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This patch fixes some issues with substitution into a C++20 template
parameter object wrapper:
* The first testcase demonstrates a situation where the same_type_p
assert in relevant case of tsubst_copy doesn't hold, because (partial)
substitution of {int,} into the wrapper's TREE_TYPE yields A<int> but
substitution into the underlying TEMPLATE_PARM_INDEX is a nop due to
tsubst's level==1/tf_partial early exit tests, hence TREE_TYPE in the
latter case remains A<T>. So this patch just gets rid of the assert;
the type mismatch doesn't seem to be a problem in practice.
* In the second testcase, dependent substitution into the underlying
TEMPLATE_PARM_INDEX yields a CALL_EXPR with empty TREE_TYPE, which
tsubst_copy doesn't expect. This patch fixes this by handling empty
TREE_TYPE the same way as a non-const TREE_TYPE. Moreover, after
this substitution we're left with a VIEW_CONVERT_EXPR wrapping a
CALL_EXPR instead of a TEMPLATE_PARM_INDEX, which during the subsequent
non-dependent substitution tsubst_copy doesn't expect either. So
this patch also relaxes tsubst_copy to accept such VIEW_CONVERT_EXPR
too.
* In the third testcase, we end up never resolving the call to
f.modify() because tsubst_copy doesn't do overload resolution.
This patch fixes this by moving the handling of these
VIEW_CONVERT_EXPR wrappers from tsubst_copy to tsubst_copy_and_build.
For good measure tsubst_copy_and_build should also handle
REF_PARENTHESIZED_P wrappers instead of delegating to tsubst_copy.
PR c++/103346
PR c++/104278
PR c++/102553
gcc/cp/ChangeLog:
* pt.cc (tsubst_copy) <case VIEW_CONVERT_EXPR>: Move the
handling of C++20 template parameter object wrappers to ...
(tsubst_copy_and_build) <case VIEW_CONVERT_EXPR>: ... here.
Accept non-TEMPLATE_PARM_INDEX inner operand. Handle empty
TREE_TYPE on substituted inner operand. Remove same_type_p
assert. Also handle REF_PARENTHESIZED_P VIEW_CONVERT_EXPRs.
gcc/testsuite/ChangeLog:
* g++.dg/cpp2a/nontype-class52a.C: New test.
* g++.dg/cpp2a/nontype-class53.C: New test.
* g++.dg/cpp2a/nontype-class54.C: New test.
* g++.dg/cpp2a/nontype-class55.C: New test.
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