Merge pull request #19376 from xedin/rdar-44569159

[CSDiagnostics] Fix requirement source lookup to support subscripts

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -102,6 +102,10 @@ ValueDecl *RequirementFailure::getDeclRef() const {
     ConstraintLocatorBuilder member(locator);
     locator =
         cs.getConstraintLocator(member.withPathElement(PathEltKind::Member));
+  } else if (isa<SubscriptExpr>(anchor)) {
+    ConstraintLocatorBuilder subscript(locator);
+    locator = cs.getConstraintLocator(
+        subscript.withPathElement(PathEltKind::SubscriptMember));
   }
 
   auto overload = getOverloadChoiceIfAvailable(locator);

test/Constraints/rdar44569159.swift

@@ -0,0 +1,18 @@
+// RUN: %target-typecheck-verify-swift
+
+protocol P {}
+struct S<V> where V: P { // expected-note {{where 'V' = 'Double'}}
+  var value: V
+}
+
+struct A {
+  subscript<T>(_ v: S<T>) -> A { // expected-note {{where 'T' = 'Double'}}
+    fatalError()
+  }
+}
+
+func foo(_ v: Double) {
+  _ = A()[S(value: v)]
+// expected-error@-1 {{subscript 'subscript' requires that 'Double' conform to 'P'}}
+// expected-error@-2 {{referencing initializer 'init(value:)' on 'S' requires that 'Double' conform to 'P'}}
+}