[ConstraintSystem] Allow placeholder vars to have initializer and nested holes

Initializers of such variables are going to be type-checked post factum,
when the type of the variable itself has been determined by the solver.

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -6505,6 +6505,13 @@ bool isOperatorDisjunction(Constraint *disjunction);
 /// or nested declarations).
 ASTNode findAsyncNode(ClosureExpr *closure);
 
+/// Check whether the given binding represents a placeholder variable that
+/// has to get its type inferred at a first use site.
+///
+/// \returns The currently assigned type if it's a placeholder,
+/// empty type otherwise.
+Type isPlaceholderVar(PatternBindingDecl *PB);
+
 } // end namespace constraints
 
 template<typename ...Args>

lib/Sema/CSApply.cpp

@@ -9053,11 +9053,9 @@ ExprWalker::rewriteTarget(SolutionApplicationTarget target) {
 
     // If this is a placeholder variable, let's override its type with
     // inferred one.
-    if (auto *var = PB->getSingleVar()) {
-      if (var->getName().hasDollarPrefix() && patternType->hasPlaceholder()) {
-        patternType = solution.getResolvedType(var);
-        options |= TypeResolutionFlags::OverrideType;
-      }
+    if (isPlaceholderVar(PB)) {
+      patternType = solution.getResolvedType(PB->getSingleVar());
+      options |= TypeResolutionFlags::OverrideType;
     }
 
     if (auto coercedPattern = TypeChecker::coercePatternToType(

lib/Sema/CSClosure.cpp

@@ -95,19 +95,12 @@ class TypeVariableRefFinder : public ASTWalker {
       // If there is no type recorded yet, let's check whether
       // it is a placeholder variable implicitly generated by the
       // compiler.
-      if (var->getName().hasDollarPrefix()) {
-        if (auto *PB = var->getParentPatternBinding()) {
-          auto patternTarget = CS.getSolutionApplicationTarget({PB, 0});
-          if (!patternTarget)
-            return {true, expr};
-
-          auto patternType = patternTarget->getTypeOfUninitializedVar();
-          if (patternType->hasPlaceholder()) {
-            auto openedTy = CS.replaceInferableTypesWithTypeVars(
-                patternType, CS.getConstraintLocator(expr));
-            inferVariables(openedTy);
-            CS.setType(var, openedTy);
-          }
+      if (auto *PB = var->getParentPatternBinding()) {
+        if (auto placeholderTy = isPlaceholderVar(PB)) {
+          auto openedTy = CS.replaceInferableTypesWithTypeVars(
+              placeholderTy, CS.getConstraintLocator(expr));
+          inferVariables(openedTy);
+          CS.setType(var, openedTy);
         }
       }
     }
@@ -553,13 +546,8 @@ class SyntacticElementConstraintGenerator
     // Keep track of this binding entry.
     cs.setSolutionApplicationTarget({patternBinding, index}, *target);
 
-    if (patternType->hasPlaceholder()) {
-      if (auto *var = patternBinding->getSingleVar()) {
-        if (var->getName().hasDollarPrefix() &&
-            !patternBinding->isExplicitlyInitialized(index))
-          return;
-      }
-    }
+    if (isPlaceholderVar(patternBinding))
+      return;
 
     if (cs.generateConstraints(*target, FreeTypeVariableBinding::Disallow)) {
       hadError = true;
@@ -1829,6 +1817,11 @@ void ConjunctionElement::findReferencedVariables(
 
   if (auto *patternBinding =
           dyn_cast_or_null<PatternBindingDecl>(element.dyn_cast<Decl *>())) {
+    // Let's not walk into placeholder variable initializers, since they
+    // are type-checked separately right now.
+    if (isPlaceholderVar(patternBinding))
+      return;
+
     if (auto patternBindingElt =
             locator
                 ->getLastElementAs<LocatorPathElt::PatternBindingElement>()) {
@@ -1842,3 +1835,21 @@ void ConjunctionElement::findReferencedVariables(
       element.is<Expr *>() || element.isStmt(StmtKind::Return))
     element.walk(refFinder);
 }
+
+Type constraints::isPlaceholderVar(PatternBindingDecl *PB) {
+  auto *var = PB->getSingleVar();
+  if (!var)
+    return Type();
+
+  if (!var->getName().hasDollarPrefix())
+    return Type();
+
+  auto *pattern = PB->getPattern(0);
+  if (auto *typedPattern = dyn_cast<TypedPattern>(pattern)) {
+    auto type = typedPattern->getType();
+    if (type && type->hasPlaceholder())
+      return type;
+  }
+
+  return Type();
+}