Improve error message when aggregate template is used as a type

Note: s.kind is just "struct" for struct templates.
Note: When td.onemember is an alias, we don't know if it resolves to a
type or not until the template is instantiated.

Author: ntrel

src/dmd/typesem.d

@@ -1156,7 +1156,12 @@ private extern (C++) final class TypeSemanticVisitor : Visitor
         {
             if (s)
             {
-                s.error(loc, "is used as a type");
+                auto td = s.isTemplateDeclaration;
+                if (td && td.onemember && td.onemember.isAggregateDeclaration)
+                    mtype.error(loc, "%s template `%s` is used as a type without instantiation",
+                        s.kind, s.toPrettyChars);
+                else
+                    mtype.error(loc, "%s `%s` is used as a type", s.kind, s.toPrettyChars);
                 //assert(0);
             }
             else

test/fail_compilation/notype.d

@@ -0,0 +1,31 @@
+struct S(int var = 3) {
+    int a;
+}
+S s;
+
+alias A() = int;
+A a;
+
+enum e() = 5;
+e val;
+
+interface I()
+{
+}
+I i;
+
+template t()
+{
+}
+t tv;
+
+/*
+TEST_OUTPUT:
+---
+fail_compilation/notype.d(4): Error: struct template `notype.S(int var = 3)` is used as a type without instantiation
+fail_compilation/notype.d(7): Error: template `notype.A()` is used as a type
+fail_compilation/notype.d(10): Error: template `notype.e()` is used as a type
+fail_compilation/notype.d(15): Error: interface template `notype.I()` is used as a type without instantiation
+fail_compilation/notype.d(20): Error: template `notype.t()` is used as a type
+---
+*/