[CSSimplify] Report unresolved base failure if leading-dot syntax base is a placeholder

If base type of the leading-dot syntax expression has been determined
to be a placeholder, this could only mean that `.<name>` couldn't be
resolved in the current context.

Resolves: rdar://104302974
(cherry picked from commit 9308796)

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -6158,6 +6158,17 @@ bool ConstraintSystem::repairFailures(
     if (rhs->isPlaceholder())
       return true;
 
+    // The base is a placeholder, let's report an unknown base issue.
+    if (lhs->isPlaceholder()) {
+      auto *baseExpr =
+          castToExpr<UnresolvedMemberChainResultExpr>(anchor)->getChainBase();
+
+      auto *fix = SpecifyBaseTypeForContextualMember::create(
+          *this, baseExpr->getName(), getConstraintLocator(locator));
+      conversionsOrFixes.push_back(fix);
+      break;
+    }
+
     if (repairViaOptionalUnwrap(*this, lhs, rhs, matchKind, conversionsOrFixes,
                                 locator))
       break;

test/Constraints/members.swift

@@ -794,3 +794,11 @@ func test_diagnose_inaccessible_member_in_ambiguous_context() {
 
   test(\.x) // expected-error {{'x' is inaccessible due to 'private' protection level}}
 }
+
+// rdar://104302974
+func test_leading_dot_syntax_unknown_base_ambiguity() {
+  func fn<S: StringProtocol, T: Hashable>(_: S, value: T?) {}
+  func fn<T: Hashable>(_: String, value: T?) {}
+
+  fn("", value: .member) // expected-error {{cannot infer contextual base in reference to member 'member'}}
+}