[ConstraintSystem] Adjust ranking rules associated with Double/CGFloat conversions

Change the conversion rule to favor any number of widenings (CGFloat -> Double)
over even a single narrowing conversion and if there is no way to avoid narrowing
(due to contextual requirements) attempt it as late as possible (the deeper in
the AST that conversion is located the higher its score).

This is a generally better rule when it comes to rounding and information
loss that would result from narrowing conversions.

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -10552,20 +10552,17 @@ ConstraintSystem::simplifyRestrictedConstraintImpl(
   case ConversionRestrictionKind::DoubleToCGFloat:
   case ConversionRestrictionKind::CGFloatToDouble: {
     // Prefer CGFloat -> Double over other way araund.
-    auto defaultImpact =
-        restriction == ConversionRestrictionKind::CGFloatToDouble ? 1 : 2;
+    auto impact =
+        restriction == ConversionRestrictionKind::CGFloatToDouble ? 1 : 10;
 
-    // Consider conversion with context to be worse
-    // by default because it means that expression
-    // as a whole couldn't get to the expected type.
-    if (auto last = locator.last()) {
-      if (last->is<LocatorPathElt::ContextualType>())
-        ++defaultImpact;
+    if (restriction == ConversionRestrictionKind::DoubleToCGFloat) {
+      if (auto *anchor = locator.trySimplifyToExpr()) {
+        if (auto depth = getExprDepth(anchor))
+          impact = (*depth + 1) * impact;
+      }
     }
 
-    auto numConversions = ImplicitValueConversions.size();
-    increaseScore(SK_ImplicitValueConversion,
-                  defaultImpact * (numConversions + 1));
+    increaseScore(SK_ImplicitValueConversion, impact);
 
     if (worseThanBestSolution())
       return SolutionKind::Error;

test/Constraints/implicit_double_cgfloat_conversion.swift

@@ -124,3 +124,52 @@ func test_static_members_are_contextually_convertible() {
     return S.testFunc() // Ok
   }
 }
+
+func test_narrowing_is_delayed(x: Double, y: CGFloat) {
+  func test(_: CGFloat) {}
+
+  func overloaded(_: Double, _: Double) -> Double {}
+  func overloaded(_: CGFloat, _: CGFloat) -> CGFloat {}
+
+  // CHECK: function_ref @$sSd12CoreGraphicsEySdAA7CGFloatVcfC
+  // CHECK: function_ref @$sSd1doiyS2d_SdtFZ
+  // CHECK: function_ref @$s12CoreGraphics7CGFloatVyACSdcfC
+  let _: CGFloat = x / y // CGFloat.init(x / Double.init(y))
+  // CHECK: function_ref @$sSd12CoreGraphicsEySdAA7CGFloatVcfC
+  // CHECK: function_ref @$sSd1doiyS2d_SdtFZ
+  // CHECK: function_ref @$sSd22_builtinIntegerLiteralSdBI_tcfC
+  // CHECK: function_ref @$sSd1poiyS2d_SdtFZ
+  // CHECK: function_ref @$s12CoreGraphics7CGFloatVyACSdcfC
+  let _: CGFloat = x / y + 1 // CGFloat.init(x / Double(y) + 1 as Double)
+  // CHECK: function_ref @$sSd12CoreGraphicsEySdAA7CGFloatVcfC
+  // CHECK: function_ref @$s34implicit_double_cgfloat_conversion25test_narrowing_is_delayed1x1yySd_12CoreGraphics7CGFloatVtF10overloadedL_yS2d_SdtF
+  // CHECK: function_ref @$s12CoreGraphics7CGFloatVyACSdcf
+  let _: CGFloat = overloaded(x, y) // Prefers `overloaded(Double, Double) -> Double`
+  // CHECK: function_ref @$sSd12CoreGraphicsEySdAA7CGFloatVcfC
+  // CHECK: function_ref @$s34implicit_double_cgfloat_conversion25test_narrowing_is_delayed1x1yySd_12CoreGraphics7CGFloatVtF10overloadedL_yS2d_SdtF
+  // CHECK: @$s34implicit_double_cgfloat_conversion25test_narrowing_is_delayed1x1yySd_12CoreGraphics7CGFloatVtF10overloadedL_yS2d_SdtF
+  // CHECK: function_ref @$s12CoreGraphics7CGFloatVyACSdcf
+  let _: CGFloat = overloaded(x, overloaded(x, y)) // Prefers `overloaded(Double, Double) -> Double` in both occurances.
+
+  // Calls should behave exactly the same as contextual conversions.
+
+  // CHECK: function_ref @$sSd12CoreGraphicsEySdAA7CGFloatVcfC
+  // CHECK: function_ref @$sSd1doiyS2d_SdtFZ
+  // CHECK: function_ref @$s12CoreGraphics7CGFloatVyACSdcfC
+  test(x / y)
+  // CHECK: function_ref @$sSd12CoreGraphicsEySdAA7CGFloatVcfC
+  // CHECK: function_ref @$sSd1doiyS2d_SdtFZ
+  // CHECK: function_ref @$sSd22_builtinIntegerLiteralSdBI_tcfC
+  // CHECK: function_ref @$sSd1poiyS2d_SdtFZ
+  // CHECK: function_ref @$s12CoreGraphics7CGFloatVyACSdcfC
+  test(x / y + 1)
+  // CHECK: function_ref @$sSd12CoreGraphicsEySdAA7CGFloatVcfC
+  // CHECK: function_ref @$s34implicit_double_cgfloat_conversion25test_narrowing_is_delayed1x1yySd_12CoreGraphics7CGFloatVtF10overloadedL_yS2d_SdtF
+  // CHECK: function_ref @$s12CoreGraphics7CGFloatVyACSdcf
+  test(overloaded(x, y))
+  // CHECK: function_ref @$sSd12CoreGraphicsEySdAA7CGFloatVcfC
+  // CHECK: function_ref @$s34implicit_double_cgfloat_conversion25test_narrowing_is_delayed1x1yySd_12CoreGraphics7CGFloatVtF10overloadedL_yS2d_SdtF
+  // CHECK: @$s34implicit_double_cgfloat_conversion25test_narrowing_is_delayed1x1yySd_12CoreGraphics7CGFloatVtF10overloadedL_yS2d_SdtF
+  // CHECK: function_ref @$s12CoreGraphics7CGFloatVyACSdcf
+  test(overloaded(x, overloaded(x, y)))
+}