fix Issue 18480 - dmd hangs with self-alias declaration

- fixed by detecting `alias sym = sym;` during semantic
- small adoption of assertion in mostVisibleOverload to deal with
  alias error expression (type = Type.terror) in overload

src/dmd/access.d

@@ -571,7 +571,8 @@ public Dsymbol mostVisibleOverload(Dsymbol s)
         // private void name(int) {}
         else if (auto ad = s.isAliasDeclaration())
         {
-            assert(ad.isOverloadable, "Non overloadable Aliasee in overload list");
+            assert(ad.isOverloadable || ad.type && ad.type.ty == Terror,
+                "Non overloadable Aliasee in overload list");
             // Yet unresolved aliases store overloads in overnext.
             if (ad.semanticRun < PASS.semanticdone)
                 next = ad.overnext;

src/dmd/dsymbolsem.d

@@ -5492,6 +5492,19 @@ void aliasSemantic(AliasDeclaration ds, Scope* sc)
         global.gag = 0;
     }
 
+    // https://issues.dlang.org/show_bug.cgi?id=18480
+    // Detect `alias sym = sym;` to prevent creating loops in overload overnext lists.
+    // Selective imports are allowed to alias to the same name `import mod : sym=sym`.
+    if (ds.type.ty == Tident && !ds._import)
+    {
+        auto tident = cast(TypeIdentifier)ds.type;
+        if (tident.ident is ds.ident && !tident.idents.dim)
+        {
+            error(ds.loc, "`alias %s = %s;` cannot alias itself, use a qualified name to create an overload set",
+                ds.ident.toChars(), tident.ident.toChars());
+            ds.type = Type.terror;
+        }
+    }
     /* This section is needed because Type.resolve() will:
      *   const x = 3;
      *   alias y = x;

test/fail_compilation/test18480.d

@@ -2,7 +2,7 @@
 /*
 TEST_OUTPUT:
 ---
-fail_compilation/imports/test18480a.d(2): Error: `alias X = X` not allowed (with `X = TestTemplate`)
+fail_compilation/imports/test18480a.d(2): Error: `alias TestTemplate = TestTemplate;` cannot alias itself, use a qualified name to create an overload set
 ---
 https://issues.dlang.org/show_bug.cgi?id=18480
 */