[ConstraintSystem] Solve where clauses of for-in loops separately

Doing so fits better into conjunction model which leads to more
granular control over what variables are brought into scope during
`where` clause expression checking.

These changes also remove "one-way bind" flag from "for-in" statement
target.

Author: xedin

include/swift/Sema/SyntacticElementTarget.h

@@ -147,7 +147,7 @@ class SyntacticElementTarget {
       ForEachStmt *stmt;
       DeclContext *dc;
       Pattern *pattern;
-      bool bindPatternVarsOneWay;
+      bool ignoreWhereClause;
       ForEachStmtInfo info;
     } forEachStmt;
 
@@ -227,11 +227,11 @@ class SyntacticElementTarget {
   }
 
   SyntacticElementTarget(ForEachStmt *stmt, DeclContext *dc,
-                         bool bindPatternVarsOneWay)
+                         bool ignoreWhereClause)
       : kind(Kind::forEachStmt) {
     forEachStmt.stmt = stmt;
     forEachStmt.dc = dc;
-    forEachStmt.bindPatternVarsOneWay = bindPatternVarsOneWay;
+    forEachStmt.ignoreWhereClause = ignoreWhereClause;
   }
 
   /// Form a target for the initialization of a pattern from an expression.
@@ -249,7 +249,7 @@ class SyntacticElementTarget {
   /// Form a target for a for-in loop.
   static SyntacticElementTarget forForEachStmt(ForEachStmt *stmt,
                                                DeclContext *dc,
-                                               bool bindPatternVarsOneWay);
+                                               bool ignoreWhereClause = false);
 
   /// Form a target for a property with an attached property wrapper that is
   /// initialized out-of-line.
@@ -469,10 +469,6 @@ class SyntacticElementTarget {
   bool shouldBindPatternVarsOneWay() const {
     if (kind == Kind::expression)
       return expression.bindPatternVarsOneWay;
-
-    if (kind == Kind::forEachStmt)
-      return forEachStmt.bindPatternVarsOneWay;
-
     return false;
   }
 
@@ -521,6 +517,11 @@ class SyntacticElementTarget {
     return expression.initialization.patternBindingIndex;
   }
 
+  bool ignoreForEachWhereClause() const {
+    assert(isForEachStmt());
+    return forEachStmt.ignoreWhereClause;
+  }
+
   const ForEachStmtInfo &getForEachStmtInfo() const {
     assert(isForEachStmt());
     return forEachStmt.info;

lib/Sema/CSGen.cpp

@@ -4546,12 +4546,9 @@ generateForEachStmtConstraints(ConstraintSystem &cs,
   }
 
   // Generate constraints for the "where" expression, if there is one.
-  if (auto *whereExpr = stmt->getWhere()) {
-    auto *boolDecl = dc->getASTContext().getBoolDecl();
-    if (!boolDecl)
-      return llvm::None;
-
-    Type boolType = boolDecl->getDeclaredInterfaceType();
+  auto *whereExpr = stmt->getWhere();
+  if (whereExpr && !target.ignoreForEachWhereClause()) {
+    Type boolType = dc->getASTContext().getBoolType();
     if (!boolType)
       return llvm::None;
 

lib/Sema/CSSyntacticElement.cpp

@@ -631,7 +631,7 @@ class SyntacticElementConstraintGenerator
   void visitForEachPattern(Pattern *pattern, ForEachStmt *forEachStmt) {
     auto target = SyntacticElementTarget::forForEachStmt(
         forEachStmt, context.getAsDeclContext(),
-        /*bindTypeVarsOneWay=*/false);
+        /*ignoreWhereClause=*/true);
 
     if (cs.generateConstraints(target)) {
       hadError = true;
@@ -959,6 +959,20 @@ class SyntacticElementConstraintGenerator
     // they would be handled together with pattern because pattern can
     // inform a type of sequence element e.g. `for i: Int8 in 0 ..< 8`
     elements.push_back(makeElement(forEachStmt->getPattern(), stmtLoc));
+
+    // Where clause if any.
+    if (auto *where = forEachStmt->getWhere()) {
+      Type boolType = cs.getASTContext().getBoolType();
+      if (!boolType) {
+        hadError = true;
+        return;
+      }
+
+      ContextualTypeInfo context(boolType, CTP_Condition);
+      elements.push_back(
+          makeElement(where, stmtLoc, context, /*isDiscarded=*/false));
+    }
+
     // Body of the `for-in` loop.
     elements.push_back(makeElement(forEachStmt->getBody(), stmtLoc));
 

lib/Sema/SyntacticElementTarget.cpp

@@ -178,9 +178,8 @@ SyntacticElementTarget SyntacticElementTarget::forInitialization(
 
 SyntacticElementTarget
 SyntacticElementTarget::forForEachStmt(ForEachStmt *stmt, DeclContext *dc,
-                                       bool bindPatternVarsOneWay) {
-  SyntacticElementTarget target(
-      stmt, dc, bindPatternVarsOneWay || bool(stmt->getWhere()));
+                                       bool ignoreWhereClause) {
+  SyntacticElementTarget target(stmt, dc, ignoreWhereClause);
   return target;
 }
 

lib/Sema/TypeCheckConstraints.cpp

@@ -901,8 +901,7 @@ bool TypeChecker::typeCheckForEachBinding(DeclContext *dc, ForEachStmt *stmt) {
     return true;
   };
 
-  auto target = SyntacticElementTarget::forForEachStmt(
-      stmt, dc, /*bindPatternVarsOneWay=*/false);
+  auto target = SyntacticElementTarget::forForEachStmt(stmt, dc);
   if (!typeCheckTarget(target))
     return failed();