c++: optimize tsubst_template_decl for function templates

r14-2655-g92d1425ca78040 made instantiate_template avoid redundantly
performing a specialization lookup when calling tsubst_decl.  This patch
applies the same optimization to the analagous tsubst_template_decl when
(partially) instantiating a function template.  This allows us to remove
an early exit test from register_specialization since we no longer try
to register the FUNCTION_DECL corresponding to a function template
partial instantiation.

gcc/cp/ChangeLog:

	* pt.cc (register_specialization): Remove now-unnecessary
	early exit for FUNCTION_DECL partial instantiation.
	(tsubst_template_decl): Pass use_spec_table=false to
	tsubst_function_decl.  Set DECL_TI_ARGS of a non-lambda
	FUNCTION_DECL specialization to the full set of arguments.
	Simplify register_specialization call accordingly.

gcc/testsuite/ChangeLog:

	* g++.dg/template/nontype12.C: Expect two instead of three
	duplicate diagnostics for A<double>::bar() specialization.

gcc/cp/pt.cc

@@ -1507,21 +1507,6 @@ register_specialization (tree spec, tree tmpl, tree args, bool is_friend,
 	      || (TREE_CODE (tmpl) == FIELD_DECL
 		  && TREE_CODE (spec) == NONTYPE_ARGUMENT_PACK));
-  if (TREE_CODE (spec) == FUNCTION_DECL
-      && uses_template_parms (DECL_TI_ARGS (spec)))
-    /* This is the FUNCTION_DECL for a partial instantiation.  Don't
-       register it; we want the corresponding TEMPLATE_DECL instead.
-       We use `uses_template_parms (DECL_TI_ARGS (spec))' rather than
-       the more obvious `uses_template_parms (spec)' to avoid problems
-       with default function arguments.  In particular, given
-       something like this:
-
-	  template <class T> void f(T t1, T t = T())
-
-       the default argument expression is not substituted for in an
-       instantiation unless and until it is actually needed.  */
-    return spec;
-
   spec_entry elt;
   elt.tmpl = tmpl;
   elt.args = args;
@@ -14665,7 +14650,7 @@ tsubst_template_decl (tree t, tree args, tsubst_flags_t complain,
   tree in_decl = t;
   tree spec;
   tree tmpl_args;
-  tree full_args;
+  tree full_args = NULL_TREE;
   tree r;
   hashval_t hash = 0;
@@ -14756,7 +14741,8 @@ tsubst_template_decl (tree t, tree args, tsubst_flags_t complain,
   tree inner = decl;
   ++processing_template_decl;
   if (TREE_CODE (inner) == FUNCTION_DECL)
-    inner = tsubst_function_decl (inner, args, complain, lambda_fntype);
+    inner = tsubst_function_decl (inner, args, complain, lambda_fntype,
+				  /*use_spec_table=*/false);
   else
     {
       if (TREE_CODE (inner) == TYPE_DECL && !TYPE_DECL_ALIAS_P (inner))
@@ -14794,6 +14780,11 @@ tsubst_template_decl (tree t, tree args, tsubst_flags_t complain,
     }
   else
     {
+      if (TREE_CODE (inner) == FUNCTION_DECL)
+	/* Set DECL_TI_ARGS to the full set of template arguments, which
+	   tsubst_function_decl didn't do due to use_spec_table=false.  */
+	DECL_TI_ARGS (inner) = full_args;
+
       DECL_TI_TEMPLATE (inner) = r;
       DECL_TI_ARGS (r) = DECL_TI_ARGS (inner);
     }
@@ -14824,9 +14815,7 @@ tsubst_template_decl (tree t, tree args, tsubst_flags_t complain,
   if (TREE_CODE (decl) == FUNCTION_DECL && !lambda_fntype)
     /* Record this non-type partial instantiation.  */
-    register_specialization (r, t,
-			     DECL_TI_ARGS (DECL_TEMPLATE_RESULT (r)),
-			     false, hash);
+    register_specialization (r, t, full_args, false, hash);
   return r;
 }

gcc/testsuite/g++.dg/template/nontype12.C

@@ -5,7 +5,8 @@ template<typename T> struct A
 {
   template<T> int foo();                        // { dg-error "double" "" { target c++17_down } }
   template<template<T> class> int bar();        // { dg-bogus {double[^\n]*\n[^\n]*C:7:[^\n]*double} "" { xfail c++17_down } }
-  // { dg-error "double" "" { target c++17_down } .-1 }
+  // The above xfailed dg-bogus test means that we issue two errors
+  // for the invalid 'double' NTTP, and preferably we'd issue just one.
   template<T> struct X;                         // { dg-error "double" "" { target c++17_down } }
 };