[Bug c++/80684] New: poor error message and fix-it hint for a function with an argument of undeclared type

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80684

            Bug ID: 80684
           Summary: poor error message and fix-it hint for a function with
                    an argument of undeclared type
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

G++ issues the following confusing error messages for a function declaration
involving the undeclared type string (where std::string from <string> was meant
but the header was not included).  The suggested alternative only makes things
worse.

$ cat y.C && gcc -S -Wall -Wextra -Wpedantic y.C
void f (string);

y.C:1:15: error: variable or field ‘f’ declared void
 void f (string);
               ^
y.C:1:9: error: ‘string’ was not declared in this scope
 void f (string);
         ^~~~~~
y.C:1:9: note: suggested alternative: ‘struct’
 void f (string);
         ^~~~~~
         struct

In contrast, clang simply prints the much clearer:

y.C:1:9: error: unknown type name 'string'