Fix issue 21001: alias protection is ignored if used before declaration.

Author: FeepingCreature

src/dmd/dsymbolsem.d

@@ -6498,6 +6498,12 @@ void aliasSemantic(AliasDeclaration ds, Scope* sc)
 {
     //printf("AliasDeclaration::semantic() %s\n", ds.toChars());
 
+    // as DsymbolSemanticVisitor::visit(AliasDeclaration), in case we're called first.
+    // see https://issues.dlang.org/show_bug.cgi?id=21001
+    ds.storage_class |= sc.stc & STC.deprecated_;
+    ds.protection = sc.protection;
+    ds.userAttribDecl = sc.userAttribDecl;
+
     // TypeTraits needs to know if it's located in an AliasDeclaration
     const oldflags = sc.flags;
     sc.flags |= SCOPE.alias_;

test/fail_compilation/fail21001.d

@@ -0,0 +1,12 @@
+/*
+TEST_OUTPUT:
+---
+fail_compilation/fail21001.d(12): Error: undefined identifier `Alias`
+---
+*/
+
+module fail21001;
+
+import imports.fail21001b;
+
+void main() { Alias var; }

test/fail_compilation/imports/fail21001b.d

@@ -0,0 +1,5 @@
+module imports.fail21001b;
+
+private struct S { Alias member; }
+
+private alias Alias = int;