[Sema] Don't diagnose DynamicSubscriptExpr's arg as tuple

Register DynamicSubscriptExpr's index expression
as a call argument in MiscDiagnostics to avoid it
being diagnosed as a single element labelled tuple.

Resolves SR-12799.

Author: hamishknight

lib/Sema/MiscDiagnostics.cpp

@@ -137,6 +137,9 @@ static void diagSyntacticUseRestrictions(const Expr *E, const DeclContext *DC,
       if (auto *SE = dyn_cast<SubscriptExpr>(E))
         CallArgs.insert(SE->getIndex());
 
+      if (auto *DSE = dyn_cast<DynamicSubscriptExpr>(E))
+        CallArgs.insert(DSE->getIndex());
+
       if (auto *KPE = dyn_cast<KeyPathExpr>(E)) {
         for (auto Comp : KPE->getComponents()) {
           if (auto *Arg = Comp.getIndexExpr())

test/Constraints/dynamic_lookup.swift

@@ -371,6 +371,10 @@ class C1 {
   @objc subscript(unambiguousSubscript _: String) -> Int { return 0 } // expected-note {{found this candidate}}
 }
 
+class C2 {
+  @objc subscript(singleCandidate _: Int) -> Int { return 0 }
+}
+
 func testAnyObjectAmbiguity(_ x: AnyObject) {
   _ = x.ambiguousProperty // expected-error {{ambiguous use of 'ambiguousProperty'}}
   _ = x.unambiguousProperty
@@ -384,6 +388,9 @@ func testAnyObjectAmbiguity(_ x: AnyObject) {
   _ = x.ambiguousMethodParam // expected-error {{ambiguous use of 'ambiguousMethodParam'}}
   _ = x.unambiguousMethodParam
 
+  // SR-12799: Don't emit a "single-element" tuple error.
+  _ = x[singleCandidate: 0]
+
   _ = x[ambiguousSubscript: 0] // expected-error {{ambiguous use of 'subscript(ambiguousSubscript:)'}}
 
   // FIX-ME(SR-8611): This is currently ambiguous but shouldn't be.