[Diagnostics] Add a note if missing parameter is associated with generic subscript

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -927,6 +927,8 @@ NOTE(note_call_to_operator,none,
      "in call to operator %0", (DeclName))
 NOTE(note_call_to_func,none,
      "in call to function %0", (DeclName))
+NOTE(note_call_to_subscript,none,
+     "in call to %0", (DeclName))
 NOTE(note_call_to_initializer,none,
      "in call to initializer", ())
 NOTE(note_init_parameter,none,

lib/Sema/CSDiagnostics.cpp

@@ -2918,6 +2918,11 @@ bool MissingGenericArgumentsFailure::diagnoseForAnchor(
     return false;
 
   auto *DC = getDeclContext();
+  if (auto *SD = dyn_cast<SubscriptDecl>(DC)) {
+    emitDiagnostic(SD, diag::note_call_to_subscript, SD->getFullName());
+    return true;
+  }
+
   if (auto *AFD = dyn_cast<AbstractFunctionDecl>(DC)) {
     if (isa<ConstructorDecl>(AFD)) {
       emitDiagnostic(AFD, diag::note_call_to_initializer);
@@ -2974,11 +2979,8 @@ bool MissingGenericArgumentsFailure::diagnoseParameter(
   } else if (auto *TAD = dyn_cast<TypeAliasDecl>(DC)) {
     baseTyForNote = TAD->getUnboundGenericType();
   } else {
-    baseTyForNote = DC->getDeclaredInterfaceType();
-  }
-
-  if (!baseTyForNote)
     return true;
+  }
 
   emitDiagnostic(GP->getDecl(), diag::archetype_declared_in_type, GP,
                  baseTyForNote);

test/Constraints/generics.swift

@@ -747,3 +747,14 @@ func sr_7003() {
   // expected-error@-1 {{generic parameter 'T' could not be inferred}}
   // expected-note@-2 {{explicitly specify the generic arguments to fix this issue}} {{17-17=<Any>}}
 }
+
+func test_generic_subscript_with_missing_arg() {
+  struct S<T> {
+    subscript<U>(_: T) -> S<U> { fatalError() }
+    // expected-note@-1 {{in call to 'subscript(_:)'}}
+  }
+
+  func test(_ s: S<Int>) {
+    _ = s[0] // expected-error {{generic parameter 'U' could not be inferred}}
+  }
+}