c++: Explain fn template argument type/value mismatches [PR66439]

In fn_type_unifcation, we are passing NULL_TREE as the 'in_decl'
parameter to coerce_template_parms, and this is causing template
type/value mismatch error messages to get suppressed regardless of the
value of 'complain'.

This means that when substitution into a function template fails due to
a type/value mismatch between a template parameter and the provided
template argument, we just say "template argument deduction/substitution
failed:" without a followup explanation of the failure.

Fix this by passing 'fn' instead of NULL_TREE to coerce_template_parms.

gcc/cp/ChangeLog:

	PR c++/66439
	* pt.c (fn_type_unification): Pass 'fn' instead of NULL_TREE as
	the 'in_decl' parameter to coerce_template_parms.

gcc/testsuite/ChangeLog:

	PR c++/66439
	* g++.dg/cpp2a/concepts-ts4.C: Expect a "type/value mismatch"
	diagnostic.
	* g++.dg/cpp2a/concepts-ts6.C: Likewise.
	* g++.dg/template/error56.C: Likewise.
	* g++.dg/template/error59.C: New test.

libstdc++-v3/ChangeLog:

	PR c++/66439
	* testsuite/20_util/pair/astuple/get_neg.cc: Prune "type/value
	mismatch" messages.
	* testsuite/20_util/tuple/element_access/get_neg.cc: Likewise.

gcc/cp/ChangeLog

@@ -1,3 +1,9 @@
+2020-05-19  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/66439
+	* pt.c (fn_type_unification): Pass 'fn' instead of NULL_TREE as
+	the 'in_decl' parameter to coerce_template_parms.
+
 2020-05-18  Marek Polacek  <polacek@redhat.com>
 
 	PR c++/94955

gcc/cp/pt.c

@@ -20989,7 +20989,7 @@ fn_type_unification (tree fn,
       /* Adjust any explicit template arguments before entering the
 	 substitution context.  */
       explicit_targs
-	= (coerce_template_parms (tparms, explicit_targs, NULL_TREE,
+	= (coerce_template_parms (tparms, explicit_targs, fn,
 				  complain|tf_partial,
 				  /*require_all_args=*/false,
 				  /*use_default_args=*/false));

gcc/testsuite/ChangeLog

@@ -1,3 +1,12 @@
+2020-05-19  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/66439
+	* g++.dg/cpp2a/concepts-ts4.C: Expect a "type/value mismatch"
+	diagnostic.
+	* g++.dg/cpp2a/concepts-ts6.C: Likewise.
+	* g++.dg/template/error56.C: Likewise.
+	* g++.dg/template/error59.C: New test.
+
 2020-05-18  Marek Polacek  <polacek@redhat.com>
 
 	PR c++/94955

gcc/testsuite/g++.dg/cpp2a/concepts-ts4.C

@@ -31,4 +31,6 @@ void driver()
   fn<0>(); // OK
   fn<-1>(); // { dg-error "" }
   fn<int>(); // { dg-error "no matching function" }
+  // { dg-error "type/value mismatch at argument 1" "" { target *-*-* } .-1 }
+  // { dg-message "expected a constant of type .int., got .int." "" { target *-*-* } .-2 }
 }

gcc/testsuite/g++.dg/cpp2a/concepts-ts6.C

@@ -25,6 +25,8 @@ void driver1() {
 
   f<X>();
   f<int>(); // { dg-error "no matching function for call" }
+  // { dg-error "type/value mismatch at argument 1" "" { target *-*-* } .-1 }
+  // { dg-message "expected a class template, got .int." "" { target *-*-* } .-2 }
 
   S2<int> s2a;
   S2<char, signed char, unsigned char> s2b;
@@ -69,4 +71,4 @@ void driver2()
   S6<void, void> s6a;
   S6<void, int> s6c; // { dg-error "template constraint failure" }
   S6<void, void, void> s6b; // { dg-error "wrong number of template arguments" }
-}
+}

gcc/testsuite/g++.dg/template/error56.C

@@ -9,4 +9,6 @@ struct A
 int main()
 {
   A().f<1>();			// { dg-error "f<1>" }
+  // { dg-error "type/value mismatch at argument 1" "" { target *-*-* } .-1 }
+  // { dg-message "expected a type, got .1." "" { target *-*-* } .-2 }
 }

gcc/testsuite/g++.dg/template/error59.C

@@ -0,0 +1,11 @@
+template<int N> struct S { };
+
+template<template<typename> class TT>
+void foo();
+
+void bar()
+{
+  foo<S>(); // { dg-error "no matching function" }
+  // { dg-error "type/value mismatch at argument 1" "" { target *-*-* } .-1 }
+  // { dg-message "expected a template of type .template<class> class TT., got .template<int N> struct S." "" { target *-*-* } .-2 }
+}

libstdc++-v3/ChangeLog

@@ -1,3 +1,10 @@
+2020-05-19  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/66439
+	* testsuite/20_util/pair/astuple/get_neg.cc: Prune "type/value
+	mismatch" messages.
+	* testsuite/20_util/tuple/element_access/get_neg.cc: Likewise.
+
 2020-05-15  H.J. Lu  <hongjiu.lu@intel.com>
 
 	PR bootstrap/95147

libstdc++-v3/testsuite/20_util/pair/astuple/get_neg.cc

@@ -27,3 +27,4 @@ void test01()
 }
 
 // { dg-prune-output "tuple_element<2" }
+// { dg-prune-output "type/value mismatch" }

libstdc++-v3/testsuite/20_util/tuple/element_access/get_neg.cc

@@ -61,3 +61,4 @@ test03()
 }
 
 // { dg-prune-output "no type named .type" }
+// { dg-prune-output "type/value mismatch" }