fix Bugzilla 23803 - compile-time error for concatenation in -betterC

compiler/src/dmd/e2ir.d

@@ -1742,7 +1742,7 @@
          */
         if (!irs.params.useGC)
         {
-            irs.eSink.error(ce.loc, "array concatenation of expression `%s` requires the GC which is not available with -betterC", ce.toChars());
+            irs.eSink.error(ce.loc, "`%s` is not allowed in -betterC mode because it requires the garbage collector.", ce.toChars());
             return el_long(TYint, 0);
         }
 

compiler/test/fail_compilation/test18312.d

@@ -2,7 +2,7 @@
 REQUIRED_ARGS: -betterC
 TEST_OUTPUT:
 ---
-fail_compilation/test18312.d(14): Error: array concatenation of expression `"[" ~ s ~ "]"` requires the GC which is not available with -betterC
+fail_compilation/test18312.d(14): Error: `"[" ~ s ~ "]"` is not allowed in -betterC mode because it requires the garbage collector.
 ---
 */
 

compiler/test/fail_compilation/test23710.d

@@ -1,7 +1,7 @@
 /* REQUIRED_ARGS: -betterC
 TEST_OUTPUT:
 ---
-fail_compilation/test23710.d(111): Error: array concatenation of expression `foo ~ [1, 2, 3]` requires the GC which is not available with -betterC
+fail_compilation/test23710.d(111): Error: `foo ~ [1, 2, 3]` is not allowed in -betterC mode because it requires the garbage collector.
 ---
  */
 // https://issues.dlang.org/show_bug.cgi?id=23710

compiler/test/fail_compilation/test23803.d

@@ -0,0 +1,20 @@
+// https://issues.dlang.org/show_bug.cgi?id=23803
+
+/*
+REQUIRED_ARGS: -betterC
+TEST_OUTPUT:
+---
+fail_compilation/test23803.d(14): Error: `a ~ b` is not allowed in -betterC mode because it requires the garbage collector.
+---
+*/
+
+string foo()
+{
+    string a, b;
+    return a ~ b;
+}
+
+extern(C) void main()
+{
+    foo();
+}