In fn_type_unifcation, we are passing NULL_TREE as the 'in_decl'
parameter to coerce_template_parms, and this is causing template
type/value mismatch error messages to get suppressed regardless of the
value of 'complain'.
This means that when substitution into a function template fails due to
a type/value mismatch between a template parameter and the provided
template argument, we just say "template argument deduction/substitution
failed:" without a followup explanation of the failure.
Fix this by passing 'fn' instead of NULL_TREE to coerce_template_parms.
gcc/cp/ChangeLog:
PR c++/66439
* pt.c (fn_type_unification): Pass 'fn' instead of NULL_TREE as
the 'in_decl' parameter to coerce_template_parms.
gcc/testsuite/ChangeLog:
PR c++/66439
* g++.dg/cpp2a/concepts-ts4.C: Expect a "type/value mismatch"
diagnostic.
* g++.dg/cpp2a/concepts-ts6.C: Likewise.
* g++.dg/template/error56.C: Likewise.
* g++.dg/template/error59.C: New test.
libstdc++-v3/ChangeLog:
PR c++/66439
* testsuite/20_util/pair/astuple/get_neg.cc: Prune "type/value
mismatch" messages.
* testsuite/20_util/tuple/element_access/get_neg.cc: Likewise.
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