[Sema] Fix crash when diagnosing ambiguous trailing closure inits

Fixes #85376.

This fixes a compiler crash that occurred when diagnosing an ambiguous call
using trailing closure syntax, where one of the candidates was a function or
initializer with no parameters.

Author: naveen-seth

lib/Sema/CSDiagnostics.cpp

@@ -2142,6 +2142,8 @@ bool TrailingClosureAmbiguityFailure::diagnoseAsNote() {
       return false;
 
     const ParameterList *paramList = callee->getParameters();
+    if (paramList->getArray().empty())
+      return false;
     const ParamDecl *param = paramList->getArray().back();
 
     // Soundness-check that the trailing closure corresponds to this parameter.

test/Sema/ambiguous_init_trailing_closure.swift

@@ -0,0 +1,27 @@
+// RUN: %target-typecheck-verify-swift -swift-version 6
+// Checks that the compiler correctly diagnoses ambiguous initializers using
+// trailing closures, rather than crashing.
+//
+// Fixes #85364
+
+struct S1 {  // expected-note {{found this candidate}} \
+             // expected-note {{'S1' previously declared here}}
+  var c: () -> Void
+}
+
+S1 {}  // expected-error {{ambiguous use of 'init'}}
+
+struct S1 {  // expected-note {{found this candidate}} \
+             // expected-error {{invalid redeclaration of 'S1'}}
+  func callAsFunction(_: () -> Void) {}
+}
+
+struct S2 {
+  init() {}  // expected-note {{found this candidate}}
+
+  init(_ block: () -> Void) { block() }  // expected-note {{found this candidate}}
+
+  func callAsFunction(_ block: () -> Void) { block() }
+}
+
+S2 {}  // expected-error {{ambiguous use of 'init'}}