c++: error recovery during C++20 template-id ADL failure

c++: error recovery during C++20 template-id ADL failure

When diagnosing ADL failure we try to perform a second unqualified
lookup for backwards compatibility with legacy code (via -fpermissive),
and for better diagnostics.

But for C++20 template-id ADL, the backwards compatibility code
sometimes causes confusing subsequent diagnostics such as in the
testcase below where we end up diagnosing deduction failure after
emitting the helpful "no declarations were found by ADL".  This happens
because the code just discards the arguments of the template-id callee
when replacing it with the later-declared template, which leads to
overload resolution failure:

  <stdin>: In instantiation of ‘void f() [with T = int]’:
  <stdin>:12:22:   required from here
  <stdin>:5:9: error: ‘g’ was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
  <stdin>:10:6: note: ‘template<class T> void g(int)’ declared here, later in the translation unit
  <stdin>:5:9: error: no matching function for call to ‘g(int)’
  <stdin>:10:6: note: candidate: ‘template<class T> void g(int)’
  <stdin>:10:6: note:   template argument deduction/substitution failed:
  <stdin>:5:9: note:   couldn’t deduce template parameter ‘T’

So for C++20 template-id ADL, this patch disables the backwards
compatibility code while keeping the helpful "no declarations were
found by ADL" diagnostic.

gcc/cp/ChangeLog:

	* pt.c (tsubst_copy_and_build) <case CALL_EXPR>: Disable the
	-fpermissive fallback for C++20 template-id ADL, but keep the
	diagnostic.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/fn-template25.C: New test.