[CSSimplify] Forego any contextual score increases while checking erased member type

`resolveOverload` introduces a conversion if there were any adjustments
to a member type on existential base. This conversion exists only to
check adjustments in the member type, so the fact that adjustments also
cause a function conversion is unrelated.

Resolves: rdar://135974645

Author: xedin

include/swift/Sema/ConstraintLocator.h

@@ -1281,6 +1281,21 @@ class ConstraintLocatorBuilder {
     return false;
   }
 
+  bool isForExistentialMemberAccessConversion() const {
+    for (auto prev = this; prev;
+         prev = prev->previous.dyn_cast<ConstraintLocatorBuilder *>()) {
+      if (auto elt = prev->element) {
+        if (elt->is<LocatorPathElt::ExistentialMemberAccessConversion>())
+          return true;
+      }
+
+      if (auto locator = prev->previous.dyn_cast<ConstraintLocator *>())
+        return bool(locator->findLast<
+                    LocatorPathElt::ExistentialMemberAccessConversion>());
+    }
+    return false;
+  }
+
   std::optional<std::pair</*witness=*/ValueDecl *, GenericTypeParamType *>>
   isForWitnessGenericParameterRequirement() const {
     SmallVector<LocatorPathElt, 2> path;

lib/Sema/CSSimplify.cpp

@@ -14083,6 +14083,12 @@ ConstraintSystem::simplifyRestrictedConstraintImpl(
     // This induces conversions to occur within closures instead of
     // outside of them wherever possible.
     if (locator.isFunctionConversion()) {
+      // This conversion exists only to check adjustments in the member
+      // type, so the fact that adjustments also cause a function conversion
+      // is unrelated.
+      if (locator.isForExistentialMemberAccessConversion())
+        return;
+
       increaseScore(SK_FunctionConversion, locator);
     }
   };

test/decl/protocol/existential_member_accesses_self_assoctype.swift

@@ -1062,4 +1062,11 @@ protocol PackExpansionResult {
 func packExpansionResult(p: any PackExpansionResult) {
   p.foo(t: 1, "hi")
   // expected-error@-1 {{member 'foo' cannot be used on value of type 'any PackExpansionResult'; consider using a generic constraint instead}}
-}
+}
+
+// rdar://135974645 - invalid error: heterogeneous collection literal could only be inferred to '[Any]'
+extension StringProtocol {
+  func test(target: any StringProtocol, idx: Int) {
+    let _ = [target.prefix(idx), target.suffix(idx)] // Ok
+  }
+}