Fix Issue 24078 - Fold constants on array concatenation only for strings

Without this limitation, the code could incorrectly concatenate
`["c"] ~ "a" ~ "b"` as `["c"] ~ "ab"`, which was incorrect.

Signed-off-by: Teodor Dutu <teodor.dutu@gmail.com>

compiler/src/dmd/optimize.d

@@ -1280,10 +1280,16 @@ Expression Expression_optimize(Expression e, int result, bool keepLvalue)
         //printf("CatExp::optimize(%d) %s\n", result, e.toChars());
         if (binOptimize(e, result))
             return;
+
+        if (e.type == Type.tstring)
             if (auto ce1 = e.e1.isCatExp())
             {
                 // https://issues.dlang.org/show_bug.cgi?id=12798
                 // optimize ((expr ~ str1) ~ str2)
+                // https://issues.dlang.org/show_bug.cgi?id=24078
+                // This optimization is only valid if `expr` is a string.
+                // Otherwise it leads to:
+                // `["c"] ~ "a" ~ "b"` becoming `["c"] ~ "ab"`
                 scope CatExp cex = new CatExp(e.loc, ce1.e2, e.e2);
                 cex.type = e.type;
                 Expression ex = Expression_optimize(cex, result, false);

compiler/test/runnable/test24078.d

@@ -0,0 +1,6 @@
+//https://issues.dlang.org/show_bug.cgi?id=24078
+
+void main()
+{
+    assert(["c"] ~ "a" ~ "b" == ["c", "a", "b"]);
+}