c++: too many errors with sneaky template [PR118516]

Since C++20 P0846, a name followed by a < can be treated as a template-name
even though name lookup did not find a template-name.  That happens
in this test with "i < foo ()":

  for (int id = 0; i < foo(); ++id);

and results in a raft of errors about non-constant foo().  The problem
is that the require_potential_constant_expression call in
cp_parser_template_argument emits errors even when we're parsing
tentatively.  So we repeat the error when we're trying to parse
as a nested-name-specifier, type-name, etc.

Guarding the call with !cp_parser_uncommitted_to_tentative_parse_p would
mean that require_potential_constant_expression never gets called.  But
we don't need the call at all as far as I can tell.  Stuff like

  template<int N> struct S { };
  int foo () { return 4; }
  void
  g ()
  {
    S<foo()> s;
  }

gets diagnosed in convert_nontype_argument.  In fact, with this patch,
we only emit "call to non-constexpr function" once.  (That is, in C++17
only; C++14 uses a different path.)

	PR c++/118516

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_template_argument): Don't call
	require_potential_constant_expression.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/fn-template11.C:
	* g++.dg/template/fn-template1.C: New test.
	* g++.dg/template/fn-template2.C: New test.

4 files changed, 22 insertions, 2 deletions

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 9f619b1..69c27aa 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -20315,7 +20315,6 @@ cp_parser_template_argument (cp_parser* parser)
       /* With C++17 generalized non-type template arguments we need to handle
 	 lvalue constant expressions, too.  */
       argument = cp_parser_assignment_expression (parser);
-      require_potential_constant_expression (argument);
     }
 
   if (!maybe_type_id)
diff --git a/gcc/testsuite/g++.dg/cpp2a/fn-template11.C b/gcc/testsuite/g++.dg/cpp2a/fn-template11.C
index 1a6b688..ca25403 100644
--- a/gcc/testsuite/g++.dg/cpp2a/fn-template11.C
+++ b/gcc/testsuite/g++.dg/cpp2a/fn-template11.C
@@ -7,5 +7,5 @@ int nonconst ();
 int foo ()
 {
   return blah < // { dg-error "not declared" }
-    nonconst (), nonconst (); // { dg-error "call to non-.constexpr. function" }
+    nonconst (), nonconst ();
 }
diff --git a/gcc/testsuite/g++.dg/template/fn-template1.C b/gcc/testsuite/g++.dg/template/fn-template1.C
new file mode 100644
index 0000000..14b9883
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/fn-template1.C
@@ -0,0 +1,12 @@
+// PR c++/118516
+// { dg-do compile }
+// Like cpp2a/fn-template11.C but with blah declared.
+
+int nonconst ();
+
+int foo ()
+{
+  int blah = 20;
+  return blah <
+    nonconst (), nonconst ();
+}
diff --git a/gcc/testsuite/g++.dg/template/fn-template2.C b/gcc/testsuite/g++.dg/template/fn-template2.C
new file mode 100644
index 0000000..c7c31dd
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/fn-template2.C
@@ -0,0 +1,9 @@
+// PR c++/118516
+// { dg-do compile }
+
+int foo();
+int main()
+{
+    for (int id = 0; i <		// { dg-error "not declared in this scope" }
+			 foo(); ++id); // { dg-bogus "call to non-.constexpr." }
+}