Enable recursive differentiation of nested calls

Now, if a function `f` that is being differentiated contains a call to a function `g`, clad will proceed to try to differentiate `g` even if it has no
overload in `custom_derivatives`.

Example:
```
double f(double x) {
  return g(double x);
}
```
On `clad::differentiate(f, 0)`, clad will proceed to differentiate `g` and so on, even if it is not in `custom_derivatives` namespace.

include/clad/Differentiator/DerivativeBuilder.h

@@ -31,17 +31,21 @@ namespace clad {
   namespace utils {
     class StmtClone;
   }
+  class DiffRequest;
+  namespace plugin {
+    class CladPlugin;
+    clang::FunctionDecl* ProcessDiffRequest(CladPlugin& P, DiffRequest& request);
+  }
 }
 
 namespace clad {
   /// A pair of FunctionDecl and potential enclosing context, e.g. a function
   // in nested namespaces
   using DeclWithContext = std::pair<clang::FunctionDecl*, clang::Decl*>;
 
-  using DiffParams = llvm::SmallVector<clang::VarDecl*, 16>;
+  using DiffParams = llvm::SmallVector<const clang::VarDecl*, 16>;
 
   static clang::SourceLocation noLoc{};
-  class DiffPlan;
   /// The main builder class which then uses either ForwardModeVisitor or
   /// ReverseModeVisitor based on the required mode.
   class DerivativeBuilder {
@@ -51,6 +55,7 @@ namespace clad {
     friend class ReverseModeVisitor;
 
     clang::Sema& m_Sema;
+    plugin::CladPlugin& m_CladPlugin;
     clang::ASTContext& m_Context;
     std::unique_ptr<utils::StmtClone> m_NodeCloner;
     clang::NamespaceDecl* m_BuiltinDerivativesNSD;
@@ -71,7 +76,7 @@ namespace clad {
         stream << arg;
     }
   public:
-    DerivativeBuilder(clang::Sema& S);
+    DerivativeBuilder(clang::Sema& S, plugin::CladPlugin& P);
     ~DerivativeBuilder();
 
     ///\brief Produces the derivative of a given function
@@ -82,7 +87,8 @@ namespace clad {
     ///\returns The differentiated function and potentially created enclosing
     /// context.
     ///
-    DeclWithContext Derive(clang::FunctionDecl* FD, const DiffPlan & plan);
+    DeclWithContext Derive(const clang::FunctionDecl* FD,
+                           const DiffRequest & request);
   };
 
   /// A base class for all common functionality for visitors
@@ -91,6 +97,7 @@ namespace clad {
     VisitorBase(DerivativeBuilder& builder) :
       m_Builder(builder),
       m_Sema(builder.m_Sema),
+      m_CladPlugin(builder.m_CladPlugin),
       m_Context(builder.m_Context),
       m_CurScope(m_Sema.TUScope),
       m_DerivativeInFlight(false),
@@ -101,17 +108,18 @@ namespace clad {
 
     DerivativeBuilder& m_Builder;
     clang::Sema& m_Sema;
+    plugin::CladPlugin& m_CladPlugin;
     clang::ASTContext& m_Context;
     clang::Scope* m_CurScope;
     bool m_DerivativeInFlight;
     /// The Derivative function that is being generated.
     clang::FunctionDecl* m_Derivative;
     /// The function that is currently differentiated.
-    clang::FunctionDecl* m_Function;
+    const clang::FunctionDecl* m_Function;
 
     /// Map used to keep track of variable declarations and match them
     /// with their derivatives.
-    std::unordered_map<clang::VarDecl*, clang::Expr*> m_Variables;
+    std::unordered_map<const clang::VarDecl*, clang::Expr*> m_Variables;
     /// Map contains variable declarations replacements. If the original
     /// function contains a declaration which name collides with something
     /// already created inside derivative's body, the declaration is replaced
@@ -204,7 +212,7 @@ namespace clad {
     clang::DeclStmt* BuildDeclStmt(llvm::MutableArrayRef<clang::Decl*> DS);
 
     /// Builds a DeclRefExpr to a given Decl.
-    clang::DeclRefExpr* BuildDeclRef(clang::VarDecl* D);
+    clang::DeclRefExpr* BuildDeclRef(clang::DeclaratorDecl* D);
 
