[CS] Invalidate unresolved variables in ExprPatterns early

Make sure we invalidate when we initially visit the ExprPattern to
ensure that we don't run into issues when generating constraints for
a `where` clause before the constraints for the ExprPattern. We ought
to change the constraint generation there to use a conjunction to
ensure that the pattern is solved before the `where` clause, but I
want to keep this as a quick low risk fix that we can cherry-pick. I'll
switch it to a conjunction in a follow-up.

Author: hamishknight

lib/Sema/CSGen.cpp

@@ -3153,6 +3153,16 @@ namespace {
       }
 
       case PatternKind::Expr: {
+        // Make sure we invalidate any nested VarDecls early since generating
+        // constraints for a `where` clause may happen before we've generated
+        // constraints for the ExprPattern. We'll record a fix when visiting
+        // the UnresolvedPatternExpr.
+        // FIXME: We ought to use a conjunction for switch cases, then we
+        // wouldn't need this logic.
+        auto *EP = cast<ExprPattern>(pattern);
+        EP->getSubExpr()->forEachUnresolvedVariable([&](VarDecl *VD) {
+          CS.setType(VD, ErrorType::get(CS.getASTContext()));
+        });
         // We generate constraints for ExprPatterns in a separate pass. For
         // now, just create a type variable.
         return setType(CS.createTypeVariable(CS.getConstraintLocator(locator),

validation-test/compiler_crashers_fixed/ConstraintSystem-getTypeOfReferencePre-60045c.swift

@@ -0,0 +1,9 @@
+// {"kind":"typecheck","signature":"swift::constraints::ConstraintSystem::getTypeOfReferencePre(swift::constraints::OverloadChoice, swift::DeclContext*, swift::constraints::ConstraintLocatorBuilder, swift::constraints::PreparedOverloadBuilder*)","signatureAssert":"Assertion failed: (func->isOperator() && \"Lookup should only find operators\"), function getTypeOfReferencePre"}
+// RUN: not %target-swift-frontend -typecheck %s
+{
+  switch if .random()  else {
+  }
+  {
+  case let !a where a:
+  }
+}

…traintWalker-walkToExprPost-c956e0.swift …traintWalker-walkToExprPost-c956e0.swiftvalidation-test/compiler_crashers/ConstraintWalker-walkToExprPost-c956e0.swift renamed to validation-test/compiler_crashers_fixed/ConstraintWalker-walkToExprPost-c956e0.swift validation-test/compiler_crashers/ConstraintWalker-walkToExprPost-c956e0.swift renamed to validation-test/compiler_crashers_fixed/ConstraintWalker-walkToExprPost-c956e0.swift

@@ -1,5 +1,5 @@
 // {"kind":"typecheck","original":"1562769e","signature":"(anonymous namespace)::ConstraintWalker::walkToExprPost(swift::Expr*)"}
-// RUN: not --crash %target-swift-frontend -typecheck %s
+// RUN: not %target-swift-frontend -typecheck %s
 enum c
   func d()  e {
     if let

validation-test/compiler_crashers_fixed/TypeVarRefCollector-walkToStmtPre-e56ccf.swift

@@ -0,0 +1,10 @@
+// {"kind":"typecheck","signature":"swift::constraints::TypeVarRefCollector::walkToStmtPre(swift::Stmt*)","signatureAssert":"Assertion failed: (result), function getClosureType"}
+// RUN: not %target-swift-frontend -typecheck %s
+{
+  switch if <#expression#> {
+    return
+  }
+  {
+  case let !a where a:
+  }
+}