Merge pull request #29209 from xedin/rdar-56221372

[ConstraintSystem] Fix `getArgumentExpr` to check number of tuple ele…

Author: xedin

lib/Sema/ConstraintSystem.cpp

@@ -3384,8 +3384,12 @@ Expr *constraints::getArgumentExpr(Expr *expr, unsigned index) {
     return PE->getSubExpr();
   }
 
-  assert(isa<TupleExpr>(argExpr));
-  return cast<TupleExpr>(argExpr)->getElement(index);
+  if (auto *tuple = dyn_cast<TupleExpr>(argExpr)) {
+    return (tuple->getNumElements() > index) ? tuple->getElement(index)
+                                             : nullptr;
+  }
+
+  return nullptr;
 }
 
 bool constraints::isAutoClosureArgument(Expr *argExpr) {

test/Constraints/function_builder_diags.swift

@@ -160,9 +160,9 @@ struct Text : P {
   init(_: T) {}
 }
 
-struct Label<L> : P where L : P { // expected-note {{'L' declared as parameter to type 'Label'}}
+struct Label<L> : P where L : P { // expected-note 2 {{'L' declared as parameter to type 'Label'}}
   typealias T = L
-  init(@Builder _: () -> L) {}
+  init(@Builder _: () -> L) {} // expected-note {{'init(_:)' declared here}}
 }
 
 func test_51167632() -> some P {
@@ -173,6 +173,15 @@ func test_51167632() -> some P {
   })
 }
 
+func test_56221372() -> some P {
+  AnyP(G {
+    Text("hello")
+    Label() // expected-error {{generic parameter 'L' could not be inferred}}
+    // expected-error@-1 {{missing argument for parameter #1 in call}}
+    // expected-note@-2 {{explicitly specify the generic arguments to fix this issue}} {{10-10=<<#L: P#>>}}
+  })
+}
+
 struct SR11440 {
   typealias ReturnsTuple<T> = () -> (T, T)
   subscript<T, U>(@TupleBuilder x: ReturnsTuple<T>) -> (ReturnsTuple<U>) -> Void { //expected-note {{in call to 'subscript(_:)'}}