Fix #18342 - wstring literals cannot be implicitly converted to const(wchar)*

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: dkorpel

compiler/src/dmd/dcast.d

@@ -823,6 +823,8 @@ MATCH implicitConvTo(Expression e, Type t)
                     break;
                 }
             }
+            else if (tn.ty == tyn)
+                return m; // e.g. "foo"w -> const(wchar)*
             break;
 
         default:

compiler/test/runnable/implicit.d

@@ -474,6 +474,19 @@ void test15778()
     static assert(!__traits(compiles, (ds[] = "a"w)));
 }
 
+/***********************************/
+// https://github.com/dlang/dmd/issues/18342
+const(char)*  cptr  = "tic"c;
+const(wchar)* wcptr = "tac"w;
+immutable(dchar)* dcptr = "toe"d;
+
+void test18342()
+{
+    assert( cptr[0 .. 4] == "tic\0"c);
+    assert(wcptr[0 .. 4] == "tac\0"w);
+    assert(dcptr[0 .. 4] == "toe\0"d);
+}
+
 /***********************************/
 
 void main()
@@ -490,6 +503,7 @@ void main()
     testDIP29_5();
     testDIP29_6();
     test15778();
+    test18342();
 
     printf("Success\n");
 }