[PATCH] c++: redundant targ coercion for var/alias tmpls

[Patrick Palka <ppalka@redhat.com>]

When stepping through the variable/alias template specialization code
paths, I noticed we perform template argument coercion twice: first from
instantiate_alias_template / finish_template_variable and again from
tsubst_decl (during instantiate_template).  It should suffice to perform
coercion once.

To that end patch elides this second coercion from tsubst_decl when
possible.  We can't get rid of it completely because we don't always
specialize a variable template from finish_template_variable: we could
also be doing so directly from instantiate_template during variable
template partial specialization selection, in which case the coercion
from tsubst_decl would be the first and only coercion.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?  This reduces memory usage of range-v3's zip.cpp by ~0.5%.

gcc/cp/ChangeLog:

	* pt.cc (tsubst_decl) <case TYPE_/VAR_DECL>: Call
	coercion_template_parms only if DECL_TEMPLATE_SPECIALIZATION
	is set.
---
 gcc/cp/pt.cc | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index be86051abad..dd10409ce18 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -15232,10 +15232,17 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain)
 		argvec = tsubst (DECL_TI_ARGS (t), args, complain, in_decl);
 		if (argvec != error_mark_node
 		    && PRIMARY_TEMPLATE_P (gen_tmpl)
-		    && TMPL_ARGS_DEPTH (args) >= TMPL_ARGS_DEPTH (argvec))
-		  /* We're fully specializing a template declaration, so
-		     we need to coerce the innermost arguments corresponding to
-		     the template.  */
+		    && TMPL_ARGS_DEPTH (args) >= TMPL_ARGS_DEPTH (argvec)
+		    && DECL_TEMPLATE_SPECIALIZATION (t))
+		  /* We're fully specializing an alias or variable template, so
+		     coerce the innermost arguments if necessary.  We expect
+		     instantiate_alias_template and finish_template_variable to
+		     already have done this relative to the primary template, in
+		     which case this coercion is unnecessary, but we can also
+		     get here when substituting a partial variable template
+		     specialization (directly from instantiate_template), in
+		     which case DECL_TEMPLATE_SPECIALIZATION is set and coercion
+		     is necessary.  */
 		  argvec = (coerce_template_parms
 			    (DECL_TEMPLATE_PARMS (gen_tmpl),
 			     argvec, tmpl, complain));
-- 
2.41.0.113.g6640c2d06d