[Constraint solver] Remove expression from the constructor.

Rather than setting up the constraint solver with a single expression
(that gets recorded for parents/depths), record each expression that
goes through constraint generation.

Author: DougGregor

lib/Sema/CSGen.cpp

@@ -3754,6 +3754,8 @@ namespace {
 } // end anonymous namespace
 
 Expr *ConstraintSystem::generateConstraints(Expr *expr, DeclContext *dc) {
+  InputExprs.insert(expr);
+
   // Remove implicit conversions from the expression.
   expr = expr->walk(SanitizeExpr(*this));
 

lib/Sema/CSSolver.cpp

@@ -574,7 +574,7 @@ bool ConstraintSystem::Candidate::solve(
   };
 
   // Allocate new constraint system for sub-expression.
-  ConstraintSystem cs(DC, None, E);
+  ConstraintSystem cs(DC, None);
   cs.baseCS = &BaseCS;
 
   // Set up expression type checker timer for the candidate.

lib/Sema/ConstraintSystem.cpp

@@ -75,16 +75,11 @@ ExpressionTimer::~ExpressionTimer() {
 
 
 ConstraintSystem::ConstraintSystem(DeclContext *dc,
-                                   ConstraintSystemOptions options,
-                                   Expr *expr)
+                                   ConstraintSystemOptions options)
   : Context(dc->getASTContext()), DC(dc), Options(options),
     Arena(dc->getASTContext(), Allocator),
     CG(*new ConstraintGraph(*this))
 {
-  if (expr) {
-    InputExprs.insert(expr);
-  }
-
   assert(DC && "context required");
 }
 

lib/Sema/ConstraintSystem.h

@@ -1664,8 +1664,7 @@ class ConstraintSystem {
   };
 
   ConstraintSystem(DeclContext *dc,
-                   ConstraintSystemOptions options,
-                   Expr *expr = nullptr);
+                   ConstraintSystemOptions options);
   ~ConstraintSystem();
 
   /// Retrieve the type checker associated with this constraint system.

lib/Sema/TypeCheckConstraints.cpp

@@ -2198,7 +2198,7 @@ Type TypeChecker::typeCheckExpressionImpl(Expr *&expr, DeclContext *dc,
   if (options.contains(TypeCheckExprFlags::SubExpressionDiagnostics))
     csOptions |= ConstraintSystemFlags::SubExpressionDiagnostics;
 
-  ConstraintSystem cs(dc, csOptions, expr);
+  ConstraintSystem cs(dc, csOptions);
   cs.baseCS = baseCS;
 
   // Verify that a purpose was specified if a convertType was.  Note that it is