diff --git a/stdlib/public/core/FloatingPointParsing.swift.gyb b/stdlib/public/core/FloatingPointParsing.swift.gyb
index f65c8c2b7018..aef8227f40b7 100644
--- a/stdlib/public/core/FloatingPointParsing.swift.gyb
+++ b/stdlib/public/core/FloatingPointParsing.swift.gyb
@@ -256,15 +256,21 @@ extension ${Self}: LosslessStringConvertible {
   // Use the all-Swift `parse_float${bits}()` implementation for Float16/32/64
   @available(SwiftStdlib 5.3, *)
   public init?(_ text: Substring) {
-    // TODO: Someday, this whole function should simplify down to just:
-    // ${Self}(text.utf8.span)
   #if _pointerBitWidth(_16)
     // Always fail on 16-bit targets
     return nil
   #else
-    // Work around span availability limits
-    let parsed = unsafe text.base._guts.withFastUTF8 { chars -> ${Self}? in
-      unsafe parse_float${bits}(chars.span)
+    let parsed: ${Self}?
+    if text.base._guts.isFastUTF8 {
+      parsed = unsafe text.base._guts.withFastUTF8 { chars -> ${Self}? in
+        unsafe parse_float${bits}(chars.span.extracting(text._offsetRange))
+      }
+    } else {
+      // Otherwise, make a copy so we have contiguous UTF8...
+      let string = String(text)
+      parsed = unsafe string._guts.withFastUTF8 { chars -> ${Self}? in
+        unsafe parse_float${bits}(chars.span)
+      }
     }
 
     if let parsed {
diff --git a/test/stdlib/ParseFloat64.swift b/test/stdlib/ParseFloat64.swift
index 4683a5495150..10b1a895fe31 100644
--- a/test/stdlib/ParseFloat64.swift
+++ b/test/stdlib/ParseFloat64.swift
@@ -328,6 +328,14 @@ tests.test("Decimal Floats") {
   expectParse("999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999.999999999999999999999999999", Float64.infinity)
 }
 
+tests.test("Substring") {
+  let s1 = "1.02.03.0"
+  let s1sub = s1[s1.firstIndex(of: "2")!..<s1.firstIndex(of: "3")!]
+  let parsed = Float64(s1sub)
+  expectNotNil(parsed)
+  expectEqual(parsed!.bitPattern, (2.0).bitPattern)
+}
+
 @_extern(c, "_swift_stdlib_strtod_clocale")
 func _swift_stdlib_strtod_clocale(
   _: Optional<UnsafePointer<CChar>>,