[CSDiagnostics] Diagnose conformance failures related to pack expansion patterns

(cherry picked from commit fe6cfe2)

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -8393,6 +8393,13 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyConformsToConstraint(
     if (path.back().is<LocatorPathElt::PackElement>())
       path.pop_back();
 
+    // This is similar to `PackElement` but locator points to the requirement
+    // associted with pack expansion pattern (i.e. `repeat each T: P`) where
+    // the path is something like:
+    // `... -> type req # -> pack expansion pattern`.
+    if (path.back().is<LocatorPathElt::PackExpansionPattern>())
+      path.pop_back();
+
     if (auto req = path.back().getAs<LocatorPathElt::AnyRequirement>()) {
       // If this is a requirement associated with `Self` which is bound
       // to `Any`, let's consider this "too incorrect" to continue.

test/Constraints/pack-expansion-expressions.swift

@@ -533,8 +533,10 @@ func test_pack_expansion_to_void_conv_for_closure_result<each T>(x: repeat each
 // rdar://109539394 - crash on passing multiple variadic lists to singly variadic callee
 do {
   func test1<each T>(_: repeat each T) {}
+  func test2<each T>(_: repeat each T) where repeat each T: RawRepresentable {} // expected-note {{where 'each T' = 'each T2'}}
 
   func caller<each T1, each T2>(t1: repeat each T1, t2: repeat each T2) {
     test1(repeat each t1, repeat each t2) // Ok
+    test2(repeat each t2, repeat each t1) // expected-error {{local function 'test2' requires that 'each T2' conform to 'RawRepresentable'}}
   }
 }