[ConstraintSystem] Narrowly disable tryOptimizeGenericDisjunction when some of the arguments are number literals

Don't attempt this optimization if call has number literals.
This is intended to narrowly fix situations like:

```swift
func test<T: FloatingPoint>(_: T) { ... }
func test<T: Numeric>(_: T) { ... }

test(42)
```

The call should use `<T: Numeric>` overload even though the
`<T: FloatingPoint>` is a more specialized version because
selecting `<T: Numeric>` doesn't introduce non-default literal
types.

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -497,6 +497,10 @@ class TypeVariableType::Implementation {
   /// literal (represented by `ArrayExpr` and `DictionaryExpr` in AST).
   bool isCollectionLiteralType() const;
 
+  /// Determine whether this type variable represents a literal such
+  /// as an integer value, a floating-point value with and without a sign.
+  bool isNumberLiteralType() const;
+
   /// Determine whether this type variable represents a result type of a
   /// function call.
   bool isFunctionResult() const;

lib/Sema/CSSolver.cpp

@@ -1402,6 +1402,27 @@ tryOptimizeGenericDisjunction(ConstraintSystem &cs, Constraint *disjunction,
       return nullptr;
   }
 
+  // Don't attempt this optimization if call has number literals.
+  // This is intended to narrowly fix situations like:
+  //
+  // func test<T: FloatingPoint>(_: T) { ... }
+  // func test<T: Numeric>(_: T) { ... }
+  //
+  // test(42)
+  //
+  // The call should use `<T: Numeric>` overload even though the
+  // `<T: FloatingPoint>` is a more specialized version because
+  // selecting `<T: Numeric>` doesn't introduce non-default literal
+  // types.
+  if (auto *argFnType = cs.getAppliedDisjunctionArgumentFunction(disjunction)) {
+    if (llvm::any_of(
+            argFnType->getParams(), [](const AnyFunctionType::Param &param) {
+              auto *typeVar = param.getPlainType()->getAs<TypeVariableType>();
+              return typeVar && typeVar->getImpl().isNumberLiteralType();
+            }))
+      return nullptr;
+  }
+
   llvm::SmallVector<Constraint *, 4> choices;
   for (auto *choice : constraints) {
     if (choices.size() > 2)

lib/Sema/TypeCheckConstraints.cpp

@@ -204,6 +204,10 @@ bool TypeVariableType::Implementation::isCollectionLiteralType() const {
                      locator->directlyAt<DictionaryExpr>());
 }
 
+bool TypeVariableType::Implementation::isNumberLiteralType() const {
+  return locator && locator->directlyAt<NumberLiteralExpr>();
+}
+
 bool TypeVariableType::Implementation::isFunctionResult() const {
   return locator && locator->isLastElement<LocatorPathElt::FunctionResult>();
 }

test/Constraints/old_hack_related_ambiguities.swift

@@ -178,3 +178,12 @@ do {
   }
 }
 
+// `tryOptimizeGenericDisjunction` is too aggressive sometimes, make sure that `<T: FloatingPoint>`
+// overload is _not_ selected in this case.
+do {
+  func test<T: FloatingPoint>(_ expression1: @autoclosure () throws -> T, accuracy: T) -> T {}
+  func test<T: Numeric>(_ expression1: @autoclosure () throws -> T, accuracy: T) -> T {}
+
+  let result = test(10, accuracy: 1)
+  let _: Int = result
+}