    static assert(!hasIndirections!int);
+    static assert(!hasIndirections!(const int));
+
+    static assert( hasIndirections!(int*));
+    static assert( hasIndirections!(const int*));
+
+    static assert( hasIndirections!(int[]));
+    static assert(!hasIndirections!(int[42]));
+    static assert(!hasIndirections!(int[0]));
+
+    static assert( hasIndirections!(int*));
+    static assert( hasIndirections!(int*[]));
+    static assert( hasIndirections!(int*[42]));
+    static assert(!hasIndirections!(int*[0]));
+
+    static assert( hasIndirections!string);
+    static assert( hasIndirections!(string[]));
+    static assert( hasIndirections!(string[42]));
+    static assert(!hasIndirections!(string[0]));
+
+    static assert( hasIndirections!(void[]));
+    static assert( hasIndirections!(void[17]));
+    static assert(!hasIndirections!(void[0]));
+
+    static assert( hasIndirections!(string[int]));
+    static assert( hasIndirections!(string[int]*));
+    static assert( hasIndirections!(string[int][]));
+    static assert( hasIndirections!(string[int][12]));
+    static assert(!hasIndirections!(string[int][0]));
+
+    static assert(!hasIndirections!(int function(string)));
+    static assert( hasIndirections!(int delegate(string)));
+    static assert(!hasIndirections!(const(int function(string))));
+    static assert( hasIndirections!(const(int delegate(string))));
+    static assert(!hasIndirections!(immutable(int function(string))));
+    static assert( hasIndirections!(immutable(int delegate(string))));
+
+    static class C {}
+    static assert( hasIndirections!C);
+
+    static interface I {}
+    static assert( hasIndirections!I);
+
+    {
+        static struct S {}
+        static assert(!hasIndirections!S);
+    }
+    {
+        static struct S { int i; }
+        static assert(!hasIndirections!S);
+    }
+    {
+        static struct S { C c; }
+        static assert( hasIndirections!S);
+    }
+    {
+        static struct S { int[] arr; }
+        static assert( hasIndirections!S);
+    }
+    {
+        int local;
+        struct S { void foo() { ++local; } }
+        static assert( hasIndirections!S);
+    }
+
+    {
+        static union U {}
+        static assert(!hasIndirections!U);
+    }
+    {
+        static union U { int i; }
+        static assert(!hasIndirections!U);
+    }
+    {
+        static union U { C c; }
+        static assert( hasIndirections!U);
+    }
+    {
+        static union U { int[] arr; }
+        static assert( hasIndirections!U);
+    }
+}
+
+// https://issues.dlang.org/show_bug.cgi?id=12000
+@safe unittest
+{
+    static struct S(T)
+    {
+        static assert(hasIndirections!T);
+    }
+
+    static class A(T)
+    {
+        S!A a;
+    }
+
+    A!int dummy;
+}
+
 template hasUnsharedIndirections(T)
 {
     static if (is(T == immutable))