feat: allow template specialization

The syntax is `specialize<specialization_arguments>`
after the optional template parameter list.

Some examples from the tests:
```Cpp2
std: namespace = {
    common_type: @struct @print specialize<outer, outer> type = {
        type: type == outer;
    }
}
v: <T> const i32 = 1;
v: <> specialize<void> const i32 = 2;
v: specialize<i64> const i32 = 3;
v: <T> specialize<* T> std::optional<int> == 6;
```

regression-tests/pure2-alias-1-error.cpp2

@@ -0,0 +1 @@
+a1: <T> namespace == { }

regression-tests/pure2-alias-2-error.cpp2

@@ -0,0 +1 @@
+a2: namespace requires true == { }

regression-tests/pure2-alias-3-error.cpp2

@@ -0,0 +1 @@
+a3: type requires true = { }

regression-tests/pure2-alias-4-error.cpp2

@@ -0,0 +1 @@
+a4: specialize<> type == int;

regression-tests/pure2-alias-5-error.cpp2

@@ -0,0 +1 @@
+a5: specialize<> namespace == std;

regression-tests/pure2-namespace-1-error.cpp2

@@ -0,0 +1 @@
+n1: <T> namespace = { }

regression-tests/pure2-namespace-2-error.cpp2

@@ -0,0 +1 @@
+n2: namespace requires true = { }

regression-tests/pure2-print.cpp2

@@ -104,6 +104,15 @@ outer: @print type = {
 
 }
 
+std: namespace = {
+    common_type: @struct @print specialize<outer, outer> type = {
+        type: type == outer;
+    }
+    numbers: namespace = {
+        pi_v: /*@print*/ specialize<outer> const double = pi_v<double>;
+    }
+}
+
 main: () = {
     outer::test();
 }

regression-tests/pure2-template-specialization.cpp2

@@ -0,0 +1,47 @@
+t: @cpp1_rule_of_zero <T> type = {
+  public a: i32 = 1;
+}
+t: @cpp1_rule_of_zero <T> specialize<T> type requires std::is_void_v<T> = {
+  public b: i32 = 2;
+}
+t: @cpp1_rule_of_zero specialize<i64> type = {
+  public c: i32 = 3;
+}
+t: @cpp1_rule_of_zero specialize<* i8> type = {
+  public f: () = 17;
+  public v: int == 29;
+}
+t: @cpp1_rule_of_zero <T> specialize<* T> type = {
+  public v: int == 17;
+  public f: () = 29;
+}
+u: @cpp1_rule_of_zero <T...> type = {
+  public a: i32 = 1;
+}
+u: @cpp1_rule_of_zero specialize<> type = {
+  public a: i32 = 2;
+}
+u: specialize<i16> type = { }
+v: <T> const i32 = 1;
+v: <> specialize<void> const i32 = 2;
+v: specialize<i64> const i32 = 3;
+v: specialize<i16> std::optional<i32> == 4;
+v: <> specialize<i8> std::optional<i8> == 5;
+v: <T> specialize<* T> std::optional<int> == 6;
+main: ()   = {
+  assert(t<i32>().a == 1);
+  assert(t<void>().b == 2);
+  assert(t<i64>().c == 3);
+  assert(t<* i8>::f() == 17);
+  assert(t<* i8>::v == 29);
+  assert(t<* i16>::v == 17);
+  assert(t<* i16>::f() == 29);
+  assert(u<i32>().a == 1);
+  assert(u<>().a == 1);
+  assert(v<i32> == 1);
+  assert(v<void> == 2);
+  assert(v<i64> == 3);
+  static_assert(v<i16> == 4);
+  static_assert(v<i8> == 5);
+  static_assert(v<* int> == 6);
+}

regression-tests/test-results/pure2-alias-1-error.cpp2.output

@@ -0,0 +1,3 @@
+pure2-alias-1-error.cpp2...
+pure2-alias-1-error.cpp2(1,19): error: a namespace or namespace alias cannot have template parameters
+

regression-tests/test-results/pure2-alias-2-error.cpp2.output

@@ -0,0 +1,3 @@
+pure2-alias-2-error.cpp2...
+pure2-alias-2-error.cpp2(1,15): error: 'requires' is not allowed on a namespace alias (at 'requires')
+

