[Concepts] Make constraint expressions unevaluated until satisfaction checking

As per P1980R0, constraint expressions are unevaluated operands, and their constituent atomic
constraints only become constant evaluated during satisfaction checking.

Change the evaluation context during parsing and instantiation of constraints to unevaluated.

(cherry picked from commit 73eaf62463b4a29adf4194685af12d1a5d172987)

1 files changed, 3 insertions, 2 deletions

diff --git a/clang/test/CXX/expr/expr.prim/expr.prim.req/nested-requirement.cpp b/clang/test/CXX/expr/expr.prim/expr.prim.req/nested-requirement.cpp
index bc093a0fc50..b45b57f6b92 100644
--- a/clang/test/CXX/expr/expr.prim/expr.prim.req/nested-requirement.cpp
+++ b/clang/test/CXX/expr/expr.prim/expr.prim.req/nested-requirement.cpp
@@ -39,8 +39,9 @@ namespace std_example {
   using dc1 = D_check<short>; // expected-error{{constraints not satisfied for class template 'D_check' [with T = short]}}
 
   template<typename T>
-  concept C2 = requires (T a) { // expected-note{{'a' declared here}}
+  concept C2 = requires (T a) {
       requires sizeof(a) == 4; // OK
-      requires a == 0; // expected-error{{constraint variable 'a' cannot be used in an evaluated context}}
+      requires a == 0; // expected-note{{because 'a == 0' would be invalid: constraint variable 'a' cannot be used in an evaluated context}}
     };
+  static_assert(C2<int>); // expected-note{{because 'int' does not satisfy 'C2'}} expected-error{{static_assert failed}}
 }
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