c++: Infinite diagnostic loop with decltype([]{}) [PR94521]

We are hitting a recursive loop when printing the signature of a function
containing a decltype([]{}) type.  The loop is

  dump_function_decl -> dump_substitution
		     -> dump_template_bindings
		     -> dump_type
		     -> dump_aggr_type
		     -> dump_scope -> dump_function_decl

and we loop because dump_template_bindings wants to print the resolved type of
decltype([]{}) (i.e. just a lambda type), so it calls dump_aggr_type, which
wants to print the function scope of the lambda type.  But the function scope of
the lambda type is the function which we're in the middle of printing.

This patch breaks the loop by passing TFF_NO_FUNCTION_ARGUMENTS to
dump_function_decl from dump_scope, so that we avoid recursing into
dump_substitution and ultimately looping.

This also means we no longer emit the "[with ...]" clause when printing a
function template scope, and we instead just emit its template argument list in
a more natural way, e.g. instead of
    foo(int, char) [with T=bool]::x
we would now print
    foo<bool>::x
which seems like an improvement on its own.

The full signature of the function 'spam' in the below testcase is now
  void spam(decltype (<lambda>)*) [with T = int; decltype (<lambda>) = spam<int>::<lambda()>]

gcc/cp/ChangeLog:

	PR c++/94521
	* error.c (dump_scope): Pass TFF_NO_FUNCTION_ARGUMENTS to
	dump_function_decl when printing a function template instantiation as a
	scope.

gcc/testsuite/ChangeLog:

	PR c++/94521
	* g++.dg/cpp2a/lambda-uneval12.C: New test.

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 3da61dbcfd96033f2623b2d26a0558d9bdc27c67..65b648a3edce4ac4e86863f0c36a61ece1d9c063 100644 (file)
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,5 +1,10 @@
 2020-04-13  Patrick Palka  <ppalka@redhat.com>
 
+       PR c++/94521
+       * error.c (dump_scope): Pass TFF_NO_FUNCTION_ARGUMENTS to
+       dump_function_decl when printing a function template instantiation as a
+       scope.
+
        PR c++/94470
        * constexpr.c (get_or_insert_ctor_field): Set default value of parameter
        'pos_hint' to -1.

diff --git a/gcc/cp/error.c b/gcc/cp/error.c
index 61d1218dc9006270f815dc46a763bbe165095e24..98c163db5723474fbc3c10242afb4c83fd5cea30 100644 (file)
--- a/gcc/cp/error.c
+++ b/gcc/cp/error.c
@@ -211,6 +211,8 @@ dump_scope (cxx_pretty_printer *pp, tree scope, int flags)
     }
   else if ((flags & TFF_SCOPE) && TREE_CODE (scope) == FUNCTION_DECL)
     {
+      if (DECL_USE_TEMPLATE (scope))
+       f |= TFF_NO_FUNCTION_ARGUMENTS;
       dump_function_decl (pp, scope, f);
       pp_cxx_colon_colon (pp);
     }

diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 74a7fa1016376e092c85e9af56e48b3da9f85b19..8e2df1b6eda1145e74eade4da3533fb453611dea 100644 (file)
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,5 +1,8 @@
 2020-04-13  Patrick Palka  <ppalka@redhat.com>
 
+       PR c++/94521
+       * g++.dg/cpp2a/lambda-uneval12.C: New test.
+
        PR c++/94470
        * g++.dg/cpp1y/constexpr-nsdmi8.C: New test.
        * g++.dg/cpp1y/constexpr-nsdmi9.C: New test.

diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-uneval12.C b/gcc/testsuite/g++.dg/cpp2a/lambda-uneval12.C
new file mode 100644 (file)
index 0000000..24d2e70
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/lambda-uneval12.C
@@ -0,0 +1,13 @@
+// PR c++/94521
+// { dg-do compile { target c++2a } }
+
+template <typename T>
+void spam(decltype([]{}) *s)
+{
+  static_assert(__is_same(int, decltype(s))); // { dg-error "static assertion failed" }
+}
+
+void foo()
+{
+  spam<int>(nullptr);
+}