[CSOptimizer] Extend candidate/parameter matching to support array literals

Match `[...]` to Array<...> and/or `ExpressibleByArrayLiteral`
conforming types. This is very helpful for expressions like:
`[...] + [...]`.

Author: xedin

lib/Sema/CSOptimizer.cpp

@@ -15,6 +15,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "TypeChecker.h"
+#include "swift/AST/ConformanceLookup.h"
 #include "swift/AST/ExistentialLayout.h"
 #include "swift/AST/GenericSignature.h"
 #include "swift/Basic/OptionSet.h"
@@ -554,6 +555,30 @@ static void determineBestChoicesInContext(
         }
       }
 
+      // Match `[...]` to Array<...> and/or `ExpressibleByArrayLiteral`
+      // conforming types.
+      if (options.contains(MatchFlag::OnParam) &&
+          options.contains(MatchFlag::Literal) &&
+          isUnboundArrayType(candidateType)) {
+        // If an exact match is requested favor only `[...]` to `Array<...>`
+        // since everything else is going to increase to score.
+        if (options.contains(MatchFlag::ExactOnly))
+          return paramType->isArrayType() ? 1 : 0;
+
+        Type type = paramType->lookThroughAllOptionalTypes();
+        // Otherwise, check if the other side conforms to
+        // `ExpressibleByArrayLiteral` protocol (in some way).
+        if (auto *nominal = type->getAnyNominal()) {
+          auto &ctx = cs.getASTContext();
+          SmallVector<ProtocolConformance *> conformances;
+
+          nominal->lookupConformance(
+              ctx.getProtocol(KnownProtocolKind::ExpressibleByArrayLiteral),
+              conformances);
+          return conformances.empty() ? 0 : 0.3;
+        }
+      }
+
       if (options.contains(MatchFlag::ExactOnly))
         return areEqual(candidateType, paramType) ? 1 : 0;
 

validation-test/Sema/type_checker_perf/fast/array_count_property_vs_method.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift -solver-expression-time-threshold=5
+// RUN: %target-typecheck-verify-swift -solver-scope-threshold=11000
 // REQUIRES: tools-release,no_asan
 
 func f(n: Int, a: [Int]) {