[Sema] Fix crash when diagnosing ambiguous trailing closure inits

lib/Sema/CSDiagnostics.cpp

@@ -2142,7 +2142,9 @@ bool TrailingClosureAmbiguityFailure::diagnoseAsNote() {
       return false;
 
     const ParameterList *paramList = callee->getParameters();
-    const ParamDecl *param = paramList->getArray().back();
+    if (!paramList || paramList->size() == 0)
+      return false;
+    const ParamDecl *param = paramList->back();
 
     // Soundness-check that the trailing closure corresponds to this parameter.
     if (!param->hasInterfaceType() ||

validation-test/compiler_crashers/TrailingClosureAmbiguityFailure-diagnoseAsNote-d62041.swift → validation-test/compiler_crashers_fixed/TrailingClosureAmbiguityFailure-diagnoseAsNote-d62041.swift

@@ -1,5 +1,5 @@
 // {"kind":"typecheck","signature":"swift::constraints::TrailingClosureAmbiguityFailure::diagnoseAsNote()","signatureAssert":"Assertion failed: (!empty()), function back"}
-// RUN: not --crash %target-swift-frontend -typecheck %s
+// RUN: not %target-swift-frontend -typecheck %s
 {
   struct a {
     init ();

validation-test/compiler_crashers_fixed/issue-85364.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'}}