On 12/15/21 12:49, Patrick Palka wrote:
This patch makes us remember the function selected by overload
resolution during ahead of time processing of a non-dependent call
expression, so that we avoid repeating most of the work of overload
resolution at instantiation time.  This mirrors what we already do for
non-dependent operator expressions via build_min_non_dep_op_overload.

Some caveats:

  * When processing ahead of time a non-dependent call to a member
    function template inside a class template (as in
    g++.dg/template/deduce4.C), we end up generating an "inverted" partial
    instantiation such as S<T>::foo<int, int>(), the kinds of which we're
    apparently not prepared to fully instantiate (e.g. tsubst_baselink
    mishandles it).  So this patch disables this optimization for such
    functions and adds a FIXME.

I wonder if it would be worthwhile to build a TEMPLATE_ID_EXPR to remember the deduced template args, even if we are failing to remember the actual function?

  * WHen trying to make the instantiation machinery handle these partial
    instantiations, I made a couple of changes in register_specialization
    and tsubst_function_decl that get us closer to handling such partial
    instantiations and that seem like improvements on their own, so this
    patch includes these changes.

The tsubst_function_decl change makes me nervous; surely there was some reason that function wasn't that way in the first place. Let's hold these changes for stage 1 if they aren't actually fixing anything.

   * This change triggered a latent FUNCTION_DECL pretty printing issue
     in cpp0x/error2.C -- since we now resolve the call to foo<0> ahead
     of time, the error now looks like:

       error: expansion pattern ‘foo()()=0’ contains no parameter pack

     where the FUNCTION_DECL foo is clearly misprinted.  But this
     pretty-printing issue could be reproduced without this patch if
     we replace foo with an ordinary function.  Since this testcase was
     added to verify pretty printing of TEMPLATE_ID_EXPR, I work around
     this test failure by making the call to foo type-dependent and thus
     immune to this ahead of time pruning.

   * We now reject parts of cpp0x/fntmp-equiv1.C because we notice that
     the call d(f, b) in

       template <unsigned long f, unsigned b, typename> e<d(f, b)> d();

     isn't constexpr because the (resolved) d isn't.  I tried fixing this
     by making d constexpr, but then the call to d from main becomes
     ambiguous.  So I settled with removing this part of the testcase.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?  Also tested on cmcstl2 and range-v3.

gcc/cp/ChangeLog:

        * call.c (build_new_method_call): For a non-dependent call
        expression inside a template, returning a templated tree
        whose overload set contains just the selected function.
        * pt.c (register_specialization): Check only the innermost
        template args for dependence in the early exit test.
        (tsubst_function_decl): Simplify obtaining the template arguments
        for a partial instantiation.
        * semantics.c (finish_call_expr): As with build_new_method_call.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp0x/error2.C: Make the call to foo type-dependent in
        order to avoid latent pretty-printing issue for FUNCTION_DECL
        inside MODOP_EXPR.
        * g++.dg/cpp0x/fntmp-equiv1.C: Remove ill-formed parts of
        testcase that we now diagnose.
        * g++.dg/template/non-dependent16.C: New test.
        * g++.dg/template/non-dependent16a.C: New test.
---
  gcc/cp/call.c                                 | 17 +++++++++
  gcc/cp/pt.c                                   | 18 ++-------
  gcc/cp/semantics.c                            | 15 ++++++++
  gcc/testsuite/g++.dg/cpp0x/error2.C           |  4 +-
  gcc/testsuite/g++.dg/cpp0x/fntmp-equiv1.C     |  4 --
  .../g++.dg/template/non-dependent16.C         | 37 +++++++++++++++++++
  .../g++.dg/template/non-dependent16a.C        | 36 ++++++++++++++++++
  7 files changed, 111 insertions(+), 20 deletions(-)
  create mode 100644 gcc/testsuite/g++.dg/template/non-dependent16.C
  create mode 100644 gcc/testsuite/g++.dg/template/non-dependent16a.C

