IDE: Don't store TypeVariableTypes into CodeCompletionExprs

Instead, generate the type variable in ConstraintGenerator.
However, we only want to generate it if we're type checking
from inside TypeChecker::typeCheckCompletionSequence(), so
add an isActivated() flag to CodeCompletionExpr. If it is
not set, constraint generation will simply fail on an
expression containing a CodeCompletionExpr.

Author: slavapestov

include/swift/AST/Expr.h

@@ -717,13 +717,20 @@ class ErrorExpr : public Expr {
 /// can help us preserve the context of the code completion position.
 class CodeCompletionExpr : public Expr {
   SourceRange Range;
+  bool Activated;
+
 public:
   CodeCompletionExpr(SourceRange Range, Type Ty = Type()) :
       Expr(ExprKind::CodeCompletion, /*Implicit=*/true, Ty),
-      Range(Range) {}
+      Range(Range) {
+    Activated = false;
+  }
 
   SourceRange getSourceRange() const { return Range; }
 
+  bool isActivated() const { return Activated; }
+  void setActivated() { Activated = true; }
+
   static bool classof(const Expr *E) {
     return E->getKind() == ExprKind::CodeCompletion;
   }

lib/Sema/CSApply.cpp

@@ -1912,6 +1912,8 @@ namespace {
 
     Expr *visitCodeCompletionExpr(CodeCompletionExpr *expr) {
       // Do nothing with code completion expressions.
+      auto toType = simplifyType(cs.getType(expr));
+      cs.setType(expr, toType);
       return expr;
     }
 

lib/Sema/CSGen.cpp

@@ -1164,8 +1164,11 @@ namespace {
     }
 
     virtual Type visitCodeCompletionExpr(CodeCompletionExpr *E) {
-      // If the expression has already been assigned a type; just use that type.
-      return E->getType();
+      if (!E->isActivated())
+        return Type();
+
+      return CS.createTypeVariable(CS.getConstraintLocator(E),
+                                   TVO_CanBindToLValue);
     }
 
     Type visitLiteralExpr(LiteralExpr *expr) {

lib/Sema/TypeCheckConstraints.cpp

@@ -2089,9 +2089,7 @@ bool TypeChecker::typeCheckCompletionSequence(Expr *&expr, DeclContext *DC) {
   assert(exprAsBinOp == expr && "found wrong expr?");
 
   // Add type variable for the code-completion expression.
-  auto tvRHS =
-      CS.createTypeVariable(CS.getConstraintLocator(CCE), TVO_CanBindToLValue);
-  CCE->setType(tvRHS);
+  CCE->setActivated();
 
   if (auto generated = CS.generateConstraints(expr)) {
     expr = generated;