Merge pull request #26086 from xedin/rdar-50987089

[ConstraintSystem] `trySimplifyToExpr` shouldn't rely on locator bein…

Author: xedin

lib/Sema/TypeCheckConstraints.cpp

@@ -180,6 +180,12 @@ bool constraints::computeTupleShuffle(ArrayRef<TupleTypeElt> fromTuple,
 Expr *ConstraintLocatorBuilder::trySimplifyToExpr() const {
   SmallVector<LocatorPathElt, 4> pathBuffer;
   Expr *anchor = getLocatorParts(pathBuffer);
+  // Locators are not guaranteed to have an anchor
+  // if constraint system is used to verify generic
+  // requirements.
+  if (!anchor)
+    return nullptr;
+
   ArrayRef<LocatorPathElt> path = pathBuffer;
 
   SourceRange range;

test/Constraints/sr10728.swift

@@ -0,0 +1,23 @@
+// RUN: %target-typecheck-verify-swift
+
+typealias T1 = Int
+typealias T2 = Float
+typealias T3 = Bool
+
+protocol P {
+  associatedtype R
+  static var foo: (T1, (R) -> T2) { get }
+}
+
+extension P {
+  static func bind() -> (T1, (R) -> T3) {
+    return (1, { _ in true })
+  }
+}
+
+struct S: P {
+  typealias R = T3
+
+  static let foo: (T1, (R) -> T2) = bind()
+  // expected-error@-1 {{cannot convert value of type '(T1, (S.R) -> T3)' (aka '(Int, (Bool) -> Bool)') to specified type '(T1, (S.R) -> T2)' (aka '(Int, (Bool) -> Float)')}}
+}