[CS] Allow ExprPatterns to be type-checked in the solver

Previously we would wait until CSApply, which
would trigger their type-checking in
`coercePatternToType`. This caused a number of
bugs, and hampered solver-based completion, which
does not run CSApply. Instead, form a conjunction
of all the ExprPatterns present, which preserves
some of the previous isolation behavior (though
does not provide complete isolation).

We can then modify `coercePatternToType` to accept
a closure, which allows the solver to take over
rewriting the ExprPatterns it has already solved.

This then sets the stage for the complete removal
of `coercePatternToType`, and doing all pattern
type-checking in the solver.

Author: hamishknight

include/swift/Sema/ConstraintSystem.h

@@ -706,6 +706,11 @@ T *getAsPattern(ASTNode node) {
   return nullptr;
 }
 
+template <typename T = Pattern>
+T *castToPattern(ASTNode node) {
+  return cast<T>(node.get<Pattern *>());
+}
+
 template <typename T = Stmt> T *castToStmt(ASTNode node) {
   return cast<T>(node.get<Stmt *>());
 }
@@ -1515,6 +1520,10 @@ class Solution {
   llvm::SmallMapVector<const CaseLabelItem *, CaseLabelItemInfo, 4>
       caseLabelItems;
 
+  /// A map of expressions to the ExprPatterns that they are being solved as
+  /// a part of.
+  llvm::SmallMapVector<Expr *, ExprPattern *, 2> exprPatterns;
+
   /// The set of parameters that have been inferred to be 'isolated'.
   llvm::SmallVector<ParamDecl *, 2> isolatedParams;
 
@@ -1700,6 +1709,16 @@ class Solution {
         : nullptr;
   }
 
+  /// Retrieve the solved ExprPattern that corresponds to provided
+  /// sub-expression.
+  NullablePtr<ExprPattern> getExprPatternFor(Expr *E) const {
+    auto result = exprPatterns.find(E);
+    if (result == exprPatterns.end())
+      return nullptr;
+
+    return result->second;
+  }
+
   /// This method implements functionality of `Expr::isTypeReference`
   /// with data provided by a given solution.
   bool isTypeReference(Expr *E) const;
@@ -2163,6 +2182,10 @@ class ConstraintSystem {
   llvm::SmallMapVector<const CaseLabelItem *, CaseLabelItemInfo, 4>
       caseLabelItems;
 
+  /// A map of expressions to the ExprPatterns that they are being solved as
+  /// a part of.
+  llvm::SmallMapVector<Expr *, ExprPattern *, 2> exprPatterns;
+
   /// The set of parameters that have been inferred to be 'isolated'.
   llvm::SmallSetVector<ParamDecl *, 2> isolatedParams;
 
@@ -2754,6 +2777,9 @@ class ConstraintSystem {
     /// The length of \c caseLabelItems.
     unsigned numCaseLabelItems;
 
+    /// The length of \c exprPatterns.
+    unsigned numExprPatterns;
+
     /// The length of \c isolatedParams.
     unsigned numIsolatedParams;
 
@@ -3175,6 +3201,15 @@ class ConstraintSystem {
     caseLabelItems[item] = info;
   }
 
+  /// Record a given ExprPattern as the parent of its sub-expression.
+  void setExprPatternFor(Expr *E, ExprPattern *EP) {
+    assert(E);
+    assert(EP);
+    auto inserted = exprPatterns.insert({E, EP}).second;
+    assert(inserted && "Mapping already defined?");
+    (void)inserted;
+  }
+
   Optional<CaseLabelItemInfo> getCaseLabelItemInfo(
       const CaseLabelItem *item) const {
     auto known = caseLabelItems.find(item);
@@ -4299,6 +4334,11 @@ class ConstraintSystem {
   /// \returns \c true if constraint generation failed, \c false otherwise
   bool generateConstraints(SingleValueStmtExpr *E);
 
+  /// Generate constraints for an array of ExprPatterns, forming a conjunction
+  /// that solves each expression in turn.
+  void generateConstraints(ArrayRef<ExprPattern *> exprPatterns,
+                           ConstraintLocatorBuilder locator);
+
   /// Generate constraints for the given (unchecked) expression.
   ///
   /// \returns a possibly-sanitized expression, or null if an error occurred.

