d: Fix infinite loop in isAliasThisTuple

This reverts a change in the upstream D implementation of the compiler,
as the refactoring introduced a regression.

gcc/d/ChangeLog:

	* dmd/MERGE: Merge upstream dmd 51816cd01d.

Reviewed-on: https://github.com/dlang/dmd/pull/21155

gcc/d/dmd/MERGE

@@ -1,4 +1,4 @@
-ed17b3e95dc3fc3264a4c91843da824f5541f3e1
+51816cd01deee5cc1d7d2c6e1e24788ec655b73e
 
 The first line of this file holds the git revision number of the last
 merge done from the dlang/dmd repository.

gcc/d/dmd/expressionsem.d

@@ -641,21 +641,25 @@ TupleDeclaration isAliasThisTuple(Expression e)
     Type t = e.type.toBasetype();
     while (true)
     {
-        Dsymbol s = t.toDsymbol(null);
-        if (!s)
-            return null;
-        auto ad = s.isAggregateDeclaration();
-        if (!ad)
-            return null;
-        s = ad.aliasthis ? ad.aliasthis.sym : null;
-        if (s && s.isVarDeclaration())
+        if (Dsymbol s = t.toDsymbol(null))
         {
-            TupleDeclaration td = s.isVarDeclaration().toAlias().isTupleDeclaration();
-            if (td && td.isexp)
-                return td;
+            if (auto ad = s.isAggregateDeclaration())
+            {
+                s = ad.aliasthis ? ad.aliasthis.sym : null;
+                if (s && s.isVarDeclaration())
+                {
+                    TupleDeclaration td = s.isVarDeclaration().toAlias().isTupleDeclaration();
+                    if (td && td.isexp)
+                        return td;
+                }
+                if (Type att = t.aliasthisOf())
+                {
+                    t = att;
+                    continue;
+                }
+            }
         }
-        if (Type att = t.aliasthisOf())
-            t = att;
+        return null;
     }
 }
 

gcc/testsuite/gdc.test/compilable/test21153.d

@@ -0,0 +1,8 @@
+// https://github.com/dlang/dmd/issues/21153
+alias AliasSeq(TList...) = TList;
+class DataClass;
+void reduce(DataClass[] r)
+{
+    alias Args = AliasSeq!(DataClass);
+    Args result = r[0];
+}