Add related span to original declaration on disagreeing variable/property types.

Author: DanielRosenwasser

src/compiler/checker.ts

@@ -5352,7 +5352,7 @@ namespace ts {
                     return type;
                 }
                 else if (declaredType !== errorType && type !== errorType && !isTypeIdenticalTo(declaredType, type)) {
-                    errorNextVariableOrPropertyDeclarationMustHaveSameType(declaredType, declaration, type);
+                    errorNextVariableOrPropertyDeclarationMustHaveSameType(/*firstDeclaration*/ undefined, declaredType, declaration, type);
                 }
             }
             return declaredType;
@@ -26839,7 +26839,7 @@ namespace ts {
                 if (type !== errorType && declarationType !== errorType &&
                     !isTypeIdenticalTo(type, declarationType) &&
                     !(symbol.flags & SymbolFlags.Assignment)) {
-                    errorNextVariableOrPropertyDeclarationMustHaveSameType(type, node, declarationType);
+                    errorNextVariableOrPropertyDeclarationMustHaveSameType(symbol.valueDeclaration, type, node, declarationType);
                 }
                 if (node.initializer) {
                     checkTypeAssignableToAndOptionallyElaborate(checkExpressionCached(node.initializer), declarationType, node, node.initializer, /*headMessage*/ undefined);
@@ -26859,17 +26859,24 @@ namespace ts {
             }
         }
 
-        function errorNextVariableOrPropertyDeclarationMustHaveSameType(firstType: Type, nextDeclaration: Declaration, nextType: Type): void {
+        function errorNextVariableOrPropertyDeclarationMustHaveSameType(firstDeclaration: Declaration | undefined, firstType: Type, nextDeclaration: Declaration, nextType: Type): void {
             const nextDeclarationName = getNameOfDeclaration(nextDeclaration);
             const message = nextDeclaration.kind === SyntaxKind.PropertyDeclaration || nextDeclaration.kind === SyntaxKind.PropertySignature
                 ? Diagnostics.Subsequent_property_declarations_must_have_the_same_type_Property_0_must_be_of_type_1_but_here_has_type_2
                 : Diagnostics.Subsequent_variable_declarations_must_have_the_same_type_Variable_0_must_be_of_type_1_but_here_has_type_2;
-            error(
+            const declName = declarationNameToString(nextDeclarationName);
+            const err = error(
                 nextDeclarationName,
                 message,
-                declarationNameToString(nextDeclarationName),
+                declName,
                 typeToString(firstType),
-                typeToString(nextType));
+                typeToString(nextType)
+            );
+            if (firstDeclaration) {
+                addRelatedInfo(err,
+                    createDiagnosticForNode(firstDeclaration, Diagnostics._0_was_also_declared_here, declName)
+                );
+            }
         }
 
         function areDeclarationFlagsIdentical(left: Declaration, right: Declaration) {