[ConstraintSystem] Fix ConstraintGenerator to change phase to "constraint generation" and back

Author: xedin

lib/Sema/CSGen.cpp

@@ -917,6 +917,7 @@ namespace {
   class ConstraintGenerator : public ExprVisitor<ConstraintGenerator, Type> {
     ConstraintSystem &CS;
     DeclContext *CurDC;
+    ConstraintSystemPhase CurrPhase;
     SmallVector<DeclContext*, 4> DCStack;
 
     static const unsigned numEditorPlaceholderVariables = 2;
@@ -1108,9 +1109,16 @@ namespace {
 
   public:
     ConstraintGenerator(ConstraintSystem &CS, DeclContext *DC)
-      : CS(CS), CurDC(DC ? DC : CS.DC) { }
+        : CS(CS), CurDC(DC ? DC : CS.DC), CurrPhase(CS.getPhase()) {
+      // Although constraint system is initialized in `constraint
+      // generation` phase, we have to set it here manually because e.g.
+      // function builders could generate constraints for its body
+      // in the middle of the solving.
+      CS.setPhase(ConstraintSystemPhase::ConstraintGeneration);
+    }
 
     virtual ~ConstraintGenerator() {
+      CS.setPhase(CurrPhase);
       // We really ought to have this assertion:
       //   assert(DCStack.empty() && CurDC == CS.DC);
       // Unfortunately, ASTWalker is really bad at letting us establish

lib/Sema/ConstraintSystem.h

@@ -1536,16 +1536,24 @@ class ConstraintSystem {
   };
 
 public:
+  ConstraintSystemPhase getPhase() const { return Phase; }
+
   /// Move constraint system to a new phase of its lifetime.
   void setPhase(ConstraintSystemPhase newPhase) {
+    if (Phase == newPhase)
+      return;
+
 #ifndef NDEBUG
     switch (Phase) {
     case ConstraintSystemPhase::ConstraintGeneration:
       assert(newPhase == ConstraintSystemPhase::Solving);
       break;
 
     case ConstraintSystemPhase::Solving:
-      assert(newPhase == ConstraintSystemPhase::Diagnostics ||
+      // We can come back to constraint generation phase while
+      // processing function builder body.
+      assert(newPhase == ConstraintSystemPhase::ConstraintGeneration ||
+             newPhase == ConstraintSystemPhase::Diagnostics ||
              newPhase == ConstraintSystemPhase::Finalization);
       break;