lib/IDE/TypeCheckCompletionCallback.cpp

@@ -81,7 +81,13 @@ Type swift::ide::getTypeForCompletion(const constraints::Solution &S,
 /// \endcode
 /// If the code completion expression occurs in such an AST, return the
 /// declaration of the \c $match variable, otherwise return \c nullptr.
-static VarDecl *getMatchVarIfInPatternMatch(Expr *E, ConstraintSystem &CS) {
+static VarDecl *getMatchVarIfInPatternMatch(Expr *E, const Solution &S) {
+  if (auto EP = S.getExprPatternFor(E))
+    return EP.get()->getMatchVar();
+
+  // TODO: Once ExprPattern type-checking is fully moved into the solver,
+  // the below can be deleted.
+  auto &CS = S.getConstraintSystem();
   auto &Context = CS.getASTContext();
 
   auto *Binary = dyn_cast_or_null<BinaryExpr>(CS.getParentExpr(E));
@@ -109,20 +115,21 @@ static VarDecl *getMatchVarIfInPatternMatch(Expr *E, ConstraintSystem &CS) {
 }
 
 Type swift::ide::getPatternMatchType(const constraints::Solution &S, Expr *E) {
-  if (auto MatchVar = getMatchVarIfInPatternMatch(E, S.getConstraintSystem())) {
-    Type MatchVarType;
-    // If the MatchVar has an explicit type, it's not part of the solution. But
-    // we can look it up in the constraint system directly.
-    if (auto T = S.getConstraintSystem().getVarType(MatchVar)) {
-      MatchVarType = T;
-    } else {
-      MatchVarType = getTypeForCompletion(S, MatchVar);
-    }
-    if (MatchVarType) {
-      return MatchVarType;
-    }
-  }
-  return nullptr;
+  auto MatchVar = getMatchVarIfInPatternMatch(E, S);
+  if (!MatchVar)
+    return nullptr;
+
+  if (S.hasType(MatchVar))
+    return S.getResolvedType(MatchVar);
+
+  // If the ExprPattern wasn't solved as part of the constraint system, it's
+  // not part of the solution.
+  // TODO: This can be removed once ExprPattern type-checking is fully part
+  // of the constraint system.
+  if (auto T = S.getConstraintSystem().getVarType(MatchVar))
+    return T;
+
+  return getTypeForCompletion(S, MatchVar);
 }
 
 void swift::ide::getSolutionSpecificVarTypes(

lib/Sema/CSApply.cpp

@@ -8574,6 +8574,9 @@ namespace {
       return Action::SkipChildren();
     }
 
+    NullablePtr<Pattern>
+    rewritePattern(Pattern *pattern, DeclContext *DC);
+
     /// Rewrite the target, producing a new target.
     Optional<SyntacticElementTarget>
     rewriteTarget(SyntacticElementTarget target);
@@ -8820,12 +8823,68 @@ static Expr *wrapAsyncLetInitializer(
   return resultInit;
 }
 
+static Pattern *rewriteExprPattern(const SyntacticElementTarget &matchTarget,
+                                   Type patternTy,
+                                   RewriteTargetFn rewriteTarget) {
+  auto *EP = matchTarget.getExprPattern();
+
+  // See if we can simplify to another kind of pattern.
+  if (auto simplified = TypeChecker::trySimplifyExprPattern(EP, patternTy))
+    return simplified.get();
+
+  auto resultTarget = rewriteTarget(matchTarget);
+  if (!resultTarget)
+    return nullptr;
+
+  EP->setMatchExpr(resultTarget->getAsExpr());
+  EP->getMatchVar()->setInterfaceType(patternTy->mapTypeOutOfContext());
+  EP->setType(patternTy);
+  return EP;
+}
+
+/// Attempt to rewrite either an ExprPattern, or a pattern that was solved as
+/// an ExprPattern, e.g an EnumElementPattern that could not refer to an enum
+/// case.
+static Optional<Pattern *>
+tryRewriteExprPattern(Pattern *P, Solution &solution, Type patternTy,
+                      RewriteTargetFn rewriteTarget) {
+  // See if we have a match expression target.
+  auto matchTarget = solution.getTargetFor(P);
+  if (!matchTarget)
+    return None;
+
+  return rewriteExprPattern(*matchTarget, patternTy, rewriteTarget);
+}
+
+NullablePtr<Pattern> ExprWalker::rewritePattern(Pattern *pattern,
+                                                DeclContext *DC) {
+  auto &solution = Rewriter.solution;
+
+  // Figure out the pattern type.
+  auto patternTy = solution.getResolvedType(pattern);
+  patternTy = patternTy->reconstituteSugar(/*recursive=*/false);
+
+  // Coerce the pattern to its appropriate type.
+  TypeResolutionOptions patternOptions(TypeResolverContext::InExpression);
+  patternOptions |= TypeResolutionFlags::OverrideType;
+
+  auto tryRewritePattern = [&](Pattern *EP, Type ty) {
+    return ::tryRewriteExprPattern(
+        EP, solution, ty, [&](auto target) { return rewriteTarget(target); });
+  };
+
+  auto contextualPattern = ContextualPattern::forRawPattern(pattern, DC);
+  return TypeChecker::coercePatternToType(contextualPattern, patternTy,
+                                          patternOptions, tryRewritePattern);
+}
+
 /// Apply the given solution to the initialization target.
 ///
 /// \returns the resulting initialization expression.
 static Optional<SyntacticElementTarget>
 applySolutionToInitialization(Solution &solution, SyntacticElementTarget target,
-                              Expr *initializer) {
+                              Expr *initializer,
+                              RewriteTargetFn rewriteTarget) {
   auto wrappedVar = target.getInitializationWrappedVar();
   Type initType;
   if (wrappedVar) {
@@ -8890,10 +8949,14 @@ applySolutionToInitialization(Solution &solution, SyntacticElementTarget target,
 
