Improve diagnoses of generic specializations

Always add constraints, find fixes during simplify.
New separate fix for allow generic function specialization.
Improve parse heuristic for isGenericTypeDisambiguatingToken.

lib/Sema/CSSimplify.cpp

@@ -13926,6 +13926,8 @@ ConstraintSystem::simplifyExplicitGenericArgumentsConstraint(
 
   // Bail out if we haven't selected an overload yet.
   auto simplifiedBoundType = simplifyType(type1, flags);
+  if (simplifiedBoundType->isPlaceholder())
+    return SolutionKind::Solved;
   if (simplifiedBoundType->isTypeVariableOrMember())
     return formUnsolved();
 
@@ -14018,13 +14020,12 @@ ConstraintSystem::simplifyExplicitGenericArgumentsConstraint(
     }
   }
 
-  if (!decl->getAsGenericContext())
-    return SolutionKind::Error;
-
   auto genericParams = getGenericParams(decl);
-  if (!genericParams) {
-    // FIXME: Record an error here that we're ignoring the parameters.
-    return SolutionKind::Solved;
+  if (!decl->getAsGenericContext() || !genericParams) {
+    return recordFix(AllowConcreteTypeSpecialization::create(
+               *this, type1, getConstraintLocator(locator)))
+               ? SolutionKind::Error
+               : SolutionKind::Solved;
   }
 
   // Map the generic parameters we have over to their opened types.
@@ -14057,12 +14058,14 @@ ConstraintSystem::simplifyExplicitGenericArgumentsConstraint(
     }
   }
 
-  if (openedGenericParams.empty()) {
+  // FIXME: We could support explicit function specialization.
+  if (openedGenericParams.empty() ||
+      (isa<AbstractFunctionDecl>(decl) && !hasParameterPack)) {
     if (!shouldAttemptFixes())
       return SolutionKind::Error;
 
-    return recordFix(AllowConcreteTypeSpecialization::create(
-               *this, type1, getConstraintLocator(locator)))
+    return recordFix(AllowGenericFunctionSpecialization::create(
+               *this, decl, getConstraintLocator(locator)))
                ? SolutionKind::Error
                : SolutionKind::Solved;
   }
@@ -15193,6 +15196,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
   case FixKind::AllowAssociatedValueMismatch:
   case FixKind::GenericArgumentsMismatch:
   case FixKind::AllowConcreteTypeSpecialization:
+  case FixKind::AllowGenericFunctionSpecialization:
   case FixKind::IgnoreGenericSpecializationArityMismatch:
   case FixKind::IgnoreKeyPathSubscriptIndexMismatch:
   case FixKind::AllowMemberRefOnExistential: {