[Type checker] Don't re-insert node types when merging solutions.

Merging partial solutions can end up assigning the same type to a
particular typed node (expression, parameter, etc.), which can lead to
unbalanced set/clear when exploring the solution space (and later on,
crashes). Don't re-insert such information.

This is the same approach taken for type variable bindings, but it's
all pretty unfortunate: partial solutions should only record
information relative to their part of the constraint system, which
would save time and memory during solving. Howver, that's too big a
change for right now.

Fixes rdar://problem/50853028.

lib/Sema/CSSolver.cpp

@@ -251,7 +251,8 @@ void ConstraintSystem::applySolution(const Solution &solution) {
 
   // Add the node types back.
   for (auto &nodeType : solution.addedNodeTypes) {
-    setType(nodeType.first, nodeType.second);
+    if (!hasType(nodeType.first))
+      setType(nodeType.first, nodeType.second);
   }
 
   // Register the conformances checked along the way to arrive to solution.

lib/Sema/ConstraintSystem.h

@@ -571,7 +571,9 @@ struct Score {
 };
 
 /// An AST node that can gain type information while solving.
-using TypedNode = llvm::PointerUnion3<Expr *, TypeLoc *, ParamDecl *>;
+using TypedNode =
+    llvm::PointerUnion3<const Expr *, const TypeLoc *,
+                        const ParamDecl *>;
 
 /// Display a score.
 llvm::raw_ostream &operator<<(llvm::raw_ostream &out, const Score &score);
@@ -1756,12 +1758,12 @@ public:
     assert(type && "Expected non-null type");
 
     // Record the type.
-    if (auto expr = node.dyn_cast<Expr *>()) {
+    if (auto expr = node.dyn_cast<const Expr *>()) {
       ExprTypes[expr] = type.getPointer();
-    } else if (auto typeLoc = node.dyn_cast<TypeLoc *>()) {
+    } else if (auto typeLoc = node.dyn_cast<const TypeLoc *>()) {
       TypeLocTypes[typeLoc] = type.getPointer();
     } else {
-      auto param = node.get<ParamDecl *>();
+      auto param = node.get<const ParamDecl *>();
       ParamTypes[param] = type.getPointer();
     }
 
@@ -1775,26 +1777,18 @@ public:
   /// map is used throughout the expression type checker in order to
   /// avoid mutating expressions until we know we have successfully
   /// type-checked them.
-  void setType(Expr *E, Type T) {
-    setType(TypedNode(E), T);
-  }
-
   void setType(TypeLoc &L, Type T) {
     setType(TypedNode(&L), T);
   }
 
-  void setType(ParamDecl *P, Type T) {
-    setType(TypedNode(P), T);
-  }
-
   /// Erase the type for the given node.
   void eraseType(TypedNode node) {
-    if (auto expr = node.dyn_cast<Expr *>()) {
+    if (auto expr = node.dyn_cast<const Expr *>()) {
       ExprTypes.erase(expr);
-    } else if (auto typeLoc = node.dyn_cast<TypeLoc *>()) {
+    } else if (auto typeLoc = node.dyn_cast<const TypeLoc *>()) {
       TypeLocTypes.erase(typeLoc);
     } else {
-      auto param = node.get<ParamDecl *>();
+      auto param = node.get<const ParamDecl *>();
       ParamTypes.erase(param);
     }
   }
@@ -1812,12 +1806,20 @@ public:
   }
 
   bool hasType(const TypeLoc &L) const {
-    return TypeLocTypes.find(&L) != TypeLocTypes.end();
+    return hasType(TypedNode(&L));
   }
 
-  bool hasType(const ParamDecl *P) const {
-    assert(P != nullptr && "Expected non-null parameter!");
-    return ParamTypes.find(P) != ParamTypes.end();
+  /// Check to see if we have a type for a node.
+  bool hasType(TypedNode node) const {
+    assert(!node.isNull() && "Expected non-null node");
+    if (auto expr = node.dyn_cast<const Expr *>()) {
+      return ExprTypes.find(expr) != ExprTypes.end();
+    } else if (auto typeLoc = node.dyn_cast<const TypeLoc *>()) {
+      return TypeLocTypes.find(typeLoc) != TypeLocTypes.end();
+    } else {
+      auto param = node.get<const ParamDecl *>();
+      return ParamTypes.find(param) != ParamTypes.end();
+    }
   }
 
   bool hasType(const KeyPathExpr *KP, unsigned I) const {

test/Constraints/function_builder_diags.swift

@@ -117,5 +117,12 @@ func testOverloading(name: String) {
   _ = overloadedTuplify(true) { b in
     b ? "Hello, \(name)" : "Goodbye"
     42
+    overloadedTuplify(false) {
+      $0 ? "Hello, \(name)" : "Goodbye"
+      42
+      if b {
+        "Hello, \(name)" 
+      }
+    }
   }
 }