Add warning for interpolated strings where type is not given

src/Compiler/Checking/CheckFormatStrings.fs

@@ -458,6 +458,8 @@ let parseFormatStringInternal
 
             // residue of hole "...{n}..." in interpolated strings become %P(...)
             | 'P' when isInterpolated ->
+                let (code, message) = FSComp.SR.alwaysUseTypedStringInterpolation()
+                warning(DiagnosticWithText(code, message, m))
                 checkOtherFlags ch
                 let i = requireAndSkipInterpolationHoleFormat (i+1)
                 // Note, the fragCol doesn't advance at all as these are magically inserted.

src/Compiler/Driver/CompilerDiagnostics.fs

@@ -377,13 +377,14 @@ type PhasedDiagnostic with
         | 1182 -> false // chkUnusedValue - off by default
         | 3180 -> false // abImplicitHeapAllocation - off by default
         | 3186 -> false // pickleMissingDefinition - off by default
-        | 3366 -> false //tcIndexNotationDeprecated - currently off by default
+        | 3366 -> false // tcIndexNotationDeprecated - currently off by default
         | 3517 -> false // optFailedToInlineSuggestedValue - off by default
         | 3388 -> false // tcSubsumptionImplicitConversionUsed - off by default
         | 3389 -> false // tcBuiltInImplicitConversionUsed - off by default
         | 3390 -> false // xmlDocBadlyFormed - off by default
         | 3395 -> false // tcImplicitConversionUsedForMethodArg - off by default
         | 3559 -> false // typrelNeverRefinedAwayFromTop - off by default
+        | 3579 -> false // alwaysUseTypedStringInterpolation - off by default
         | _ ->
             match x.Exception with
             | DiagnosticEnabledWithLanguageFeature (_, _, _, enabled) -> enabled

src/Compiler/FSComp.txt

@@ -1721,3 +1721,4 @@ featureAccessorFunctionShorthand,"underscore dot shorthand for accessor only fun
 3577,tcOverrideUsesMultipleArgumentsInsteadOfTuple,"This override takes a tuple instead of multiple arguments. Try to add an additional layer of parentheses at the method definition (e.g. 'member _.Foo((x, y))'), or remove parentheses at the abstract method declaration (e.g. 'abstract member Foo: 'a * 'b -> 'c')."
 featureUnmanagedConstraintCsharpInterop,"Interop between C#'s and F#'s unmanaged generic constraint (emit additional modreq)"
 3578,chkCopyUpdateSyntaxInAnonRecords,"This expression is an anonymous record, use {{|...|}} instead of {{...}}."
+3579,alwaysUseTypedStringInterpolation,"Prefer to use typed interpolated strings over untyped one."

tests/FSharp.Compiler.ComponentTests/ErrorMessages/TypedInterpolatedStringsTests.fs

@@ -0,0 +1,38 @@
+// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
+
+namespace ErrorMessages
+
+open Xunit
+open FSharp.Test.Compiler
+
+module ``Typed interpolated strings`` =
+
+    [<Fact>]
+    let ``Untyped interpolated strings produce warning``() =
+        FSharp """
+let hello = "Hello World"
+printf $"{hello}"
+        """
+        |> withWarnOn 3579
+        |> typecheck
+        |> shouldFail
+        |> withSingleDiagnostic (Warning 3579, Line 3, Col 8, Line 3, Col 18, "Prefer to use typed interpolated strings over untyped one.")
+
+    [<Fact>]
+    let ``Do not warn on untyped interpolated strings by default``() =
+        FSharp """
+let hello = "Hello World"
+printf $"{hello}"
+        """
+        |> typecheck
+        |> shouldSucceed
+
+    [<Fact>]
+    let ``Typed interpolated strings over interpolated one``() =
+        FSharp """
+let hello = "Hello World"
+printf $"%s{hello}"
+        """
+        |> withWarnOn 3579
+        |> typecheck
+        |> shouldSucceed

tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj

@@ -143,6 +143,7 @@
     <Compile Include="EmittedIL\Structure\Structure.fs" />
     <Compile Include="EmittedIL\TestFunctions\TestFunctions.fs" />
     <Compile Include="EmittedIL\Tuples\Tuples.fs" />
+    <Compile Include="ErrorMessages\TypedInterpolatedStringsTests.fs" />
     <!--<Compile Include="EmittedIL\StructDefensiveCopy\StructDefensiveCopy.fs" />-->
     <Compile Include="ErrorMessages\UnsupportedAttributes.fs" />
     <Compile Include="ErrorMessages\TailCallAttribute.fs" />