[Sema] Continue type-checking for body when preamble fails

Skipping type-checking the body when the preamble fails to type-check
seems to be more of a historical artifact than intentional behavior.
Certain elements of the body may still get type-checked through
request evaluation, and as such may introduce autoclosures that won't
be properly contextualized. 

Make sure we continue type-checking the body even if the preamble
fails. We already invalidate any variables bound in the element
pattern, so downstream type-checking should be able to handle it
just fine. This ensures autoclosures get contextualized, and that
we're still able to provide semantic diagnostics for other issues in
the body.

rdar://136500008

Author: hamishknight

lib/Sema/TypeCheckStmt.cpp

@@ -1433,8 +1433,7 @@ class StmtChecker : public StmtVisitor<StmtChecker, Stmt*> {
   }
 
   Stmt *visitForEachStmt(ForEachStmt *S) {
-    if (TypeChecker::typeCheckForEachPreamble(DC, S))
-      return nullptr;
+    TypeChecker::typeCheckForEachPreamble(DC, S);
 
     // Type-check the body of the loop.
     auto sourceFile = DC->getParentSourceFile();

test/Parse/recovery.swift

@@ -210,8 +210,10 @@ func missingControllingExprInForEach() {
 // The #if block is used to provide a scope for the for stmt to force it to end
 // where necessary to provoke the crash.
 #if true  // <rdar://problem/21679557> compiler crashes on "for{{"
-  // expected-error @+2 {{expected pattern}}
-  // expected-error @+1 {{expected Sequence expression for for-each loop}}
+  // expected-error @+4 {{expected pattern}}
+  // expected-error @+3 {{expected Sequence expression for for-each loop}}
+  // expected-error @+2 {{closure expression is unused}}
+  // expected-note @+1 {{did you mean to use a 'do' statement?}}
   for{{ // expected-note 2 {{to match this opening '{'}}
 #endif  // expected-error {{expected '}' at end of closure}} expected-error {{expected '}' at end of brace statement}}
 

test/stmt/c_style_for.swift

@@ -32,9 +32,9 @@ for ; other<count; other+=1 { // expected-error {{C-style for statement was remo
 }
 
 for (var number : Int8 = start; number < count; number+=1) { // expected-error {{C-style for statement was removed in Swift 3}} {{none}}
-  print(number)
+  print(number) // expected-error {{cannot find 'number' in scope}}
 }
 
 for (var m : Int8 = start; m < count; m+=1) { // expected-error {{C-style for statement was removed in Swift 3}} {{none}}
-  m += 3
+  m += 3 // expected-error {{cannot find 'm' in scope}}
 }

test/stmt/foreach.swift

@@ -351,3 +351,16 @@ do {
     }
   }
 }
+
+// Make sure the bodies still type-check okay if the preamble is invalid.
+func testInvalidPreamble() {
+  func takesAutoclosure(_ x: @autoclosure () -> Int) -> Int { 0 }
+
+  for _ in undefined { // expected-error {{cannot find 'undefined' in scope}}
+    let a = takesAutoclosure(0) // Fine
+  }
+  for x in undefined { // expected-error {{cannot find 'undefined' in scope}}
+    let b: Int = x  // No type error, `x` is invalid.
+    _ = "" as Int // expected-error {{cannot convert value of type 'String' to type 'Int' in coercion}}
+  }
+}