     /// Stores the result of an expression in a temporary variable (of the same
     /// type as is the result of the expression) and returns a reference to it.
@@ -223,14 +231,16 @@ namespace clad {
     clang::Expr* StoreAndRef(clang::Expr* E, clang::QualType Type, Stmts& block,
                              llvm::StringRef prefix = "_t",
                              bool forceDeclCreation = false);
-
+    /// A flag for silencing warnings/errors output by diag function.
+    bool silenceDiags = false;
     /// Shorthand to issues a warning or error.
     template <std::size_t N>
     void diag(clang::DiagnosticsEngine::Level level, // Warning or Error
               clang::SourceLocation loc,
               const char (&format)[N],
               llvm::ArrayRef<llvm::StringRef> args = {}) {
-      m_Builder.diag(level, loc, format, args);
+      if (!silenceDiags)
+        m_Builder.diag(level, loc, format, args);
     }
 
     /// Conuter used to create unique identifiers for temporaries
@@ -247,7 +257,6 @@ namespace clad {
     clang::Stmt* Clone(const clang::Stmt* S);
     /// A shorthand to simplify cloning of expressions.
     clang::Expr* Clone(const clang::Expr* E);
-
     /// Parses the argument expression for the clad::differentiate/clad::gradient
     /// call. The argument is used to specify independent parameter(s) for
     /// differentiation. There are three valid options for the argument expression:
@@ -258,7 +267,8 @@ namespace clad {
     ///      parameter corresponding to literal's value index.
     ///   3) If no argument is provided, a default argument is used. The function
     ///      will be differentiated w.r.t. to its every parameter.
-    DiffParams parseDiffArgs(const clang::Expr* diffArgs, clang::FunctionDecl* FD);
+    DiffParams parseDiffArgs(const clang::Expr* diffArgs,
+                             const clang::FunctionDecl* FD);
   };
 
   /// A class that represents the result of Visit of ForwardModeVisitor.
@@ -313,7 +323,7 @@ namespace clad {
     : public clang::ConstStmtVisitor<ForwardModeVisitor, StmtDiff>,
       public VisitorBase {
   private:
-    clang::VarDecl* m_IndependentVar = nullptr;
+    const clang::VarDecl* m_IndependentVar = nullptr;
     unsigned m_DerivativeOrder = ~0;
     unsigned m_ArgIndex = ~0;
 
@@ -328,7 +338,8 @@ namespace clad {
     ///\returns The differentiated and potentially created enclosing
     /// context.
     ///
-    DeclWithContext Derive(clang::FunctionDecl* FD, const DiffPlan& plan);
+    DeclWithContext Derive(const clang::FunctionDecl* FD,
+                           const DiffRequest& request);
     StmtDiff VisitBinaryOperator(const clang::BinaryOperator* BinOp);
     StmtDiff VisitCallExpr(const clang::CallExpr* CE);
     StmtDiff VisitCompoundStmt(const clang::CompoundStmt* CS);
@@ -468,7 +479,8 @@ namespace clad {
     /// Improved naming scheme is required. Hence, we append the indices to of the
     /// requested parameters to 'f_grad', i.e. in the previous example "x, y" will
     /// give 'f_grad_0_1' and "x, z" will give 'f_grad_0_2'.
-    DeclWithContext Derive(clang::FunctionDecl* FD, const DiffPlan& plan);
+    DeclWithContext Derive(const clang::FunctionDecl* FD,
+                           const DiffRequest& request);
     StmtDiff VisitArraySubscriptExpr(const clang::ArraySubscriptExpr* ASE);
     StmtDiff VisitBinaryOperator(const clang::BinaryOperator* BinOp);
     StmtDiff VisitCallExpr(const clang::CallExpr* CE);

include/clad/Differentiator/DiffPlanner.h

@@ -19,72 +19,49 @@ namespace clad {
     reverse
   };
 
