fix(parser): Initial support for the decltype specifier

Author: robertoraggi

src/cxx/semantics.cc

@@ -1315,6 +1315,7 @@ void Semantics::visit(DecltypeAutoSpecifierAST* ast) {
 void Semantics::visit(DecltypeSpecifierAST* ast) {
   ExpressionSem expression;
   this->expression(ast->expression, &expression);
+  specifiers_->type.mergeWith(expression.type);
 }
 
 void Semantics::visit(TypeofSpecifierAST* ast) {

tests/unit_tests/parser/is_same_as.001.cc

@@ -38,3 +38,24 @@ static_assert(__is_same_as(const int, volatile int)); // expected-error{{static_
 static_assert(__is_same_as(volatile int, volatile int)); // expected-error{{static_assert failed}}
 
 static_assert(__is_same_as(const volatile int, volatile const int));
+
+struct vec2i {
+  int x;
+  int y;
+};
+
+struct string {
+  const char* c_str();
+};
+
+void test_1() {
+  vec2i v;
+  static_assert(__is_same_as(decltype(v), vec2i));
+  static_assert(__is_same_as(decltype(v.x), int));
+  static_assert(__is_same_as(decltype(v.x), unsigned int)); // expected-error{{static_assert failed}}
+  static_assert(__is_same_as(decltype(v.y), int));
+
+  string str;
+  static_assert(__is_same_as(decltype(str.c_str()), const char*));
+  static_assert(__is_same_as(decltype(str.c_str()), char*)); // expected-error{{static_assert failed}}
+}