[CSGen] Handle incorrect patterns (e.g. referencing unknown types)

Currently `generateInitPatternConstraints` assumes that all
patterns have types but it's not the case for patterns that
e.g. reference unknown types or have other structural issues.

Let's fail `generateInitPatternConstraints` if constraint
generation fails.

Resolves: rdar://problem/64157451
(cherry picked from commit caab4f4)

Author: xedin

lib/Sema/CSGen.cpp

@@ -2321,12 +2321,18 @@ namespace {
 
         return setType(ParenType::get(CS.getASTContext(), underlyingType));
       }
-      case PatternKind::Var:
+      case PatternKind::Var: {
+        auto *subPattern = cast<VarPattern>(pattern)->getSubPattern();
+        auto type = getTypeForPattern(subPattern, locator, externalPatternType,
+                                      bindPatternVarsOneWay);
+
+        if (!type)
+          return Type();
+
         // Var doesn't affect the type.
-        return setType(
-            getTypeForPattern(
-              cast<VarPattern>(pattern)->getSubPattern(), locator,
-              externalPatternType, bindPatternVarsOneWay));
+        return setType(type);
+      }
+
       case PatternKind::Any: {
         return setType(
             CS.createTypeVariable(CS.getConstraintLocator(locator),
@@ -4235,7 +4241,9 @@ static bool generateInitPatternConstraints(
       pattern, locator, target.shouldBindPatternVarsOneWay(),
       target.getInitializationPatternBindingDecl(),
       target.getInitializationPatternBindingIndex());
-  assert(patternType && "All patterns have a type");
+
+  if (!patternType)
+    return true;
 
   if (auto wrappedVar = target.getInitializationWrappedVar()) {
     // Add an equal constraint between the pattern type and the

test/Constraints/patterns.swift

@@ -495,3 +495,14 @@ func rdar63510989() {
     // expected-warning@-1 {{immutable value 'v' was never used; consider replacing with '_' or removing it}}
   }
 }
+
+// rdar://problem/64157451 - compiler crash when using undefined type in pattern
+func rdar64157451() {
+  enum E {
+  case foo(Int)
+  }
+
+  func test(e: E) {
+    if case .foo(let v as DoeNotExist) = e {} // expected-error {{cannot find type 'DoeNotExist' in scope}}
+  }
+}