fix Issue 15778 - polysemous string type doesn't work in array operation

src/expression.d

@@ -12428,7 +12428,18 @@ public:
                     e2x = e2x.castTo(sc, e1.type.constOf());
             }
             else
-                e2x = e2x.implicitCastTo(sc, e1.type);
+            {
+                /* Bugzilla 15778: A string literal has an array type of immutable
+                 * elements by default, and normally it cannot be convertible to
+                 * array type of mutable elements. But for element-wise assignment,
+                 * elements need to be const at best. So we should give a chance
+                 * to change code unit size for polysemous string literal.
+                 */
+                if (e2x.op == TOKstring)
+                    e2x = e2x.implicitCastTo(sc, e1.type.constOf());
+                else
+                    e2x = e2x.implicitCastTo(sc, e1.type);
+            }
         }
         else
         {

test/runnable/implicit.d

@@ -436,6 +436,30 @@ struct S13640
     }
 }
 
+/***********************************/
+// 15778
+
+void test15778()
+{
+     char[] cs = new  char[](3);
+    wchar[] ws = new wchar[](3);
+    dchar[] ds = new dchar[](3);
+
+    cs[] = "abc";   assert(cs == "abc");
+    cs[] = "def"c;  assert(cs == "def");
+    ws[] = "abc";   assert(ws == "abc");
+    ws[] = "def"w;  assert(ws == "def");
+    ds[] = "abc";   assert(ds == "abc");
+    ds[] = "def"d;  assert(ds == "def");
+
+    static assert(!__traits(compiles, (cs[] = "a"w)));
+    static assert(!__traits(compiles, (cs[] = "a"d)));
+    static assert(!__traits(compiles, (ws[] = "a"c)));
+    static assert(!__traits(compiles, (ws[] = "a"d)));
+    static assert(!__traits(compiles, (ds[] = "a"c)));
+    static assert(!__traits(compiles, (ds[] = "a"w)));
+}
+
 /***********************************/
 
 void main()
@@ -451,6 +475,7 @@ void main()
     testDIP29_4();
     testDIP29_5();
     testDIP29_6();
+    test15778();
 
     writefln("Success");
 }