c++: Nondeterministic concepts diagnostics [PR94830]

This patch makes the order in which template parameters appear in the
TREE_LIST returned by find_template_parameters deterministic between
runs.

The current nondeterminism is semantically harmless, but it has the
undesirable effect of causing some concepts diagnostics which print a
constraint's parameter mapping via pp_cxx_parameter_mapping to also be
nondeterministic, as in the testcases below.

gcc/cp/ChangeLog:

	PR c++/94830
	* pt.c (find_template_parameter_info::parm_list): New field.
	(keep_template_parm): Use the new field to build up the
	parameter list here instead of ...
	(find_template_parameters): ... here.  Return ftpi.parm_list.

gcc/testsuite/ChangeLog:

	PR c++/94830
	* g++.dg/concepts/diagnostics12.C: Clarify the dg-message now
	that the corresponding diagnostic is deterministic.
	* g++.dg/concepts/diagnostics13.C: New test.

2 files changed, 15 insertions, 1 deletions

diff --git a/gcc/testsuite/g++.dg/concepts/diagnostic12.C b/gcc/testsuite/g++.dg/concepts/diagnostic12.C
index a757342..548ba9c 100644
--- a/gcc/testsuite/g++.dg/concepts/diagnostic12.C
+++ b/gcc/testsuite/g++.dg/concepts/diagnostic12.C
@@ -3,7 +3,7 @@
 
 template<typename T, typename... Args>
   concept c1 = requires (T t, Args... args) { *t; };
-// { dg-message "in requirements with .T t., .Args ... args. .with.* Args = \{\}" "" { target *-*-* } .-1 }
+// { dg-message "in requirements with .T t., .Args ... args. .with Args = \{\}; T = int" "" { target *-*-* } .-1 }
 
 static_assert(c1<int>); // { dg-error "failed" }
 
diff --git a/gcc/testsuite/g++.dg/concepts/diagnostic13.C b/gcc/testsuite/g++.dg/concepts/diagnostic13.C
new file mode 100644
index 0000000..accd8a6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/concepts/diagnostic13.C
@@ -0,0 +1,14 @@
+// PR c++/94830
+// { dg-do compile { target concepts } }
+
+template<typename T, typename R>
+  concept c = __is_same(T, R); // { dg-message "with T = int; R = char" }
+
+template<typename T, typename R>
+  requires c<T,R>
+void foo() { }
+
+void bar()
+{
+  foo<int, char>(); // { dg-error "unsatisfied constraints" }
+}