-  ///\brief The list of the dependent functions which also need differentiation
-  /// because they are called by the function we are asked to differentitate.
-  ///
-  class DiffPlan {
-  private:
-    typedef llvm::SmallVector<clang::FunctionDecl*, 16> Functions;
-    Functions m_Functions;
-    clang::CallExpr* m_CallToUpdate = nullptr;
-    unsigned m_RequestedDerivativeOrder = 1;
-    unsigned m_CurrentDerivativeOrder = 1;
-    clang::Expr* m_DiffArgs = nullptr;
-    DiffMode m_Mode = DiffMode::unknown;
-  public:
-    typedef Functions::iterator iterator;
-    typedef Functions::const_iterator const_iterator;
+  /// A struct containing information about request to differentiate a function.
+  struct DiffRequest {
+    /// Function to be differentiated.
+    const clang::FunctionDecl* Function = nullptr;
+    /// Name of the base function to be differentiated. Can be different from
+    /// function->getNameAsString() when higher-order derivatives are computed.
+    std::string BaseFunctionName = {};
+    /// Current derivative order to be computed.
+    unsigned CurrentDerivativeOrder = 1;
+    /// Highest requested derivative order.
+    unsigned RequestedDerivativeOrder = 1;
+    /// Context in which the function is being called, or a call to
+    /// clad::gradient/differentiate, where function is the first arg.
+    clang::CallExpr* CallContext = nullptr;
+    /// Args provided to the call to clad::gradient/differentiate.
+    const clang::Expr* Args = nullptr;
+    /// Requested differentiation mode, forward or reverse.
+    DiffMode Mode = DiffMode::unknown;
+    /// If function appears in the call to clad::gradient/differentiate,
+    /// the call must be updated and the first arg replaced by the derivative.
+    bool CallUpdateRequired = false;
+    /// A flag to enable/disable diag warnings/errors during differentiation.
+    bool VerboseDiags = false;
 
-    DiffMode getMode() const {
-      assert(m_Mode != DiffMode::unknown && "Must be set!");
-      return m_Mode;
-    }
-    void setMode(DiffMode mode) {
-      m_Mode = mode;
-    }
-    unsigned getRequestedDerivativeOrder() const {
-      return m_RequestedDerivativeOrder;
-   }
-    void setCurrentDerivativeOrder(unsigned val) {
-      m_CurrentDerivativeOrder = val;
-    }
-    unsigned getCurrentDerivativeOrder() const {
-      return m_CurrentDerivativeOrder;
-    }
-    void push_back(clang::FunctionDecl* FD) { m_Functions.push_back(FD); }
-    iterator begin() { return m_Functions.begin(); }
-    iterator end() { return m_Functions.end(); }
-    const_iterator begin() const { return m_Functions.begin(); }
-    const_iterator end() const { return m_Functions.end(); }
-    size_t size() const { return m_Functions.size(); }
-    void setCallToUpdate(clang::CallExpr* CE) { m_CallToUpdate = CE; }
     void updateCall(clang::FunctionDecl* FD, clang::Sema& SemaRef);
-    clang::Expr* getArgs() const { return m_DiffArgs; }
-    LLVM_DUMP_METHOD void dump();
-
-    friend class DiffCollector;
   };
 
-  typedef llvm::SmallVector<DiffPlan, 16> DiffPlans;
+  using DiffSchedule = llvm::SmallVector<DiffRequest, 16>;
 
   class DiffCollector: public clang::RecursiveASTVisitor<DiffCollector> {
   private:
     ///\brief The diff step-by-step plan for differentiation.
     ///
-    DiffPlans& m_DiffPlans;
+    DiffSchedule& m_DiffPlans;
 
     ///\brief If set it means that we need to find the called functions and
     /// add them for implicit diff.
     ///
-    clang::FunctionDecl* m_TopMostFD;
-
+    const clang::FunctionDecl* m_TopMostFD = nullptr;
     clang::Sema& m_Sema;
 
-    DiffPlan& getCurrentPlan() { return m_DiffPlans.back(); }
-
   public:
-    DiffCollector(clang::DeclGroupRef DGR, DiffPlans& plans, clang::Sema& S);
-    void UpdatePlan(clang::FunctionDecl* FD, DiffPlan* plan);
+    DiffCollector(clang::DeclGroupRef DGR, DiffSchedule& plans, clang::Sema& S);
     bool VisitCallExpr(clang::CallExpr* E);
   };
 }