   finalPatternType = finalPatternType->reconstituteSugar(/*recursive =*/false);
 
+  auto tryRewritePattern = [&](Pattern *EP, Type ty) {
+    return ::tryRewriteExprPattern(EP, solution, ty, rewriteTarget);
+  };
+
   // Apply the solution to the pattern as well.
   auto contextualPattern = target.getContextualPattern();
   if (auto coercedPattern = TypeChecker::coercePatternToType(
-          contextualPattern, finalPatternType, options)) {
+          contextualPattern, finalPatternType, options, tryRewritePattern)) {
     resultTarget.setPattern(coercedPattern);
   } else {
     return None;
@@ -9040,10 +9103,15 @@ static Optional<SyntacticElementTarget> applySolutionToForEachStmt(
     TypeResolutionOptions options(TypeResolverContext::ForEachStmt);
     options |= TypeResolutionFlags::OverrideType;
 
+    auto tryRewritePattern = [&](Pattern *EP, Type ty) {
+      return ::tryRewriteExprPattern(EP, solution, ty, rewriteTarget);
+    };
+
     // Apply the solution to the pattern as well.
     auto contextualPattern = target.getContextualPattern();
     auto coercedPattern = TypeChecker::coercePatternToType(
-        contextualPattern, forEachStmtInfo.initType, options);
+        contextualPattern, forEachStmtInfo.initType, options,
+        tryRewritePattern);
     if (!coercedPattern)
       return None;
 
@@ -9131,7 +9199,8 @@ ExprWalker::rewriteTarget(SyntacticElementTarget target) {
     switch (target.getExprContextualTypePurpose()) {
     case CTP_Initialization: {
       auto initResultTarget = applySolutionToInitialization(
-          solution, target, rewrittenExpr);
+          solution, target, rewrittenExpr,
+          [&](auto target) { return rewriteTarget(target); });
       if (!initResultTarget)
         return None;
 
@@ -9222,47 +9291,11 @@ ExprWalker::rewriteTarget(SyntacticElementTarget target) {
     ConstraintSystem &cs = solution.getConstraintSystem();
     auto info = *cs.getCaseLabelItemInfo(*caseLabelItem);
 
