[NFC] Hoist getTypeSourceRangeForDiagnostics()

Allows code to get this for any AbstractStorageDecl.

Author: beccadax

include/swift/AST/Decl.h

@@ -5892,6 +5892,13 @@ class AbstractStorageDecl : public ValueDecl {
   /// Return the interface type of the stored value.
   Type getValueInterfaceType() const;
 
+  /// Retrieve the source range of the variable type, or an invalid range if the
+  /// variable's type is not explicitly written in the source.
+  ///
+  /// Only for use in diagnostics.  It is not always possible to always
+  /// precisely point to the variable type because of type aliases.
+  SourceRange getTypeSourceRangeForDiagnostics() const;
+
   /// Determine how this storage is implemented.
   StorageImplInfo getImplInfo() const;
 
@@ -6326,13 +6333,6 @@ class VarDecl : public AbstractStorageDecl {
   /// and not just getInterfaceType().
   Type getTypeInContext() const;
 
-  /// Retrieve the source range of the variable type, or an invalid range if the
-  /// variable's type is not explicitly written in the source.
-  ///
-  /// Only for use in diagnostics.  It is not always possible to always
-  /// precisely point to the variable type because of type aliases.
-  SourceRange getTypeSourceRangeForDiagnostics() const;
-
   /// Determine the mutability of this variable declaration when
   /// accessed from a given declaration context.
   StorageMutability mutability(

lib/AST/Decl.cpp

@@ -7758,14 +7758,19 @@ SourceRange VarDecl::getSourceRange() const {
   return getNameLoc();
 }
 
-SourceRange VarDecl::getTypeSourceRangeForDiagnostics() const {
+SourceRange AbstractStorageDecl::getTypeSourceRangeForDiagnostics() const {
+  // Subscripts always have an explicitly-written type.
+  if (auto *SD = dyn_cast<SubscriptDecl>(this))
+    return SD->getElementTypeSourceRange();
+
   // For a parameter, map back to its parameter to get the TypeLoc.
   if (auto *PD = dyn_cast<ParamDecl>(this)) {
     if (auto typeRepr = PD->getTypeRepr())
       return typeRepr->getSourceRange();
   }
-  
-  Pattern *Pat = getParentPattern();
+
+  auto *VD = cast<VarDecl>(this);
+  Pattern *Pat = VD->getParentPattern();
   if (!Pat || Pat->isImplicit())
     return SourceRange();
 

lib/Sema/AssociatedTypeInference.cpp

@@ -3724,15 +3724,12 @@ bool AssociatedTypeInference::diagnoseNoSolutions(
               failed.Result.getKind() != CheckTypeWitnessResult::Superclass) {
             Type resultType;
             SourceRange typeRange;
-            if (auto *var = dyn_cast<VarDecl>(failed.Witness)) {
-              resultType = var->getValueInterfaceType();
-              typeRange = var->getTypeSourceRangeForDiagnostics();
+            if (auto *storage = dyn_cast<AbstractStorageDecl>(failed.Witness)) {
+              resultType = storage->getValueInterfaceType();
+              typeRange = storage->getTypeSourceRangeForDiagnostics();
             } else if (auto *func = dyn_cast<FuncDecl>(failed.Witness)) {
               resultType = func->getResultInterfaceType();
               typeRange = func->getResultTypeSourceRange();
-            } else if (auto *subscript = dyn_cast<SubscriptDecl>(failed.Witness)) {
-              resultType = subscript->getElementInterfaceType();
-              typeRange = subscript->getElementTypeSourceRange();
             }
 
             // If the type witness was inferred from an existential