d: Fix qualifier ignored in alias definition if parentheses are not present

Fixes regression where the qualifier was ignored in an alias definition
if parentheses were not present.

Reviewed-on: https://github.com/dlang/dmd/pull/12504

gcc/d/ChangeLog:

	* dmd/MERGE: Merge upstream dmd b7d146c4c.

gcc/d/dmd/MERGE

@@ -1,4 +1,4 @@
-0450061c8de71328815da9323bd35c92b37d51d2
+b7d146c4c34469f876a63f26ff19091a7f9d54d7
 
 The first line of this file holds the git revision number of the last
 merge done from the dlang/dmd repository.

gcc/d/dmd/dsymbolsem.c

@@ -4880,8 +4880,11 @@ static void aliasInstanceSemantic(TemplateInstance *tempinst, Scope *sc, Templat
 
     TemplateTypeParameter *ttp = (*tempdecl->parameters)[0]->isTemplateTypeParameter();
     Type *ta = isType(tempinst->tdtypes[0]);
-    Declaration *d = new AliasDeclaration(tempinst->loc, ttp->ident, ta->addMod(tempdecl->onemember->isAliasDeclaration()->type->mod));
-    d->storage_class |= STCtemplateparameter;
+    AliasDeclaration *ad = tempdecl->onemember->isAliasDeclaration();
+
+    // Note: qualifiers can be in both 'ad.type.mod' and 'ad.storage_class'
+    Declaration *d = new AliasDeclaration(tempinst->loc, ttp->ident, ta->addMod(ad->type->mod));
+    d->storage_class |= STCtemplateparameter | ad->storage_class;
     dsymbolSemantic(d, sc);
 
     paramscope->pop();

gcc/testsuite/gdc.test/compilable/test21898.d

@@ -0,0 +1,7 @@
+// https://issues.dlang.org/show_bug.cgi?id=21898
+
+alias Works(T) = immutable(T);
+alias Fails(T) = immutable T;
+
+static assert(is(Works!int == immutable int));
+static assert(is(Fails!int == immutable int));