[Sema] Improve diagnostic for empty if expr branch

Previously we would say that it must end with a
`throw`, but a more useful diagnostic is that
there is an expression missing.

Author: hamishknight

include/swift/AST/DiagnosticsSema.def

@@ -1143,6 +1143,9 @@ ERROR(single_value_stmt_must_be_unlabeled,none,
 ERROR(if_expr_must_be_syntactically_exhaustive,none,
       "'if' must have an unconditional 'else' to be used as expression",
       ())
+ERROR(single_value_stmt_branch_empty,none,
+      "expected expression in branch of '%0' expression",
+      (StmtKind))
 ERROR(single_value_stmt_branch_must_end_in_throw,none,
       "non-expression branch of '%0' expression may only end with a 'throw'",
       (StmtKind))

lib/Sema/MiscDiagnostics.cpp

@@ -3866,6 +3866,14 @@ class SingleValueStmtUsageChecker final : public ASTWalker {
         break;
       case IsSingleValueStmtResult::Kind::UnterminatedBranches: {
         for (auto *branch : mayProduceSingleValue.getUnterminatedBranches()) {
+          if (auto *BS = dyn_cast<BraceStmt>(branch)) {
+            if (BS->empty()) {
+              Diags.diagnose(branch->getStartLoc(),
+                             diag::single_value_stmt_branch_empty,
+                             S->getKind());
+              continue;
+            }
+          }
           Diags.diagnose(branch->getEndLoc(),
                          diag::single_value_stmt_branch_must_end_in_throw,
                          S->getKind());

test/expr/unary/if_expr.swift

@@ -646,6 +646,12 @@ func nestedType() -> Int {
   }
 }
 
+func testEmptyBranch() -> Int {
+  let x = if .random() {} else { 0 }
+  // expected-error@-1:24 {{expected expression in branch of 'if' expression}}
+  return x
+}
+
 // MARK: Jumping
 
 func break1() -> Int {

test/expr/unary/switch_expr.swift

@@ -859,6 +859,21 @@ func nestedType() -> Int {
   }
 }
 
+func testEmptyBranch() -> Int {
+  // TODO: Ideally we wouldn't emit both diagnostics, the latter is the better
+  // one, but the former is currently emitted by the parser. Ideally the former
+  // one should become semantic, and we'd just avoid it for
+  // SingleValueStmtExprs.
+  let x = switch Bool.random() {
+    case true:
+    // expected-error@-1 {{'case' label in a 'switch' must have at least one executable statement}}
+    // expected-error@-2:14 {{expected expression in branch of 'switch' expression}}
+    case false:
+    0
+  }
+  return x
+}
+
 // MARK: Jumping
 
 func break1() -> Int {