Further improve UFCS diagnostics, closes #999, closes #1002

Author: hsutter

Diff for: docs/cpp2/expressions.md

@@ -49,7 +49,7 @@ For example:
 
 
 
-## <a id="wildcard"></a> `_` — the "don't care" wildcard, including explicit discard
+## <a id="wildcard"></a> <a id="discard"></a> `_` — the "don't care" wildcard, including explicit discard
 
 `_` is pronounced **"don't care"** and allowed as a wildcard in most contexts. For example:
 

Diff for: include/cpp2util.h

@@ -957,17 +957,17 @@ template <class T> struct dependent_false : std::false_type {};
         return                   MVFWD(CPP2_UFCS_REMPARENS QUALID __VA_ARGS__)(CPP2_FORWARD(obj), CPP2_FORWARD(params)...); \
     } \
     else if constexpr (requires{ obj.CPP2_UFCS_REMPARENS QUALID TEMPKW __VA_ARGS__(CPP2_FORWARD(params)...); }) { \
-        static_assert( cpp2::impl::dependent_false<Obj>::value, "this function call syntax tries 'obj.func(...)', then 'func(obj,...);' - both failed, but the first would have succeeded if obj were an lvalue - the likely problem is that this is a definite last use of the object (which automatically treats it as an rvalue / move candidate), and the function cannot accept an rvalue" ); \
+        static_assert( cpp2::impl::dependent_false<Obj>::value, "error: implicit discard of an object's modified value is not allowed - this function call modifies 'obj', but 'obj' is never used again in the function so the new value is never used - if that's what you intended, add another line '_ = obj;' afterward to explicitly discard the new value of the object" ); \
         CPP2_FORWARD(obj).CPP2_UFCS_REMPARENS QUALID TEMPKW __VA_ARGS__(CPP2_FORWARD(params)...); \
         MVFWD(CPP2_UFCS_REMPARENS QUALID __VA_ARGS__)(CPP2_FORWARD(obj), CPP2_FORWARD(params)...); \
     } \
     else if constexpr (requires{ MVFWD(CPP2_UFCS_REMPARENS QUALID __VA_ARGS__)(obj, CPP2_FORWARD(params)...); }) { \
-        static_assert( cpp2::impl::dependent_false<Obj>::value, "this function call syntax tries 'obj.func(...)', then 'func(obj,...);' - both failed, but the second would have succeeded if obj were an lvalue - the likely problem is that this is a definite last use of the object, (which automatically treats it as an rvalue / move candidate) and the function cannot accept an rvalue" ); \
+        static_assert( cpp2::impl::dependent_false<Obj>::value, "error: implicit discard of an object's modified value is not allowed - this function call modifies 'obj', but 'obj' is never used again in the function so the new value is never used - if that's what you intended, add another line '_ = obj;' afterward to explicitly discard the new value of the object" ); \
         CPP2_FORWARD(obj).CPP2_UFCS_REMPARENS QUALID TEMPKW __VA_ARGS__(CPP2_FORWARD(params)...); \
         MVFWD(CPP2_UFCS_REMPARENS QUALID __VA_ARGS__)(CPP2_FORWARD(obj), CPP2_FORWARD(params)...); \
     } \
     else { \
-        static_assert( cpp2::impl::dependent_false<Obj>::value, "this function call syntax tries 'obj.func(...)', then 'func(obj,...);' - both failed, did you spell the function name correctly?" ); \
+        static_assert( cpp2::impl::dependent_false<Obj>::value, "this function call syntax tries 'obj.func(...)', then 'func(obj,...);', but both failed - if this function call is passing a local variable that will be modified by the function, but that variable is never used again in the function so the new value is never used, that's likely the problem - if that's what you intended, add another line '_ = obj;' afterward to explicitly discard the new value of the object" ); \
         CPP2_FORWARD(obj).CPP2_UFCS_REMPARENS QUALID TEMPKW __VA_ARGS__(CPP2_FORWARD(params)...); \
         MVFWD(CPP2_UFCS_REMPARENS QUALID __VA_ARGS__)(CPP2_FORWARD(obj), CPP2_FORWARD(params)...); \
     } \