-    // Figure out the pattern type.
-    Type patternType = solution.simplifyType(solution.getType(info.pattern));
-    patternType = patternType->reconstituteSugar(/*recursive=*/false);
-
-    // Check whether this enum element is resolved via ~= application.
-    if (auto *enumElement = dyn_cast<EnumElementPattern>(info.pattern)) {
-      if (auto target = cs.getTargetFor(enumElement)) {
-        auto *EP = target->getExprPattern();
-        auto enumType = solution.getResolvedType(EP);
-
-        auto *matchCall = target->getAsExpr();
-
-        auto *result = matchCall->walk(*this);
-        if (!result)
-          return None;
-
-        {
-          auto *matchVar = EP->getMatchVar();
-          matchVar->setInterfaceType(enumType->mapTypeOutOfContext());
-        }
-
-        EP->setMatchExpr(result);
-        EP->setType(enumType);
-
-        (*caseLabelItem)->setPattern(EP, /*resolved=*/true);
-        return target;
-      }
-    }
-
-    // Coerce the pattern to its appropriate type.
-    TypeResolutionOptions patternOptions(TypeResolverContext::InExpression);
-    patternOptions |= TypeResolutionFlags::OverrideType;
-    auto contextualPattern =
-        ContextualPattern::forRawPattern(info.pattern,
-                                         target.getDeclContext());
-    if (auto coercedPattern = TypeChecker::coercePatternToType(
-            contextualPattern, patternType, patternOptions)) {
-      (*caseLabelItem)->setPattern(coercedPattern, /*resolved=*/true);
-    } else {
+    auto pattern = rewritePattern(info.pattern, target.getDeclContext());
+    if (!pattern)
       return None;
-    }
+
+    (*caseLabelItem)->setPattern(pattern.get(), /*resolved=*/true);
 
     // If there is a guard expression, coerce that.
     if (auto *guardExpr = info.guardExpr) {
@@ -9330,8 +9363,13 @@ ExprWalker::rewriteTarget(SyntacticElementTarget target) {
       options |= TypeResolutionFlags::OverrideType;
     }
 
+    auto tryRewritePattern = [&](Pattern *EP, Type ty) {
+      return ::tryRewriteExprPattern(
+          EP, solution, ty, [&](auto target) { return rewriteTarget(target); });
+    };
+
     if (auto coercedPattern = TypeChecker::coercePatternToType(
-            contextualPattern, patternType, options)) {
+            contextualPattern, patternType, options, tryRewritePattern)) {
       auto resultTarget = target;
       resultTarget.setPattern(coercedPattern);
       return resultTarget;

lib/Sema/CSGen.cpp

@@ -2393,12 +2393,6 @@ namespace {
       // function, to set the type of the pattern.
       auto setType = [&](Type type) {
         CS.setType(pattern, type);
-        if (auto PE = dyn_cast<ExprPattern>(pattern)) {
-          // Set the type of the pattern's sub-expression as well, so code
-          // completion can retrieve the expression's type in case it is a code
-          // completion token.
-          CS.setType(PE->getSubExpr(), type);
-        }
         return type;
       };
 
@@ -2816,15 +2810,12 @@ namespace {
         return setType(patternType);
       }
 
-      // Refutable patterns occur when checking the PatternBindingDecls in an
-      // if/let or while/let condition.  They always require an initial value,
-      // so they always allow unspecified types.
-      case PatternKind::Expr:
-        // TODO: we could try harder here, e.g. for enum elements to provide the
-        // enum type.
-        return setType(
-            CS.createTypeVariable(
-              CS.getConstraintLocator(locator), TVO_CanBindToNoEscape));
+      case PatternKind::Expr: {
+        // We generate constraints for ExprPatterns in a separate pass. For
+        // now, just create a type variable.
+        return setType(CS.createTypeVariable(CS.getConstraintLocator(locator),
+                                             TVO_CanBindToNoEscape));
+      }
       }
 
       llvm_unreachable("Unhandled pattern kind");
@@ -4627,8 +4618,20 @@ Type ConstraintSystem::generateConstraints(
     bool bindPatternVarsOneWay, PatternBindingDecl *patternBinding,
     unsigned patternIndex) {
   ConstraintGenerator cg(*this, nullptr);
-  return cg.getTypeForPattern(pattern, locator, bindPatternVarsOneWay,
-                              patternBinding, patternIndex);
+  auto ty = cg.getTypeForPattern(pattern, locator, bindPatternVarsOneWay,
+                                 patternBinding, patternIndex);
+  assert(ty);
+
+  // Gather the ExprPatterns, and form a conjunction for their expressions.
+  SmallVector<ExprPattern *, 4> exprPatterns;
+  pattern->forEachNode([&](Pattern *P) {
+    if (auto *EP = dyn_cast<ExprPattern>(P))
+      exprPatterns.push_back(EP);
+  });
+  if (!exprPatterns.empty())
+    generateConstraints(exprPatterns, getConstraintLocator(pattern));
+
+  return ty;
 }
 
 bool ConstraintSystem::generateConstraints(StmtCondition condition,