regression-tests/test-results/pure2-alias-3-error.cpp2.output

@@ -0,0 +1,3 @@
+pure2-alias-3-error.cpp2...
+pure2-alias-3-error.cpp2(1,10): error: 'requires' is not allowed on a type alias that does not have a template parameter list (at 'requires')
+

regression-tests/test-results/pure2-alias-4-error.cpp2.output

@@ -0,0 +1,3 @@
+pure2-alias-4-error.cpp2...
+pure2-alias-4-error.cpp2(1,18): error: a type alias cannot be specialized
+

regression-tests/test-results/pure2-alias-5-error.cpp2.output

@@ -0,0 +1,3 @@
+pure2-alias-5-error.cpp2...
+pure2-alias-5-error.cpp2(1,18): error: a namespace alias cannot be specialized
+

regression-tests/test-results/pure2-namespace-1-error.cpp2.output

@@ -0,0 +1,3 @@
+pure2-namespace-1-error.cpp2...
+pure2-namespace-1-error.cpp2(1,19): error: a namespace or namespace alias cannot have template parameters
+

regression-tests/test-results/pure2-namespace-2-error.cpp2.output

@@ -0,0 +1,3 @@
+pure2-namespace-2-error.cpp2...
+pure2-namespace-2-error.cpp2(1,15): error: 'requires' is not allowed on a namespace alias (at 'requires')
+

regression-tests/test-results/pure2-print.cpp

@@ -11,6 +11,15 @@
 #line 6 "pure2-print.cpp2"
 class outer;
 
+#line 107 "pure2-print.cpp2"
+namespace std {
+
+#line 111 "pure2-print.cpp2"
+    namespace numbers {
+
+    }
+}
+
 
 //=== Cpp2 type definitions and function declarations ===========================
 
@@ -81,6 +90,15 @@ CPP2_REQUIRES_ (cpp2::impl::cmp_greater_eq(sizeof...(Args),0u)) ;
 #line 105 "pure2-print.cpp2"
 };
 
+namespace std {
+    template<> class common_type<outer,outer> {
+        public: using type = outer;
+    };
+    namespace numbers {
+              /*@print*/
+    }
+}
+
 auto main() -> int;
 
 //=== Cpp2 function definitions =================================================
@@ -206,6 +224,15 @@ requires (cpp2::impl::cmp_greater_eq(sizeof...(Args),0u)) {
     auto outer::y([[maybe_unused]] cpp2::impl::in<CPP2_TYPEOF(0)> unnamed_param_1) -> void{}
 
 #line 107 "pure2-print.cpp2"
+namespace std {
+
+#line 111 "pure2-print.cpp2"
+    namespace numbers {
+        template<> double const pi_v<outer> {pi_v<double>}; 
+    }
+}
+
+#line 116 "pure2-print.cpp2"
 auto main() -> int{
     outer::test();
 }

regression-tests/test-results/pure2-print.cpp2.output

@@ -151,5 +151,11 @@ outer:/* @print */ type =
     {
     }
 }
+
+
+common_type:/* @struct @print */ specialize<outer, outer> type = 
+{
+    public type: type == outer;
+}
  ok (all Cpp2, passes safety checks)
 

