add tests and remove bogus assert

Author: cor3ntin

clang/lib/Sema/SemaExpr.cpp

@@ -20628,10 +20628,8 @@ Sema::ConditionResult Sema::ActOnCondition(Scope *S, SourceLocation Loc,
     break;
 
   case ConditionKind::ConstexprIf:
+    // Note: this might produce a FullExpr
     Cond = CheckBooleanCondition(Loc, SubExpr, true);
-    assert(isa<FullExpr>(Cond.get()) &&
-           "we should have converted this expression to a FullExpr before "
-           "evaluating it");
     break;
 
   case ConditionKind::Switch:
@@ -20643,10 +20641,12 @@ Sema::ConditionResult Sema::ActOnCondition(Scope *S, SourceLocation Loc,
                               {SubExpr}, PreferredConditionType(CK));
     if (!Cond.get())
       return ConditionError();
-  }
-  if (!isa<FullExpr>(Cond.get()))
+  } else if (Cond.isUsable() && !isa<FullExpr>(Cond.get()))
     Cond = ActOnFinishFullExpr(Cond.get(), Loc, /*DiscardedValue*/ false);
 
+  if (Cond.isInvalid())
+    return ConditionError();
+
   return ConditionResult(*this, nullptr, Cond,
                          CK == ConditionKind::ConstexprIf);
 }

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

@@ -252,7 +252,13 @@ struct S {
 };
 
 int f() {
-    if constexpr((S{}, true)) {
+    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}}
+        return 1;
+    }
+    if constexpr(S s; (s, true)) { // expected-warning{{left operand of comma operator has no effect}}
         return 1;
     }
     return 0;