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index 53a391cbc6b..92d96c19f5c 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -11165,6 +11165,23 @@ build_new_method_call (tree instance, tree fns, vec<tree, 
va_gc> **args,
        }
        if (INDIRECT_REF_P (call))
        call = TREE_OPERAND (call, 0);
+
+      /* Prune all but the selected function from the original overload
+        set so that we can avoid some duplicate work at instantiation time.  */
+      if (really_overloaded_fn (fns))
+       {
+         if (DECL_TEMPLATE_INFO (fn)
+             && DECL_MEMBER_TEMPLATE_P (DECL_TI_TEMPLATE (fn))
+             && dependent_type_p (DECL_CONTEXT (fn)))
+           /* FIXME: We're not prepared to fully instantiate "inverted"
+              partial instantiations such as A<T>::f<int>().  */;
+         else
+           {
+             orig_fns = copy_node (orig_fns);
+             BASELINK_FUNCTIONS (orig_fns) = fn;
+           }
+       }
+
        call = (build_min_non_dep_call_vec
              (call,
               build_min (COMPONENT_REF, TREE_TYPE (CALL_EXPR_FN (call)),
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 2340139b238..b114114e617 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -1566,18 +1566,10 @@ register_specialization (tree spec, tree tmpl, tree 
args, bool is_friend,
                  && TREE_CODE (spec) == NONTYPE_ARGUMENT_PACK));
if (TREE_CODE (spec) == FUNCTION_DECL
-      && uses_template_parms (DECL_TI_ARGS (spec)))
+      && uses_template_parms (INNERMOST_TEMPLATE_ARGS (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.  */
+       register it; we want to register the corresponding TEMPLATE_DECL
+       instead.  */
      return spec;
if (optimize_specialization_lookup_p (tmpl))
@@ -13960,9 +13952,7 @@ tsubst_function_decl (tree t, tree args, tsubst_flags_t 
complain,
/* Calculate the complete set of arguments used to
         specialize R.  */
-      argvec = tsubst_template_args (DECL_TI_ARGS
-                                    (DECL_TEMPLATE_RESULT
-                                     (DECL_TI_TEMPLATE (t))),
+      argvec = tsubst_template_args (DECL_TI_ARGS (t),
                                     args, complain, in_decl);
        if (argvec == error_mark_node)
        return error_mark_node;
diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index 7078af03d3c..57f689042b9 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -2893,6 +2893,21 @@ finish_call_expr (tree fn, vec<tree, va_gc> **args, bool 
disallow_virtual,
      {
        if (INDIRECT_REF_P (result))
        result = TREE_OPERAND (result, 0);
+
+      /* Prune all but the selected function from the original overload
+        set so that we can avoid some duplicate work at instantiation time.  */
+      if (TREE_CODE (result) == CALL_EXPR
+         && really_overloaded_fn (orig_fn))
+       {
+         orig_fn = CALL_EXPR_FN (result);
+         if (TREE_CODE (orig_fn) == COMPONENT_REF)
+           {
+             /* The result of build_new_method_call.  */
+             orig_fn = TREE_OPERAND (orig_fn, 1);
+             gcc_assert (BASELINK_P (orig_fn));
+           }
+       }
+
        result = build_call_vec (TREE_TYPE (result), orig_fn, orig_args);
        SET_EXPR_LOCATION (result, input_location);
        KOENIG_LOOKUP_P (result) = koenig_p;
diff --git a/gcc/testsuite/g++.dg/cpp0x/error2.C 
b/gcc/testsuite/g++.dg/cpp0x/error2.C
index e6af294c180..eb966362ccb 100644
--- a/gcc/testsuite/g++.dg/cpp0x/error2.C
+++ b/gcc/testsuite/g++.dg/cpp0x/error2.C
@@ -3,7 +3,7 @@
template<int> int foo(); -template<typename F> void bar(F f)
+template<typename F, int N> void bar(F f)
  {
-  f((foo<0>()=0)...); // { dg-error "pattern '\\(foo\\<0\\>\\)\\(\\)=0'" }
+  f((foo<N>()=0)...); // { dg-error "pattern '\\(foo\\<N\\>\\)\\(\\)=0'" }
  }
diff --git a/gcc/testsuite/g++.dg/cpp0x/fntmp-equiv1.C 
b/gcc/testsuite/g++.dg/cpp0x/fntmp-equiv1.C
index 833ae6fc85c..60ebad8d1d3 100644
--- a/gcc/testsuite/g++.dg/cpp0x/fntmp-equiv1.C
+++ b/gcc/testsuite/g++.dg/cpp0x/fntmp-equiv1.C
@@ -1,10 +1,7 @@
  // PR c++/86946, DR 1321
  // { dg-do compile { target c++11 } }
-int d(int, int);
  template <long> class e {};
-template <unsigned long f, unsigned b, typename> e<sizeof(d(f, b))> d();
-template <unsigned long f, unsigned b, typename> e<d(f, b)> d();
template <class T, class U> constexpr T d2(T, U) { return 42; }
