Teach AST verifier about thrown error contexts

The AST verifier was only correctly extracting thrown error types from
enclosing functions, without account for do...catch bodies. Switch it
over to using ASTScope's lookup to find the innermost enclosing caught
error context.

Fixes a AST verifier error reported by Paul Cantrell

Author: DougGregor

lib/AST/ASTVerifier.cpp

@@ -28,6 +28,7 @@
 #include "swift/AST/Initializer.h"
 #include "swift/AST/MacroDiscriminatorContext.h"
 #include "swift/AST/Module.h"
+#include "swift/AST/NameLookup.h"
 #include "swift/AST/ParameterList.h"
 #include "swift/AST/Pattern.h"
 #include "swift/AST/PrettyStackTrace.h"
@@ -1021,13 +1022,24 @@ class Verifier : public ASTWalker {
 
     void verifyChecked(ThrowStmt *S) {
       Type thrownError;
-      if (!Functions.empty()) {
-        if (auto fn = AnyFunctionRef::fromDeclContext(Functions.back()))
-          thrownError = fn->getThrownErrorType();
+      SourceLoc loc = S->getThrowLoc();
+      if (loc.isValid()) {
+        auto catchNode = ASTScope::lookupCatchNode(getModuleContext(), loc);
+
+        if (!catchNode) {
+          Out << "No catch context for throw statement\n";
+          abort();
+        }
+
+        if (auto thrown = catchNode.getThrownErrorTypeInContext(Ctx)) {
+          thrownError = *thrown;
+        } else {
+          thrownError = Ctx.getNeverType();
+        }
+      } else {
+        return;
       }
 
-      if (!thrownError)
-        thrownError = checkExceptionTypeExists("throw expression");
       checkSameType(S->getSubExpr()->getType(), thrownError, "throw operand");
       verifyCheckedBase(S);
     }

test/SILGen/typed_throws.swift

@@ -130,6 +130,23 @@ open class MyClass {
   func f() throws { }
 }
 
+
+struct Foo: Error { }
+struct Bar: Error { }
+
+// CHECK-LABEL: sil hidden [ossa] @$s12typed_throws0B22DifferentFromEnclosingyyAA3FooVYKF : $@convention(thin) () -> @error Foo
+func throwsDifferentFromEnclosing() throws(Foo) {
+  do {
+    throw Bar()
+  } catch {
+    print("Bar was barred")
+  }
+
+  // CHECK: throw [[ERROR:%.*]] : $Foo
+  throw Foo()
+}
+
+
 // CHECK-LABEL:      sil_vtable MySubclass {
 // CHECK-NEXT:   #MyClass.init!allocator: <E where E : Error> (MyClass.Type) -> (() throws(E) -> ()) throws(E) -> MyClass : @$s12typed_throws10MySubclassC4bodyACyyxYKXE_txYKcs5ErrorRzlufC [override]
 // CHECK-NEXT:  #MyClass.f: (MyClass) -> () throws -> () : @$s12typed_throws10MySubclassC1fyyAA0C5ErrorOYKFAA0C5ClassCADyyKFTV [override]