regression-tests/test-results/pure2-template-specialization.cpp

@@ -0,0 +1,94 @@
+
+#define CPP2_IMPORT_STD          Yes
+
+//=== Cpp2 type declarations ====================================================
+
+
+#include "cpp2util.h"
+
+#line 1 "pure2-template-specialization.cpp2"
+template<typename T> class t;
+#line 2 "pure2-template-specialization.cpp2"
+
+
+#line 18 "pure2-template-specialization.cpp2"
+template<typename ...T> class u;
+
+
+//=== Cpp2 type definitions and function declarations ===========================
+
+#line 1 "pure2-template-specialization.cpp2"
+template<typename T> class t {
+#line 2 "pure2-template-specialization.cpp2"
+  public: cpp2::i32 a {1}; 
+};
+template<typename T> requires( std::is_void_v<T> )
+  class t<T> {public: cpp2::i32 b {2}; 
+};
+template<> class t<cpp2::i64> {
+  public: cpp2::i32 c {3}; 
+};
+template<> class t<cpp2::i8*> {
+  public: [[nodiscard]] static auto f() -> decltype(auto);
+  public: static const int v;
+};
+template<typename T> class t<T*> {
+  public: static const int v;
+  public: [[nodiscard]] static auto f() -> decltype(auto);
+};
+template<typename ...T> class u {
+  public: cpp2::i32 a {1}; 
+};
+template<> class u<> {
+  public: cpp2::i32 a {2}; 
+};
+template<> class u<cpp2::i16> {
+      public: u() = default;
+      public: u(u const&) = delete; /* No 'that' constructor, suppress copy */
+      public: auto operator=(u const&) -> void = delete;
+};
+#line 25 "pure2-template-specialization.cpp2"
+template<typename T> extern cpp2::i32 const v;
+
+#line 28 "pure2-template-specialization.cpp2"
+template<> std::optional<cpp2::i32> inline constexpr v<cpp2::i16>{ 4 };
+template<> std::optional<cpp2::i8> inline constexpr v<cpp2::i8>{ 5 };
+template<typename T> std::optional<int> inline constexpr v<T*>{ 6 };
+auto main() -> int;
+
+//=== Cpp2 function definitions =================================================
+
+#line 1 "pure2-template-specialization.cpp2"
+
+#line 11 "pure2-template-specialization.cpp2"
+  [[nodiscard]] auto t<cpp2::i8*>::f() -> decltype(auto) { return 17;  }
+  inline CPP2_CONSTEXPR int t<cpp2::i8*>::v{ 29 };
+
+#line 15 "pure2-template-specialization.cpp2"
+  template <typename T> inline CPP2_CONSTEXPR int t<T*>::v{ 17 };
+  template <typename T> [[nodiscard]] auto t<T*>::f() -> decltype(auto) { return 29;  }
+
+#line 25 "pure2-template-specialization.cpp2"
+template<typename T> cpp2::i32 const v {1}; 
+template<> cpp2::i32 const v<void> {2}; 
+template<> cpp2::i32 const v<cpp2::i64> {3}; 
+
+#line 31 "pure2-template-specialization.cpp2"
+auto main() -> int{
+  if (cpp2::cpp2_default.is_active() && !(t<cpp2::i32>().a == 1) ) { cpp2::cpp2_default.report_violation(""); }
+  if (cpp2::cpp2_default.is_active() && !(t<void>().b == 2) ) { cpp2::cpp2_default.report_violation(""); }
+  if (cpp2::cpp2_default.is_active() && !(t<cpp2::i64>().c == 3) ) { cpp2::cpp2_default.report_violation(""); }
+  if (cpp2::cpp2_default.is_active() && !(t<cpp2::i8*>::f() == 17) ) { cpp2::cpp2_default.report_violation(""); }
+  if (cpp2::cpp2_default.is_active() && !(t<cpp2::i8*>::v == 29) ) { cpp2::cpp2_default.report_violation(""); }
+  if (cpp2::cpp2_default.is_active() && !(t<cpp2::i16*>::v == 17) ) { cpp2::cpp2_default.report_violation(""); }
+  if (cpp2::cpp2_default.is_active() && !(t<cpp2::i16*>::f() == 29) ) { cpp2::cpp2_default.report_violation(""); }
+  if (cpp2::cpp2_default.is_active() && !(u<cpp2::i32>().a == 1) ) { cpp2::cpp2_default.report_violation(""); }
+  if (cpp2::cpp2_default.is_active() && !(u<>().a == 1) ) { cpp2::cpp2_default.report_violation(""); }
+  if (cpp2::cpp2_default.is_active() && !(v<cpp2::i32> == 1) ) { cpp2::cpp2_default.report_violation(""); }
+  if (cpp2::cpp2_default.is_active() && !(v<void> == 2) ) { cpp2::cpp2_default.report_violation(""); }
+  if (cpp2::cpp2_default.is_active() && !(v<cpp2::i64> == 3) ) { cpp2::cpp2_default.report_violation(""); }
+  static_assert(v<cpp2::i16> == 4);
+  static_assert(v<cpp2::i8> == 5);
+  static_assert(v<int*> == 6);
+}
+

regression-tests/test-results/pure2-template-specialization.cpp2.output

@@ -0,0 +1,2 @@
+pure2-template-specialization.cpp2... ok (all Cpp2, passes safety checks)
+