Fix 17351 - Manifest constants can't sometimes be passed by ref in …

…CTFE

As mentioned in the comment, the fix is crude but works well.

src/dmd/dinterpret.d

@@ -2131,7 +2131,15 @@ public:
             return;
         }
 
-        if (goal == ctfeNeedLvalue)
+        // Note: This is a workaround for
+        // https://issues.dlang.org/show_bug.cgi?id=17351
+        // The aforementioned bug triggers when passing manifest constant by `ref`.
+        // If there was not a previous reference to them, they are
+        // not cached and trigger a "cannot be read at compile time".
+        // This fix is a crude solution to get it to work. A more proper
+        // approach would be to resolve the forward reference, but that is
+        // much more involved.
+        if (goal == ctfeNeedLvalue && e.var.type.isMutable())
         {
             VarDeclaration v = e.var.isVarDeclaration();
             if (v && !v.isDataseg() && !v.isCTFE() && !istate)

test/compilable/test17351.d

@@ -0,0 +1,17 @@
+bool fun(S)(ref S[3] a) { assert(a == [42, 84, 169]); return true; }
+bool fun2(S)(ref S a) { return true; }
+void main()
+{
+    static const int[3] sa = [42, 84, 169];
+    static const double sa2 = 42.42;
+    static assert(fun(sa));
+    static assert(fun2(sa2));
+}
+
+int f1(ref const int p)    { return p; }
+int f2(ref const int[2] p) { return p[0] + p[1]; }
+void test2()
+{
+    static immutable int[2] P = [ 0, 1 ];
+    static assert(f2(P) == 1);
+}