[Sema] SE-0213: Fix LinkedExprAnalyzer to not record types for lite…

lib/Sema/CSGen.cpp

@@ -304,9 +304,12 @@ namespace {
 
       // Coercion exprs have a rigid type, so there's no use in gathering info
       // about them.
-      if (isa<CoerceExpr>(expr)) {
-        LTI.collectedTypes.insert(CS.getType(expr).getPointer());
-
+      if (auto *coercion = dyn_cast<CoerceExpr>(expr)) {
+        // Let's not collect information about types initialized by
+        // coercions just like we don't for regular initializer calls,
+        // because that might lead to overly eager type variable merging.
+        if (!coercion->isLiteralInit())
+          LTI.collectedTypes.insert(CS.getType(expr).getPointer());
         return { false, expr };
       }
 

test/Constraints/rdar42750089.swift

@@ -0,0 +1,74 @@
+// RUN: %target-typecheck-verify-swift
+
+protocol P : Equatable {
+  associatedtype T = String
+}
+
+struct S : Hashable {
+  var key: String
+
+  init(_ key: String) {
+    self.key = key
+  }
+}
+
+extension S : ExpressibleByStringLiteral {
+    public init(stringLiteral value: String) {
+        self.init(value)
+    }
+}
+
+extension S : _ExpressibleByStringInterpolation {
+  init(stringInterpolation strings: S...) {
+    self.key = "foo"
+  }
+
+  init<T>(stringInterpolationSegment expr: T) {
+    self.init(String(describing: expr))
+  }
+}
+
+extension S : P {}
+
+struct ConcP<F: P, S: P> : P where F.T == S.T {
+  var lhs: F
+  var rhs: S
+}
+
+struct Z : P {
+}
+
+extension P {
+  func bar() -> Z { fatalError() }
+
+  static func +<T : P>(lhs: Self, rhs: T) -> ConcP<Self, T> {
+    return ConcP(lhs: lhs, rhs: rhs)
+  }
+}
+
+class Container<V> {
+  var value: V
+  init(_ value: V) {
+    self.value = value
+  }
+}
+
+struct A {
+  enum Value : CustomStringConvertible {
+    case foo, bar
+
+    var description: String {
+      switch self {
+        case .foo: return "foo"
+        case .bar: return "bar"
+      }
+    }
+  }
+
+  var value: Container<Value>
+
+  func foo() {
+    let value = self.value.value
+    _ = S("A") + S("\(value)").bar() + S("B") // Ok
+  }
+}