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c++: unifying identical tmpls from current inst [PR108347]

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Here more_specialized_partial_spec wrongly considers the two partial
specializations to be unordered ultimately because unify for identical
parm=arg=A<T>::C returns failure due to C being dependent.

This patch fixes this by relaxing unify's early-exit identity test to
also accept dependent decls; we can't deduce anything further from them
anyway.  In passing this patch removes the CONST_DECL case of unify:
we should never see the CONST_DECL version of a template parameter here,
and for other CONST_DECLs (such as enumerators) it seems we can rely on
them to already have been folded to their DECL_INITIAL.

	PR c++/108347

gcc/cp/ChangeLog:

	* pt.cc (unify): Return unify_success for identical dependent
	DECL_P 'arg' and 'parm'.
	<case CONST_DECL>: Remove handling.

gcc/testsuite/ChangeLog:

	* g++.dg/template/ttp41.C: New test.
This commit is contained in:
Patrick Palka 2023-09-18 14:41:07 -04:00
parent 155178ccb5
commit a6ac1fc64c
2 changed files with 21 additions and 6 deletions
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10
gcc/cp/pt.cc
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@ -24559,7 +24559,8 @@ unify (tree tparms, tree targs, tree parm, tree arg, int strict,
even if ARG == PARM, since we won't record unifications for the
template parameters. We might need them if we're trying to
figure out which of two things is more specialized. */
if (arg == parm && !uses_template_parms (parm))
if (arg == parm
&& (DECL_P (parm) || !uses_template_parms (parm)))
return unify_success (explain_p);
/* Handle init lists early, so the rest of the function can assume
@ -25278,11 +25279,8 @@ unify (tree tparms, tree targs, tree parm, tree arg, int strict,
strict, explain_p);
case CONST_DECL:
if (DECL_TEMPLATE_PARM_P (parm))
return unify (tparms, targs, DECL_INITIAL (parm), arg, strict, explain_p);
if (arg != scalar_constant_value (parm))
return unify_template_argument_mismatch (explain_p, parm, arg);
return unify_success (explain_p);
/* CONST_DECL should already have been folded to its DECL_INITIAL. */
gcc_unreachable ();
case FIELD_DECL:
case TEMPLATE_DECL:

17
gcc/testsuite/g++.dg/template/ttp41.C Normal file
View file

@ -0,0 +1,17 @@
// PR c++/108347
template<class T>
struct A {
template<class U> struct C { };
template<template<class> class TT, class U>
struct B;
template<class U>
struct B<C, U*>;
template<class U>
struct B<C, const U*> { };
};
A<int>::B<A<int>::C, const int*> b;