  template <unsigned long f, unsigned b, typename> e<d2(f, b)> d2();
@@ -17,7 +14,6 @@ template <unsigned long f, unsigned b, typename> e<sizeof(d3(f, 
b))> d3();
int main()
  {
-  d<1,2,int>();
    d2<1,2,int>();
    d3<1,2,int>();
  }
diff --git a/gcc/testsuite/g++.dg/template/non-dependent16.C 
b/gcc/testsuite/g++.dg/template/non-dependent16.C
new file mode 100644
index 00000000000..ee8ef902529
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/non-dependent16.C
@@ -0,0 +1,37 @@
+// This test verifies that after resolving a non-dependent call expression
+// ahead of time, we prune all but the selected candidate from the overload
+// set.  Without this optimization, overload resolution for the final call to
+// f<void>() would be exponential in the size of the overload set.
+
+// { dg-do compile { target c++11 } }
+
+template<class T> void f();
+template<class T> auto f() -> decltype(f<void>(), 1, *T());
+template<class T> auto f() -> decltype(f<void>(), 2, *T());
+template<class T> auto f() -> decltype(f<void>(), 3, *T());
+template<class T> auto f() -> decltype(f<void>(), 4, *T());
+template<class T> auto f() -> decltype(f<void>(), 5, *T());
+template<class T> auto f() -> decltype(f<void>(), 6, *T());
+template<class T> auto f() -> decltype(f<void>(), 7, *T());
+template<class T> auto f() -> decltype(f<void>(), 8, *T());
+template<class T> auto f() -> decltype(f<void>(), 9, *T());
+template<class T> auto f() -> decltype(f<void>(), 10, *T());
+template<class T> auto f() -> decltype(f<void>(), 11, *T());
+template<class T> auto f() -> decltype(f<void>(), 12, *T());
+template<class T> auto f() -> decltype(f<void>(), 13, *T());
+template<class T> auto f() -> decltype(f<void>(), 14, *T());
+template<class T> auto f() -> decltype(f<void>(), 15, *T());
+template<class T> auto f() -> decltype(f<void>(), 16, *T());
+template<class T> auto f() -> decltype(f<void>(), 17, *T());
+template<class T> auto f() -> decltype(f<void>(), 18, *T());
+template<class T> auto f() -> decltype(f<void>(), 19, *T());
+template<class T> auto f() -> decltype(f<void>(), 20, *T());
+template<class T> auto f() -> decltype(f<void>(), 21, *T());
+template<class T> auto f() -> decltype(f<void>(), 22, *T());
+template<class T> auto f() -> decltype(f<void>(), 23, *T());
+template<class T> auto f() -> decltype(f<void>(), 24, *T());
+template<class T> auto f() -> decltype(f<void>(), 25, *T());
+
+int main() {
+  f<void>();
+}
diff --git a/gcc/testsuite/g++.dg/template/non-dependent16a.C 
b/gcc/testsuite/g++.dg/template/non-dependent16a.C
new file mode 100644
index 00000000000..0e04d646c0b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/non-dependent16a.C
@@ -0,0 +1,36 @@
+// Like non-dependent16.C, but using member functions.
+
+// { dg-do compile { target c++11 } }
+
+struct A {
+  template<class T> static void f();
+  template<class T> static auto f() -> decltype(f<void>(), 1, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 2, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 3, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 4, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 5, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 6, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 7, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 8, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 9, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 10, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 11, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 12, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 13, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 14, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 15, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 16, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 17, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 18, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 19, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 20, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 21, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 22, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 23, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 24, *T());
+  template<class T> static auto f() -> decltype(f<void>(), 25, *T());
+};
+
+int main() {
+  A::f<void>();
+}

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