https://gcc.gnu.org/bugzilla/show_bug.cgi?id=25814
--- Comment #19 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Marek Polacek <mpola...@gcc.gnu.org>: https://gcc.gnu.org/g:5b2003105b35f8fe8e074c055a718c8f484d9d32 commit r11-4756-g5b2003105b35f8fe8e074c055a718c8f484d9d32 Author: Marek Polacek <pola...@redhat.com> Date: Fri Oct 2 09:46:30 2020 -0400 c++: Implement -Wvexing-parse [PR25814] This patch implements the -Wvexing-parse warning to warn about the sneaky most vexing parse rule in C++: the cases when a declaration looks like a variable definition, but the C++ language requires it to be interpreted as a function declaration. This warning is on by default (like clang++). From the docs: void f(double a) { int i(); // extern int i (void); int n(int(a)); // extern int n (int); } Another example: struct S { S(int); }; void f(double a) { S x(int(a)); // extern struct S x (int); S y(int()); // extern struct S y (int (*) (void)); S z(); // extern struct S z (void); } You can find more on this in [dcl.ambig.res]. I spent a fair amount of time on fix-it hints so that GCC can recommend various ways to resolve such an ambiguity. Sometimes that's tricky. E.g., suggesting default-initialization when the class doesn't have a default constructor would not be optimal. Suggesting {}-init is also not trivial because it can use an initializer-list constructor if no default constructor is available (which ()-init wouldn't do). And of course, pre-C++11, we shouldn't be recommending {}-init at all. I also uncovered a bug in cp_parser_declarator, where we were setting *parenthesized_p to true despite the comment saying the exact opposite. gcc/c-family/ChangeLog: PR c++/25814 * c.opt (Wvexing-parse): New option. gcc/cp/ChangeLog: PR c++/25814 * cp-tree.h (enum cp_tree_index): Add CPTI_EXPLICIT_VOID_LIST. (explicit_void_list_node): Define. (PARENTHESIZED_LIST_P): New macro. (struct cp_declarator): Add function::parens_loc. * decl.c (cxx_init_decl_processing): Initialize explicit_void_list_node. (grokparms): Also break when explicit_void_list_node. * parser.c (make_call_declarator): New location_t parameter. Use it to set declarator->u.function.parens_loc. (cp_parser_lambda_declarator_opt): Pass UNKNOWN_LOCATION to make_call_declarator. (warn_about_ambiguous_parse): New function. (cp_parser_init_declarator): Call warn_about_ambiguous_parse. (cp_parser_declarator): Set *parenthesized_p to false rather than to true. (cp_parser_direct_declarator): Create a location for the function's parentheses and pass it to make_call_declarator. (cp_parser_parameter_declaration_clause): Return explicit_void_list_node for (void). (cp_parser_parameter_declaration_list): Set PARENTHESIZED_LIST_P in the parameters tree. gcc/ChangeLog: PR c++/25814 * doc/invoke.texi: Document -Wvexing-parse. gcc/testsuite/ChangeLog: PR c++/25814 * g++.dg/cpp2a/fn-template16.C: Add a dg-warning. * g++.dg/cpp2a/fn-template7.C: Likewise. * g++.dg/lookup/pr80891-5.C: Likewise. * g++.dg/lto/pr79050_0.C: Add extern. * g++.dg/lto/pr84805_0.C: Likewise. * g++.dg/parse/pr58898.C: Add a dg-warning. * g++.dg/template/scope5.C: Likewise. * g++.old-deja/g++.brendan/recurse.C: Likewise. * g++.old-deja/g++.jason/template4.C: Likewise. * g++.old-deja/g++.law/arm4.C: Likewise. * g++.old-deja/g++.mike/for2.C: Likewise. * g++.old-deja/g++.other/local4.C: Likewise. * g++.old-deja/g++.pt/crash3.C: Likewise. * g++.dg/warn/Wvexing-parse.C: New test. * g++.dg/warn/Wvexing-parse2.C: New test. * g++.dg/warn/Wvexing-parse3.C: New test. * g++.dg/warn/Wvexing-parse4.C: New test. * g++.dg/warn/Wvexing-parse5.C: New test. * g++.dg/warn/Wvexing-parse6.C: New test. * g++.dg/warn/Wvexing-parse7.C: New test. libstdc++-v3/ChangeLog: PR c++/25814 * testsuite/20_util/reference_wrapper/lwg2993.cc: Add a dg-warning. * testsuite/25_algorithms/generate_n/87982_neg.cc: Likewise.
