From 86e2d75187f83495cd92b2699f93d8f4ba6e7137 Mon Sep 17 00:00:00 2001
From: Harry Gillanders <contact@harrygillanders.com>
Date: Thu, 18 Jun 2020 05:56:06 +0100
Subject: [PATCH] Fix issue 20929

std.experimental.allocator.expandArray's range-based overload fails to
instantiate for char and wchar arrays.
---
 std/experimental/allocator/package.d | 48 ++++++++++++++++++++++------
 1 file changed, 39 insertions(+), 9 deletions(-)

diff --git a/std/experimental/allocator/package.d b/std/experimental/allocator/package.d
index 7503614c7..a0655341a 100644
--- a/std/experimental/allocator/package.d
+++ b/std/experimental/allocator/package.d
@@ -2082,12 +2082,12 @@ if (isInputRange!R && !isInfinite!R)
 
 version (StdUnittest)
 {
-    private struct ForcedInputRange
+    private struct ForcedInputRange(T)
     {
-        int[]* array;
+        T[]* array;
         pure nothrow @safe @nogc:
         bool empty() { return !array || (*array).empty; }
-        ref int front() { return (*array)[0]; }
+        ref T front() { return (*array)[0]; }
         void popFront() { *array = (*array)[1 .. $]; }
     }
 }
@@ -2100,7 +2100,7 @@ version (StdUnittest)
 
     void test(A)(auto ref A alloc)
     {
-        ForcedInputRange r;
+        ForcedInputRange!int r;
         long[] a = alloc.makeArray!long(r);
         assert(a.length == 0 && a.ptr is null);
         auto arr2 = arr;
@@ -2221,13 +2221,13 @@ if (isInputRange!R)
             toFill.uninitializedFillDefault;
         }
 
-        for (; !range.empty; range.popFront, toFill.popFront)
+        for (; !range.empty; range.popFront, toFill = toFill[1 .. $])
         {
-            assert(!toFill.empty);
+            assert(toFill.length > 0);
             import std.conv : emplace;
-            emplace!T(&toFill.front, range.front);
+            emplace!T(&toFill[0], range.front);
         }
-        assert(toFill.empty);
+        assert(toFill.length == 0);
     }
     else
     {
@@ -2267,7 +2267,7 @@ if (isInputRange!R)
 @system unittest
 {
     auto arr = theAllocator.makeArray!int([1, 2, 3]);
-    ForcedInputRange r;
+    ForcedInputRange!int r;
     int[] b = [ 1, 2, 3, 4 ];
     auto temp = b;
     r.array = &temp;
@@ -2275,6 +2275,36 @@ if (isInputRange!R)
     assert(arr == [1, 2, 3, 1, 2, 3, 4]);
 }
 
+// Regression test for https://issues.dlang.org/show_bug.cgi?id=20929
+@system unittest
+{
+    static void test(Char, Allocator)(auto ref Allocator alloc)
+    {
+        auto arr = alloc.makeArray!Char(1, Char('f'));
+
+        import std.utf : byUTF;
+        auto forwardRange = "oo".byUTF!Char();
+        static assert(isForwardRange!(typeof(forwardRange)));
+        // Test the forward-range code-path.
+        assert(alloc.expandArray(arr, forwardRange));
+
+        assert(arr == "foo");
+
+        immutable(Char)[] temp = "bar";
+        auto inputRange = ForcedInputRange!(immutable(Char))(&temp);
+        // Test the input-range code-path.
+        assert(alloc.expandArray(arr, inputRange));
+
+        assert(arr == "foobar");
+    }
+
+    import std.experimental.allocator.gc_allocator : GCAllocator;
+    test!char(GCAllocator.instance);
+    test!wchar(GCAllocator.instance);
+    test!char(theAllocator);
+    test!wchar(theAllocator);
+}
+
 /**
 Shrinks an array by `delta` elements.