fix(parse): requires clause on one-liner and add error when used on non-template (#528)

* Fix parsing error of requires clause

Add handling of lack of empty line after oneliner.
Add parsing error when requires is used on non-template

* Update parse.h

Restrict new check to types

---------

Co-authored-by: Herb Sutter <[email protected]>

Author: filipsajdak

regression-tests/test-results/pure2-requires-clauses.cpp

@@ -11,8 +11,7 @@
 template<typename T, typename U> 
 
     requires( std::is_same_v<T,int> 
-              && std::is_same_v<U,int> )
-class X;
+              && std::is_same_v<U,int> )class X;
 
 //=== Cpp2 type definitions and function declarations ===========================
 
@@ -21,8 +20,8 @@ class X;
 template<typename T, typename U> 
 
     requires( std::is_same_v<T,int> 
-              && std::is_same_v<U,int> )
-class X {
+              && std::is_same_v<U,int> )class X
+ {
     public: explicit X();
 
     public: X(X const&) = delete; /* No 'that' constructor, suppress copy */

source/cppfront.cpp

@@ -5148,7 +5148,7 @@ class cppfront
                 if (n.requires_clause_expression) {
                     printer.print_cpp2("requires( ", n.requires_pos);
                     emit(*n.requires_clause_expression);
-                    printer.print_cpp2(" )\n\n", n.requires_pos);
+                    printer.print_cpp2(" )\n", n.requires_pos);
                 }
 
                 printer.print_cpp2("class ", n.position());

source/parse.h

@@ -6510,7 +6510,17 @@ class parser
         }
 
         //  Next is optionally a requires clause
-        if (curr() == "requires") {
+        if (curr() == "requires")
+        {
+            if (
+                n->is_type()
+                && !n->template_parameters
+                )
+            {
+                error("'requires' is only valid for a type with a template parameter list");
+                return {};
+            }
+
             n->requires_pos = curr().position();
             next();
             auto e = logical_or_expression();