[Sema] Relax a check in VarDeclUsageChecker

We don't want to just check the first pattern, we're interested in
the first pattern that binds the given variable. That can be determined
by checking if it's canonical or not.

Author: hamishknight

lib/Sema/MiscDiagnostics.cpp

@@ -4007,12 +4007,8 @@ VarDeclUsageChecker::~VarDeclUsageChecker() {
 
     if (auto *caseStmt =
             dyn_cast_or_null<CaseStmt>(var->getRecursiveParentPatternStmt())) {
-      // Only diagnose VarDecls from the first CaseLabelItem in CaseStmts, as
-      // the remaining items must match it anyway.
-      auto caseItems = caseStmt->getCaseLabelItems();
-      assert(!caseItems.empty() &&
-             "If we have any case stmt var decls, we should have a case item");
-      if (!caseItems.front().getPattern()->containsVarDecl(var))
+      // Only diagnose for the parent-most VarDecl.
+      if (var->getParentVarDecl())
         continue;
 
       auto *childVar = var->getCorrespondingCaseBodyVariable().get();

test/Parse/switch.swift

@@ -200,10 +200,10 @@ switch t {
 case (var a, 2), (1, _): // expected-error {{'a' must be bound in every pattern}} expected-warning {{variable 'a' was never used; consider replacing with '_' or removing it}}
   ()
 
-case (_, 2), (var a, _): // expected-error {{'a' must be bound in every pattern}}
+case (_, 2), (var a, _): // expected-error {{'a' must be bound in every pattern}} expected-warning {{variable 'a' was never used; consider replacing with '_' or removing it}}
   ()
 
-case (var a, 2), (1, var b): // expected-error {{'a' must be bound in every pattern}} expected-error {{'b' must be bound in every pattern}} expected-warning {{variable 'a' was never used; consider replacing with '_' or removing it}}
+case (var a, 2), (1, var b): // expected-error {{'a' must be bound in every pattern}} expected-error {{'b' must be bound in every pattern}} expected-warning {{variable 'a' was never used; consider replacing with '_' or removing it}} expected-warning {{variable 'b' was never used; consider replacing with '_' or removing it}}
   ()
 
 case (var a, 2): // expected-error {{'case' label in a 'switch' must have at least one executable statement}} {{17-17= break}} expected-warning {{variable 'a' was never used; consider replacing with '_' or removing it}}
@@ -221,7 +221,7 @@ case (1, var b): // expected-warning {{variable 'b' was never used; consider rep
 case (1, let b): // let bindings expected-warning {{immutable value 'b' was never used; consider replacing with '_' or removing it}}
   ()
 
-case (_, 2), (let a, _): // expected-error {{'a' must be bound in every pattern}} expected-warning {{case is already handled by previous patterns; consider removing it}}
+case (_, 2), (let a, _): // expected-error {{'a' must be bound in every pattern}} expected-warning {{case is already handled by previous patterns; consider removing it}} expected-warning {{immutable value 'a' was never used; consider replacing with '_' or removing it}}
   ()
 
 // OK