Merge pull request swiftlang#74454 from hborla/throws-never-diagnostics

[TypeCheckEffects] Diagnose type mismatches for thrown errors in contexts that throw `Never`.

lib/Parse/ParseExpr.cpp

@@ -372,6 +372,23 @@ ParserResult<Expr> Parser::parseExprSequence(Diag<> Message,
   if (SequencedExprs.size() == 1)
     return makeParserResult(SequenceStatus, SequencedExprs[0]);
 
+  // If the left-most sequence expr is a 'try', hoist it up to turn
+  // '(try x) + y' into 'try (x + y)'. This is necessary to do in the
+  // parser because 'try' nodes are represented in the ASTScope tree
+  // to look up catch nodes. The scope tree must be syntactic because
+  // it's constructed before sequence folding happens during preCheckExpr.
+  // Otherwise, catch node lookup would find the incorrect catch node for
+  // 'try x + y' at the source location for 'y'.
+  //
+  // 'try' has restrictions for where it can appear within a sequence
+  // expr. This is still diagnosed in TypeChecker::foldSequence.
+  if (auto *tryEval = dyn_cast<AnyTryExpr>(SequencedExprs[0])) {
+    SequencedExprs[0] = tryEval->getSubExpr();
+    auto *sequence = SequenceExpr::create(Context, SequencedExprs);
+    tryEval->setSubExpr(sequence);
+    return makeParserResult(SequenceStatus, tryEval);
+  }
+
   return makeParserResult(SequenceStatus,
                           SequenceExpr::create(Context, SequencedExprs));
 }

lib/Sema/TypeCheckEffects.cpp

@@ -3020,9 +3020,24 @@ class CheckEffectsCoverage : public EffectsHandlingWalker<CheckEffectsCoverage>
   Type getCaughtErrorTypeAt(SourceLoc loc) {
     auto dc = CurContext.getDeclContext();
     auto module = dc->getParentModule();
+
+    // Autoclosures can't be found via ASTScope lookup.
+    if (CurContext.isAutoClosure()) {
+      auto *closure = dyn_cast<AutoClosureExpr>(CurContext.getDeclContext());
+      if (auto type = closure->getEffectiveThrownType())
+        return *type;
+
+      // Otherwise, the closure does not throw.
+      return Ctx.getNeverType();
+    }
+
     if (CatchNode catchNode = ASTScope::lookupCatchNode(module, loc)) {
       if (auto caughtType = catchNode.getThrownErrorTypeInContext(Ctx))
         return *caughtType;
+
+      // If a catch node returns null for its thrown error type, we're
+      // in a non-throwing context.
+      return Ctx.getNeverType();
     }
 
     // Fall back to the error existential.

test/stmt/typed_throws.swift

@@ -311,3 +311,29 @@ func testDoCatchInClosure(cond: Bool, x: ThrowingMembers) {
     }
   }
 }
+
+func takesThrowingAutoclosure(_: @autoclosure () throws(MyError) -> Int) {}
+func takesNonThrowingAutoclosure(_: @autoclosure () throws(Never) -> Int) {}
+
+func getInt() throws -> Int { 0 }
+
+func throwingAutoclosures() {
+  takesThrowingAutoclosure(try getInt())
+  // expected-error@-1 {{thrown expression type 'any Error' cannot be converted to error type 'MyError'}}
+
+  takesNonThrowingAutoclosure(try getInt())
+  // expected-error@-1 {{thrown expression type 'any Error' cannot be converted to error type 'Never'}}
+}
+
+func noThrow() throws(Never) {
+  throw MyError.epicFailed
+  // expected-error@-1 {{thrown expression type 'MyError' cannot be converted to error type 'Never'}}
+
+  try doSomething()
+  // expected-error@-1 {{thrown expression type 'MyError' cannot be converted to error type 'Never'}}
+
+  do throws(Never) {
+    throw MyError.epicFailed
+    // expected-error@-1 {{thrown expression type 'MyError' cannot be converted to error type 'Never'}}
+  } catch {}
+}