[TypeCheckEffects] Diagnose type mismatches for thrown errors in contexts that

throw `Never`.

Previously, this mistake slipped through the effects checker because it used
a fallback type of `any Error` if a catch node had a null thrown error type.
This change also fixes an issue where thrown error type mismatches were not
diagnosed at all in autoclosures.

Author: hborla

lib/Sema/TypeCheckEffects.cpp

@@ -3010,9 +3010,24 @@ 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 {}
+}