[Type checker] Introduce a request for computing the type of a pattern.

Compute the "raw" type of a pattern via a request, caching the result
of TypeChecker::typeCheckPattern(). Retain typeCheckPattern() as the
general entrypoint for performing this computation so that other code
doesn't need to change yet.

Author: DougGregor

include/swift/AST/TypeCheckRequests.h

@@ -20,6 +20,7 @@
 #include "swift/AST/GenericSignature.h"
 #include "swift/AST/Type.h"
 #include "swift/AST/Evaluator.h"
+#include "swift/AST/Pattern.h"
 #include "swift/AST/SimpleRequest.h"
 #include "swift/AST/TypeResolutionStage.h"
 #include "swift/Basic/AnyValue.h"
@@ -33,6 +34,7 @@ namespace swift {
 class AbstractStorageDecl;
 class AccessorDecl;
 enum class AccessorKind;
+class ContextualPattern;
 class DefaultArgumentExpr;
 class GenericParamList;
 class PrecedenceGroupDecl;
@@ -1991,6 +1993,32 @@ class HasDynamicMemberLookupAttributeRequest
   }
 };
 
+/// Determines the type of a given pattern.
+///
+/// Note that this returns the "raw" pattern type, which can involve
+/// unresolved types and unbound generic types where type inference is
+/// allowed.
+class PatternTypeRequest
+    : public SimpleRequest<PatternTypeRequest, Type(ContextualPattern),
+                           CacheKind::Cached> {
+public:
+  using SimpleRequest::SimpleRequest;
+
+private:
+  friend SimpleRequest;
+
+  // Evaluation.
+  llvm::Expected<Type> evaluate(
+      Evaluator &evaluator, ContextualPattern pattern) const;
+
+public:
+  bool isCached() const { return true; }
+
+  SourceLoc getNearestLoc() const {
+    return std::get<0>(getStorage()).getPattern()->getLoc();
+  }
+};
+
 // Allow AnyValue to compare two Type values, even though Type doesn't
 // support ==.
 template<>

include/swift/AST/TypeCheckerTypeIDZone.def

@@ -216,3 +216,6 @@ SWIFT_REQUEST(TypeChecker, TypeWitnessRequest,
 SWIFT_REQUEST(TypeChecker, ValueWitnessRequest,
               Witness(NormalProtocolConformance *, ValueDecl *),
               SeparatelyCached, NoLocationInfo)
+SWIFT_REQUEST(TypeChecker, PatternTypeRequest,
+              Type(ContextualPattern),
+              Cached, HasNearestLocation)

lib/Sema/TypeCheckPattern.cpp

@@ -24,6 +24,7 @@
 #include "swift/AST/SourceFile.h"
 #include "swift/AST/NameLookup.h"
 #include "swift/AST/ParameterList.h"
+#include "swift/AST/TypeCheckRequests.h"
 #include "llvm/Support/SaveAndRestore.h"
 #include <utility>
 using namespace swift;
@@ -704,6 +705,14 @@ static Type validateTypedPattern(TypeResolution resolution,
 }
 
 Type TypeChecker::typeCheckPattern(ContextualPattern pattern) {
+  DeclContext *dc = pattern.getDeclContext();
+  ASTContext &ctx = dc->getASTContext();
+  return evaluateOrDefault(
+      ctx.evaluator, PatternTypeRequest{pattern}, ErrorType::get(ctx));
+}
+
+llvm::Expected<Type> PatternTypeRequest::evaluate(
+    Evaluator &evaluator, ContextualPattern pattern) const {
   Pattern *P = pattern.getPattern();
   DeclContext *dc = pattern.getDeclContext();