Make goto_programt::instructiont::validate() accept differing return types

When a function declaration and its definition appear in different translation
units they may have different return types. This commit relaxes the checks
in goto_programt::instructiont::validate() in these circumstances.

Author: chrisr-diffblue

src/goto-programs/goto_program.cpp

@@ -738,6 +738,32 @@ void goto_programt::instructiont::validate(
               symbol_expr_type_matches_symbol_table = base_type_eq(
                 goto_symbol_expr.type(), full_table_symbol_type, ns);
             }
+
+            if(
+              !symbol_expr_type_matches_symbol_table &&
+              goto_symbol_expr.type().id() == ID_code)
+            {
+              // If a function declaration and its definition are in different
+              // translation units they may have different return types,
+              // which remove_returns patches up with a typecast. If thats
+              // the case, then the return type in the symbol table may differ
+              // from the return type in the symbol expr
+              if(
+                goto_symbol_expr.type().source_location().get_file() !=
+                table_symbol->type.source_location().get_file())
+              {
+                // temporarily fixup the return types
+                auto goto_symbol_expr_type =
+                  to_code_type(goto_symbol_expr.type());
+                auto table_symbol_type = to_code_type(table_symbol->type);
+
+                goto_symbol_expr_type.return_type() =
+                  table_symbol_type.return_type();
+
+                symbol_expr_type_matches_symbol_table =
+                  base_type_eq(goto_symbol_expr_type, table_symbol_type, ns);
+              }
+            }
           }
 
           if(