[TypeChecker] Prevent pattern resolution from triggering type-checking for unresolved references

Workaround for Clang enum typaliases called `isInvalid()` and `getInterfaceType()`
on discovered declarations. That doesn't play well with result builders because
declarations/patterns used in the body of a result builder don't get a type until
a solution is applied. Calling `isInvalid()` or `getInterfaceType` on declarations
located in the body of a result builder trigger a separate type-check for the
declaration that interferes with builder transformation and leads to crashes.

Author: xedin

lib/Sema/TypeCheckPattern.cpp

@@ -84,9 +84,21 @@ filterForEnumElement(DeclContext *DC, SourceLoc UseLoc,
   for (const LookupResultEntry &result : foundElements) {
     ValueDecl *e = result.getValueDecl();
     assert(e);
-    if (e->isInvalid()) {
+
+    // Check `isInvalid` only if the declaration has been
+    // verified, otherwise this would cause a problem if
+    // this declaration is found in the body of a result
+    // builder because it wouldn't have a type set until
+    // whole body is type-checked, and `isInvalid` would
+    // trigger a separate type-check that interfers with
+    // result builder transformation and causes crashes.
+    //
+    // Note: This check cannot simply be removed because
+    // enums with re-declarationed members would trigger
+    // an ambiguity assertion below.
+    if (e->hasInterfaceType() && e->isInvalid())
       continue;
-    }
+
     // Skip if the enum element was referenced as an instance member
     if (unqualifiedLookup) {
       if (!result.getBaseDecl() ||

test/Constraints/result_builder.swift

@@ -582,6 +582,26 @@ func testSwitch(_ e: E) {
   }
 }
 
+func testExistingPatternsInCaseStatements() {
+  tuplify(true) { c in
+    switch false {
+    case (c): 1 // Ok
+    default:  0
+    }
+  }
+
+  var arr: [Int] = []
+
+  tuplify(true) { c in
+    let n = arr.endIndex
+
+    switch arr.startIndex {
+    case (n): 1 // Ok
+    default:  0
+    }
+  }
+}
+
 // CHECK: testSwitch
 // CHECK-SAME: first(main.Either<Swift.String, (Swift.Int, Swift.String)>.first("a"))
 testSwitch(getE(0))