correctly handle dependent conditions

Author: cor3ntin

clang/lib/Sema/SemaExpr.cpp

@@ -20644,7 +20644,7 @@ Sema::ConditionResult Sema::ActOnCondition(Scope *S, SourceLocation Loc,
   } else if (Cond.isUsable() && !isa<FullExpr>(Cond.get()))
     Cond = ActOnFinishFullExpr(Cond.get(), Loc, /*DiscardedValue*/ false);
 
-  if (Cond.isInvalid())
+  if (!Cond.isUsable())
     return ConditionError();
 
   return ConditionResult(*this, nullptr, Cond,

clang/lib/Sema/TreeTransform.h

@@ -4561,6 +4561,13 @@ bool TreeTransform<Derived>::TransformExprs(Expr *const *Inputs,
 template <typename Derived>
 Sema::ConditionResult TreeTransform<Derived>::TransformCondition(
     SourceLocation Loc, VarDecl *Var, Expr *Expr, Sema::ConditionKind Kind) {
+
+  EnterExpressionEvaluationContext Eval(
+      SemaRef, Sema::ExpressionEvaluationContext::ConstantEvaluated,
+      /*LambdaContextDecl=*/nullptr,
+      /*ExprContext=*/Sema::ExpressionEvaluationContextRecord::EK_Other,
+      /*ShouldEnter=*/Kind == Sema::ConditionKind::ConstexprIf);
+
   if (Var) {
     VarDecl *ConditionVar = cast_or_null<VarDecl>(
         getDerived().TransformDefinition(Var->getLocation(), Var));

clang/test/SemaCXX/cxx2a-constexpr-dynalloc.cpp

@@ -256,7 +256,7 @@ int f() {
     if constexpr((S{}, true)) { // expected-warning{{left operand of comma operator has no effect}}
         return 1;
     }
-    if constexpr(S s; (s, true)) { // expected-warning{{left operand of comma operator has no effect}}
+    if constexpr(S s; (S{}, true)) { // expected-warning{{left operand of comma operator has no effect}}
         return 1;
     }
     if constexpr(S s; (s, true)) { // expected-warning{{left operand of comma operator has no effect}}
@@ -267,8 +267,29 @@ int f() {
     }
     return 0;
 }
+
+template <typename T>
+int f2() {
+    if constexpr((T{}, true)) { // expected-warning{{left operand of comma operator has no effect}}
+        return 1;
+    }
+    if constexpr(T s; (T{}, true)) { // expected-warning{{left operand of comma operator has no effect}}
+        return 1;
+    }
+    if constexpr(T s; (s, true)) { // expected-warning{{left operand of comma operator has no effect}}
+        return 1;
+    }
+    if constexpr(constexpr int _ = T{}.i; true) {
+        return 1;
+    }
+    return 0;
+}
+
+void test() {
+  f2<S>(); // expected-note {{in instantiation}}
 }
 
+}
 namespace GH120197{
 struct NonTrivialDtor {
   NonTrivialDtor() = default;