[AST] NFC: add an assertion for invariant of CXXFoldExpr

CXXFoldExpr relies on exactly one of the two operands to have unexpanded
parameter packs. If this invariant does not holds, results of
`getPattern()`, `isLeftFold()` and other related members are incorrect.

Asserting this on construction makes debugging the problems easier as
the failure is happening closer to the code that contains the error.

Also move the constructor to the `.cpp` file to avoid potential ODR
violations from having an `assert` in the header in combination with
precompiled libraries.

Author: ilya-biryukov

clang/include/clang/AST/ExprCXX.h

@@ -4854,15 +4854,7 @@ class CXXFoldExpr : public Expr {
   CXXFoldExpr(QualType T, UnresolvedLookupExpr *Callee,
               SourceLocation LParenLoc, Expr *LHS, BinaryOperatorKind Opcode,
               SourceLocation EllipsisLoc, Expr *RHS, SourceLocation RParenLoc,
-              std::optional<unsigned> NumExpansions)
-      : Expr(CXXFoldExprClass, T, VK_PRValue, OK_Ordinary),
-        LParenLoc(LParenLoc), EllipsisLoc(EllipsisLoc), RParenLoc(RParenLoc),
-        NumExpansions(NumExpansions ? *NumExpansions + 1 : 0), Opcode(Opcode) {
-    SubExprs[SubExpr::Callee] = Callee;
-    SubExprs[SubExpr::LHS] = LHS;
-    SubExprs[SubExpr::RHS] = RHS;
-    setDependence(computeDependence(this));
-  }
+              std::optional<unsigned> NumExpansions);
 
   CXXFoldExpr(EmptyShell Empty) : Expr(CXXFoldExprClass, Empty) {}
 

clang/lib/AST/ExprCXX.cpp

@@ -1944,3 +1944,22 @@ CXXParenListInitExpr *CXXParenListInitExpr::CreateEmpty(ASTContext &C,
                          alignof(CXXParenListInitExpr));
   return new (Mem) CXXParenListInitExpr(Empty, NumExprs);
 }
+
+CXXFoldExpr::CXXFoldExpr(QualType T, UnresolvedLookupExpr *Callee,
+                                SourceLocation LParenLoc, Expr *LHS,
+                                BinaryOperatorKind Opcode,
+                                SourceLocation EllipsisLoc, Expr *RHS,
+                                SourceLocation RParenLoc,
+                                std::optional<unsigned> NumExpansions)
+    : Expr(CXXFoldExprClass, T, VK_PRValue, OK_Ordinary), LParenLoc(LParenLoc),
+      EllipsisLoc(EllipsisLoc), RParenLoc(RParenLoc),
+      NumExpansions(NumExpansions ? *NumExpansions + 1 : 0), Opcode(Opcode) {
+  // We rely on asserted invariant to distinguish left and right folds.
+  assert(((LHS && LHS->containsUnexpandedParameterPack()) !=
+          (RHS && RHS->containsUnexpandedParameterPack())) &&
+         "Exactly one of LHS or RHS should contain an unexpanded pack");
+  SubExprs[SubExpr::Callee] = Callee;
+  SubExprs[SubExpr::LHS] = LHS;
+  SubExprs[SubExpr::RHS] = RHS;
+  setDependence(computeDependence(this));
+}