[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)

4 files changed, 25 insertions, 9 deletions

diff --git a/clang/lib/Parse/ParseExpr.cpp b/clang/lib/Parse/ParseExpr.cpp
index df8388926db..1eb3ad6afd1 100644
--- a/clang/lib/Parse/ParseExpr.cpp
+++ b/clang/lib/Parse/ParseExpr.cpp
@@ -234,7 +234,7 @@ ExprResult Parser::ParseCaseExpression(SourceLocation CaseLoc) {
 /// \endverbatim
 ExprResult Parser::ParseConstraintExpression() {
   EnterExpressionEvaluationContext ConstantEvaluated(
-      Actions, Sema::ExpressionEvaluationContext::ConstantEvaluated);
+      Actions, Sema::ExpressionEvaluationContext::Unevaluated);
   ExprResult LHS(ParseCastExpression(AnyCastExpr));
   ExprResult Res(ParseRHSOfBinaryExpression(LHS, prec::LogicalOr));
   if (Res.isUsable() && !Actions.CheckConstraintExpression(Res.get())) {
@@ -256,7 +256,7 @@ ExprResult Parser::ParseConstraintExpression() {
 ExprResult
 Parser::ParseConstraintLogicalAndExpression(bool IsTrailingRequiresClause) {
   EnterExpressionEvaluationContext ConstantEvaluated(
-      Actions, Sema::ExpressionEvaluationContext::ConstantEvaluated);
+      Actions, Sema::ExpressionEvaluationContext::Unevaluated);
   bool NotPrimaryExpression = false;
   auto ParsePrimary = [&] () {
     ExprResult E = ParseCastExpression(PrimaryExprOnly,
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 92f6e0dc1c9..fbbab8f0070 100755
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -1849,7 +1849,7 @@ Decl *TemplateDeclInstantiator::VisitFunctionDecl(
   Expr *TrailingRequiresClause = D->getTrailingRequiresClause();
   if (TrailingRequiresClause) {
     EnterExpressionEvaluationContext ConstantEvaluated(
-        SemaRef, Sema::ExpressionEvaluationContext::ConstantEvaluated);
+        SemaRef, Sema::ExpressionEvaluationContext::Unevaluated);
     ExprResult SubstRC = SemaRef.SubstExpr(TrailingRequiresClause,
                                            TemplateArgs);
     if (SubstRC.isInvalid())
@@ -2189,7 +2189,7 @@ Decl *TemplateDeclInstantiator::VisitCXXMethodDecl(
   Expr *TrailingRequiresClause = D->getTrailingRequiresClause();
   if (TrailingRequiresClause) {
     EnterExpressionEvaluationContext ConstantEvaluated(
-        SemaRef, Sema::ExpressionEvaluationContext::ConstantEvaluated);
+        SemaRef, Sema::ExpressionEvaluationContext::Unevaluated);
     ExprResult SubstRC = SemaRef.SubstExpr(TrailingRequiresClause,
                                            TemplateArgs);
     if (SubstRC.isInvalid())
@@ -2529,8 +2529,6 @@ Decl *TemplateDeclInstantiator::VisitTemplateTypeParmDecl(
     TemplateArgumentListInfo InstArgs;
 
     if (TemplArgInfo) {
-      EnterExpressionEvaluationContext ConstantEvaluated(
-        SemaRef, Sema::ExpressionEvaluationContext::ConstantEvaluated);
       InstArgs.setLAngleLoc(TemplArgInfo->LAngleLoc);
       InstArgs.setRAngleLoc(TemplArgInfo->RAngleLoc);
       if (SemaRef.Subst(TemplArgInfo->getTemplateArgs(),
@@ -3736,7 +3734,7 @@ TemplateDeclInstantiator::SubstTemplateParams(TemplateParameterList *L) {
   Expr *InstRequiresClause = nullptr;
   if (Expr *E = L->getRequiresClause()) {
     EnterExpressionEvaluationContext ConstantEvaluated(
-        SemaRef, Sema::ExpressionEvaluationContext::ConstantEvaluated);
+        SemaRef, Sema::ExpressionEvaluationContext::Unevaluated);
     ExprResult Res = SemaRef.SubstExpr(E, TemplateArgs);
     if (Res.isInvalid() || !Res.isUsable()) {
       return nullptr;
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}}
 }
\ No newline at end of file
diff --git a/clang/test/SemaTemplate/cxx2a-constraint-exprs.cpp b/clang/test/SemaTemplate/cxx2a-constraint-exprs.cpp
new file mode 100644
index 00000000000..a2a7232b4b8
--- /dev/null
+++ b/clang/test/SemaTemplate/cxx2a-constraint-exprs.cpp
@@ -0,0 +1,17 @@
+// RUN:  %clang_cc1 -std=c++2a -verify %s
+
+// Make sure constraint expressions are unevaluated before being substituted
+// into during satisfaction checking.
+
+template<typename T> constexpr int f() { return T::value; }
+template<typename T> concept Foo = false && (f<int>(), true);
+bool k = Foo<int>;
+template<typename T> requires false && (f<int>(), true) struct S {};
+// expected-note@-1{{because}}
+using s = S<int>; // expected-error {{constraints not satisfied}}
+template<typename T> void foo() requires false && (f<int>(), true) { };
+// expected-note@-1{{because}} expected-note@-1{{candidate template ignored}}
+int a = (foo<int>(), 0); // expected-error{{no matching function}}
+template<typename T> void bar() requires requires { requires false && (f<int>(), true); } { };
+// expected-note@-1{{because}} expected-note@-1{{candidate template ignored}}
+int b = (bar<int>(), 0); // expected-error{{no matching function}}
\ No newline at end of file