[AST] Use ErrorType for invalid value generic parameter

If we fail to resolve the value type for a value generic parameter,
previously we would have returned a null Type, causing crashes
downstream. Instead, return an ErrorType, leaving a null Type for
cases where the generic parameter isn't a value generic at all.

rdar://154856417

Author: hamishknight

lib/AST/Decl.cpp

@@ -6121,9 +6121,13 @@ GenericTypeParamDecl *GenericTypeParamDecl::createImplicit(
 }
 
 Type GenericTypeParamDecl::getValueType() const {
-  return evaluateOrDefault(getASTContext().evaluator,
-                           GenericTypeParamDeclGetValueTypeRequest{this},
-                           Type());
+  if (!isValue())
+    return Type();
+
+  auto &ctx = getASTContext();
+  GenericTypeParamDeclGetValueTypeRequest req(this);
+  auto ty = evaluateOrDefault(ctx.evaluator, req, Type());
+  return ty ? ty : ErrorType::get(ctx);
 }
 
 SourceRange GenericTypeParamDecl::getSourceRange() const {

lib/Sema/CSGen.cpp

@@ -1745,7 +1745,10 @@ namespace {
     }
 
     Type visitTypeValueExpr(TypeValueExpr *E) {
-      return E->getParamDecl()->getValueType();
+      auto ty = E->getParamDecl()->getValueType();
+      if (!ty || ty->hasError())
+        return recordInvalidNode(E);
+      return ty;
     }
 
     Type visitDotSyntaxBaseIgnoredExpr(DotSyntaxBaseIgnoredExpr *expr) {

lib/Sema/TypeCheckRequestFunctions.cpp

@@ -488,10 +488,10 @@ Type ResultBuilderTypeRequest::evaluate(Evaluator &evaluator,
 
 Type GenericTypeParamDeclGetValueTypeRequest::evaluate(
     Evaluator &evaluator, const GenericTypeParamDecl *decl) const {
-  if (!decl->isValue())
-    return Type();
+  ASSERT(decl->isValue());
 
-  if (decl->getInherited().size() == 0) {
+  auto inherited = decl->getInherited();
+  if (inherited.empty()) {
     decl->diagnose(diag::missing_value_generic_type, decl->getName());
     return Type();
   }
@@ -502,12 +502,11 @@ Type GenericTypeParamDeclGetValueTypeRequest::evaluate(
   //
   // We should have 1 inherited type for 'N', 'Int', and have a 2nd generic
   // parameter called 'Bool'.
-  ASSERT(decl->getInherited().size() == 1);
+  ASSERT(inherited.size() == 1);
 
   // The value type of a generic parameter should never rely on the generic
   // signature of the generic parameter itself or any of the outside context.
-  return decl->getInherited().getResolvedType(0,
-                                              TypeResolutionStage::Structural);
+  return inherited.getResolvedType(0, TypeResolutionStage::Structural);
 }
 
 // Define request evaluation functions for each of the type checker requests.

lib/Sema/TypeCheckType.cpp

@@ -808,16 +808,16 @@ namespace {
       if (!paramType->isValue())
         return true;
 
-      // Both of these generic type parameters are values, but they may not have
-      // underlying value types associated with them. This can occur when a
-      // parameter doesn't declare a value type and we're going to diagnose it
-      // later.
-      if (!paramType->getValueType() || !secondType->getValueType())
+      // If either value type has an error, we've already diagnosed the issue.
+      auto paramValueTy = paramType->getValueType();
+      auto secondValueTy = secondType->getValueType();
+      if (!paramValueTy || paramValueTy->hasError() ||
+          !secondValueTy || secondValueTy->hasError()) {
         return true;
-
+      }
       // Otherwise, these are both value parameters and check that both their
       // value types are the same.
-      return paramType->getValueType()->isEqual(secondType->getValueType());
+      return paramValueTy->isEqual(secondValueTy);
     }
 
     bool alwaysMismatchTypeParameters() const { return true; }

test/Sema/value_generics.swift

@@ -2,8 +2,10 @@
 
 protocol P {}
 
-func invalid<let N>() {} // expected-error {{value generic 'N' must have an explicit value type declared}}
-                         // expected-error@-1 {{generic parameter 'N' is not used in function signature}}
+func invalid<let N>() { // expected-error {{value generic 'N' must have an explicit value type declared}}
+                        // expected-error@-1 {{generic parameter 'N' is not used in function signature}}
+  let x: String = N // Fine, we bind to a hole.
+}
 func invalid<let N>(_: A<N>) {} // expected-error {{value generic 'N' must have an explicit value type declared}}
 
 struct A<let N: Int> {

validation-test/IDE/crashers/ab16c0e38dae31b3.swift renamed to validation-test/IDE/crashers_fixed/ab16c0e38dae31b3.swift

@@ -1,4 +1,4 @@
 // {"kind":"complete","signature":"swift::Mangle::ASTMangler::appendType(swift::Type, swift::GenericSignature, swift::ValueDecl const*)"}
-// RUN: not --crash %target-swift-ide-test -code-completion --code-completion-token=COMPLETE -source-filename %s
+// RUN: %target-swift-ide-test -code-completion --code-completion-token=COMPLETE -source-filename %s
 func a<let b
 #^COMPLETE^#

validation-test/compiler_crashers_2/c232a2df5b6fffd.swift renamed to validation-test/compiler_crashers_2_fixed/c232a2df5b6fffd.swift

@@ -1,5 +1,5 @@
 // {"signature":"swift::constraints::ConstraintSystem::getTypeOfMemberReference(swift::Type, swift::ValueDecl*, swift::DeclContext*, bool, swift::FunctionRefInfo, swift::constraints::ConstraintLocator*, llvm::SmallVectorImpl<std::__1::pair<swift::GenericTypeParamType*, swift::TypeVariableType*>>*)"}
-// RUN: not --crash %target-swift-frontend -typecheck %s
+// RUN: not %target-swift-frontend -typecheck %s
 struct a < let b > {
   func
   < (c : Self) {