[PATCH] c++: Backport PR90505 fix to 9

[Jason Merrill]

On 3/4/20 4:02 PM, Marek Polacek wrote:
> While backporting our 90505 fix to 9 I noticed a bunch of concepts regressions.
> Fortunately I think the following variant of the fix is safe and still fixes
> the deduction issue.  In 9, we want to reject 'auto' when tf_partial before
> returning cp_build_qualified_type_real for TEMPLATE_TYPE_PARM and similar.
> And then to fix 90505, return early before reducing the template level.
> 
> The difference is caused by the huge concepts merge, so it's tough to figure
> out what specific change caused it.
> 
> Bootstrapped/regtested on x86_64-linux, ok for 9?

OK.

> 2020-03-04  Jason Merrill  <jason@redhat.com>
> 	    Marek Polacek  <polacek@redhat.com>
> 
> 	PR c++/90505 - mismatch in template argument deduction.
> 	* pt.c (tsubst): Don't reduce the template level of template
> 	parameters when tf_partial.
> 
> 	* g++.dg/template/deduce4.C: New test.
> 	* g++.dg/template/deduce5.C: New test.
> 	* g++.dg/template/deduce6.C: New test.
> 	* g++.dg/template/deduce7.C: New test.
> 
> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> index f233e78cc45..2de9036b647 100644
> --- a/gcc/cp/pt.c
> +++ b/gcc/cp/pt.c
> @@ -14630,6 +14630,11 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
>   	     about the template parameter in question.  */
>   	  return t;
>   
> +	/* Like with 'auto', don't reduce the level of template parameters
> +	   to avoid mismatches when deducing their types.  */
> +	if (complain & tf_partial)
> +	  return t;
> +
>   	/* If we get here, we must have been looking at a parm for a
>   	   more deeply nested template.  Make a new version of this
>   	   template parameter, but with a lower level.  */
> diff --git a/gcc/testsuite/g++.dg/template/deduce4.C b/gcc/testsuite/g++.dg/template/deduce4.C
> new file mode 100644
> index 00000000000..e2c165dc788
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/deduce4.C
> @@ -0,0 +1,17 @@
> +// PR c++/90505 - mismatch in template argument deduction.
> +// { dg-do compile }
> +
> +template <typename T>
> +struct S {
> +  template <typename U, typename V>
> +  static void foo(V) { }
> +
> +  void bar () { foo<int>(10); }
> +};
> +
> +void
> +test ()
> +{
> +  S<int> s;
> +  s.bar ();
> +}
> diff --git a/gcc/testsuite/g++.dg/template/deduce5.C b/gcc/testsuite/g++.dg/template/deduce5.C
> new file mode 100644
> index 00000000000..9d382bfe03a
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/deduce5.C
> @@ -0,0 +1,17 @@
> +// PR c++/90505 - mismatch in template argument deduction.
> +// { dg-do compile { target c++11 } }
> +
> +template <typename T>
> +struct S {
> +  template <typename U, typename V = void>
> +  static void foo(U) { }
> +
> +  void bar () { foo<int>(10); }
> +};
> +
> +void
> +test ()
> +{
> +  S<int> s;
> +  s.bar ();
> +}
> diff --git a/gcc/testsuite/g++.dg/template/deduce6.C b/gcc/testsuite/g++.dg/template/deduce6.C
> new file mode 100644
> index 00000000000..8fee6124f5a
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/deduce6.C
> @@ -0,0 +1,17 @@
> +// PR c++/90505 - mismatch in template argument deduction.
> +// { dg-do compile { target c++11 } }
> +
> +template <typename T>
> +struct S {
> +  template <typename U = int, typename V>
> +  static void foo(V) { }
> +
> +  void bar () { foo<>(10); }
> +};
> +
> +void
> +test ()
> +{
> +  S<int> s;
> +  s.bar ();
> +}
> diff --git a/gcc/testsuite/g++.dg/template/deduce7.C b/gcc/testsuite/g++.dg/template/deduce7.C
> new file mode 100644
> index 00000000000..fbc28e5150d
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/deduce7.C
> @@ -0,0 +1,10 @@
> +// PR c++/90505 - mismatch in template argument deduction.
> +// { dg-do compile { target c++11 } }
> +
> +template <typename> class a {
> +  using b = int;
> +  using c = int;
> +  b d;
> +  void e() { g<c>(d); }
> +  template <typename... f> static void g(f...);
> +};
>