fix(cpp1): do not emit _template-head_ alone in phase 2

Author: JohelEGP

regression-tests/pure2-type-and-namespace-aliases.cpp2

@@ -47,3 +47,7 @@ myclass3: @struct type = {
 }
 
 myclass4: @struct type = { }
+
+myclass6: @struct <T: type> type = {
+    v: <U> _ requires true == 0;
+}

regression-tests/test-results/pure2-type-and-namespace-aliases.cpp

@@ -23,6 +23,8 @@ class myclass3;
 #line 49 "pure2-type-and-namespace-aliases.cpp2"
 class myclass4;
 
+template<typename T> class myclass6;
+    
 
 //=== Cpp2 type definitions and function declarations ===========================
 
@@ -73,6 +75,13 @@ class myclass3 {
 
 class myclass4 {};
 
+template<typename T> class myclass6 {
+    public: template<typename U> 
+CPP2_REQUIRES_ (true)
+#line 52 "pure2-type-and-namespace-aliases.cpp2"
+    static constexpr auto v = 0;
+};
+
 
 //=== Cpp2 function definitions =================================================
 

source/cppfront.cpp

@@ -5006,8 +5006,19 @@ class cppfront
             auto& a = std::get<declaration_node::an_alias>(n.type);
             assert(a);
 
+            //  Helper for aliases that emit as a defining declaration.
+            auto const type_scope_object_alias_emits_in_phase_1_only = [&]() {
+                assert(
+                    n.parent_is_type()
+                    && n.is_object_alias()
+                );
+                return !a->type_id
+                       || a->type_id->is_wildcard()
+                       || !has_potentially_incomplete_type(n, *a->type_id);
+            };
+
             //  Namespace-scope aliases are emitted in phase 1,
-            //  type-scope object aliases in both phases 1 and 2, and
+            //  type-scope object aliases is emitted in phase 1 and maybe 2, and
             //  function-scope aliases in phase 2
             if (
                 (
@@ -5019,6 +5030,7 @@ class cppfront
                     n.parent_is_type()
                     && n.is_object_alias()
                     && printer.get_phase() == printer.phase2_func_defs
+                    && !type_scope_object_alias_emits_in_phase_1_only()
                     )
                 ||
                 (
@@ -5084,7 +5096,7 @@ class cppfront
                 //  Handle object aliases:
                 //      - at function scope, it's const&
                 //      - at namespace scope, it's inline constexpr
-                //      - at type scope, it's also inline constexpr but see note (*) below
+                //      - at type scope, it's also static constexpr but see note (*) below
                 else if (a->is_object_alias())
                 {
                     auto type = std::string{"auto"};
@@ -5096,11 +5108,7 @@ class cppfront
                     {
                         assert (n.parent_declaration->name());
 
-                        if (
-                            !a->type_id
-                            || a->type_id->is_wildcard()
-                            || !has_potentially_incomplete_type(n, *a->type_id)
-                        ) {
+                        if (type_scope_object_alias_emits_in_phase_1_only()) {
                             if (printer.get_phase() == printer.phase1_type_defs_func_decls) {
                                 printer.print_cpp2(
                                     "static constexpr "