Fix crash in member lookup when base type contains type variables.

We were crashing during performTypoCorrection if the base type contained
type variables.

rdar://problem/28387684

Author: rudkx

lib/AST/LookupVisibleDecls.cpp

@@ -718,7 +718,8 @@ class OverrideFilteringConsumer : public VisibleDeclConsumer {
     // Does it make sense to substitute types?
     bool shouldSubst = !BaseTy->hasUnboundGenericType() &&
                        !isa<AnyMetatypeType>(BaseTy.getPointer()) &&
-                       !BaseTy->isAnyExistentialType();
+                       !BaseTy->isAnyExistentialType() &&
+                       !BaseTy->hasTypeVariable();
     ModuleDecl *M = DC->getParentModule();
 
     auto FoundSignature = VD->getOverloadSignature();

lib/Sema/CSGen.cpp

@@ -3084,7 +3084,10 @@ bool swift::isExtensionApplied(DeclContext &DC, Type BaseTy,
   ConstraintSystemOptions Options;
   NominalTypeDecl *Nominal = BaseTy->getNominalOrBoundGenericNominal();
   if (!Nominal || !BaseTy->isSpecialized() ||
-      ED->getGenericRequirements().empty())
+      ED->getGenericRequirements().empty() ||
+      // We'll crash if we leak type variables from one constraint
+      // system into the new one created below.
+      BaseTy->hasTypeVariable())
     return true;
   std::unique_ptr<TypeChecker> CreatedTC;
   // If the current ast context has no type checker, create one for it.
@@ -3101,11 +3104,6 @@ bool swift::isExtensionApplied(DeclContext &DC, Type BaseTy,
   bool Failed = false;
   SmallVector<Type, 3> TypeScratch;
 
-  // Don't allow type variables from an existing constraint system to
-  // leak into the new constraint system created below.
-  assert(!BaseTy->hasTypeVariable() &&
-         "Unexpected type variable in base type!");
-
   // Prepare type substitution map.
   TypeSubstitutionMap Substitutions = BaseTy->getMemberSubstitutions(ED);
   auto resolveType = [&](Type Ty) {

test/Sema/typo_correction.swift

@@ -69,3 +69,11 @@ func test_too_many_but_some_better() {
   let match6 = 0
   let x = mtch // expected-error {{use of unresolved identifier 'mtch'}}
 }
+
+// rdar://problem/28387684
+// Don't crash performing typo correction on bound generic types with
+// type variables.
+_ = [Any]().withUnsafeBufferPointer { (buf) -> [Any] in
+  guard let base = buf.baseAddress else { return [] }
+  return (base ..< base + buf.count).m // expected-error {{value of type 'CountableRange<_>' has no member 'm'}}
+}

validation-test/IDE/crashers/109-swift-constraints-constraintgraph-change-addedtypevariable.swift

@@ -0,0 +1,4 @@
+// RUN: %target-swift-ide-test -code-completion -code-completion-token=A -source-filename=%s
+// REQUIRES: asserts
+assert(Range.b
+v#^A^#

validation-test/IDE/crashers_fixed/109-swift-constraints-constraintgraph-change-addedtypevariable.swift

@@ -5,7 +5,7 @@
 // See http://swift.org/LICENSE.txt for license information
 // See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 
-// RUN: not --crash %target-swift-frontend %s -parse
+// RUN: not %target-swift-frontend %s -parse
 // REQUIRES: asserts
 func d<T{
 var d