Don't drop aggregate type qualifier for alias parameter (#22393)

Fixes #17959.

Author: ntrel

compiler/src/dmd/templatesem.d

@@ -2778,7 +2778,9 @@ private MATCH matchArg(TemplateParameter tp, Scope* sc, RootObject oarg, size_t
             if (!sa || si != sa)
                 return matchArgNoMatch();
         }
-        dedtypes[i] = sa;
+        // Note: Dsymbol can't have type qualifiers, so use type if it has them
+        // https://github.com/dlang/dmd/issues/17959
+        dedtypes[i] = ta && ta.mod ? ta : sa;
 
         if (psparam)
         {

compiler/test/compilable/alias_param_qual.d

@@ -0,0 +1,7 @@
+alias t(alias a) = a;
+
+static assert(is(t!(const Object) == const Object));
+static assert(is(t!(shared(Object)) == shared Object));
+static assert(is(t!(immutable S) == immutable(S)));
+
+struct S {}