Handle remaining interpolated-string forms adjacent to '=' (#16696)

Extend the merged '=$"' fix to the verbatim (=$@", =@$") and extended
multi-dollar (=$$""") interpolated-string forms, by matching '=' plus the
opener and rewinding so the regular interpolated-string lexer handles the rest.
Operators containing '$' remain rejected (FS0035), unchanged.

Author: edgarfgp

docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

@@ -14,6 +14,7 @@
 * Honor `--nowarn` and `--warnaserror` for warnings emitted during command-line option parsing ([Issue #19576](https://github.com/dotnet/fsharp/issues/19576), [PR #19776](https://github.com/dotnet/fsharp/pull/19776))
 * Fix `[<return: X>]` prefix attributes being silently dropped on class members, and fix false-positive `AllowMultiple=false` errors when `[<X>]` and `[<return: X>]` are applied to the same binding. ([Issue #17904](https://github.com/dotnet/fsharp/issues/17904), [Issue #19020](https://github.com/dotnet/fsharp/issues/19020), [PR #19738](https://github.com/dotnet/fsharp/pull/19738))
 * Fix `=` adjacent to an interpolated string (e.g. `C(Name=$"value")`) being lexed as the invalid operator `=$` instead of an assignment followed by an interpolated string. ([Issue #16696](https://github.com/dotnet/fsharp/issues/16696))
+* Extend the `=` adjacent to an interpolated string fix to the verbatim (`=$@"…"`, `=@$"…"`) and extended multi-dollar (`=$$"""…"""`) interpolated-string forms. ([Issue #16696](https://github.com/dotnet/fsharp/issues/16696), [PR #19984](https://github.com/dotnet/fsharp/pull/19984))
 * Preserve type abbreviations (`string`, user-defined aliases) in the refined type of bindings introduced after a `| null` pattern in a `match` expression. ([Issue #19646](https://github.com/dotnet/fsharp/issues/19646), [PR #19745](https://github.com/dotnet/fsharp/pull/19745))
 * Fix attributes on return type of unparenthesized tuple methods being silently dropped from IL. ([Issue #462](https://github.com/dotnet/fsharp/issues/462), [PR #19714](https://github.com/dotnet/fsharp/pull/19714))
 * Fix false-positive nullness warning (FS3261) when pattern matching narrows nullness inside seq/list/array comprehensions. ([Issue #19644](https://github.com/dotnet/fsharp/issues/19644), [PR #19743](https://github.com/dotnet/fsharp/pull/19743))

src/Compiler/lex.fsl

@@ -975,11 +975,14 @@ rule token (args: LexArgs) (skip: bool) = parse
 
  | ignored_op_char* ('@'|'^')                  op_char* { checkExprOp lexbuf; INFIX_AT_HAT_OP(lexeme lexbuf) }
 
- // For '=$"' (property/named-arg initialization with an interpolated string, e.g. C(Name=$"123")):
- // match the 3 chars, but consume only the '=' and rewind so the next scan begins at '$"',
- // letting the regular interpolated-string lexer process it (including any '{...}' holes).
- // See https://github.com/dotnet/fsharp/issues/16696.
- | '=' '$' '"' {
+ // For '=' immediately followed (no space) by an interpolated-string opener, e.g. C(Name=$"123").
+ // Match the '=' together with the opener, but consume only the '=' and rewind so the next scan
+ // begins at the opener, letting the regular interpolated-string lexer process it (including any
+ // '{...}' holes). This covers the single ($"), extended/multi-dollar ($$"""), and verbatim
+ // ($@" / @$") interpolated-string forms. See https://github.com/dotnet/fsharp/issues/16696.
+ | '=' '$' '"'
+ | '=' ('$'+) '"' '"' '"'
+ | '=' ("$@" | "@$") '"' {
        lexbuf.LexemeLength <- 1
        lexbuf.EndPos <- lexbuf.StartPos.ShiftColumnBy(1)
        EQUALS }

tests/FSharp.Compiler.ComponentTests/Language/InterpolatedStringsTests.fs

@@ -393,6 +393,34 @@ if c.Name <> "42" || r.Name <> "42" then failwith "expected 42"
         |> compileExeAndRun
         |> shouldSucceed
 
+    // The verbatim ($@" / @$") forms, also adjacent to '='.
+    [<Fact>]
+    let ``Issue 16696 - '=' adjacent to a verbatim interpolated string binds it`` () =
+        Fsx """
+let n = 42
+let a =$@"{n}"
+let b =@$"{n}"
+if a <> "42" || b <> "42" then failwith "expected 42"
+        """
+        |> compileExeAndRun
+        |> shouldSucceed
+
+    [<Fact>]
+    let ``Issue 16696 - '=' adjacent to a verbatim interpolated string in named-argument and record contexts`` () =
+        Fsx """
+type C() = member val Name = "" with get, set
+type R = { Name: string }
+let n = 42
+let c = C(Name=$@"{n}")
+let r = { Name=@$"{n}" }
+if c.Name <> "42" || r.Name <> "42" then failwith "expected 42"
+        """
+        |> compileExeAndRun
+        |> shouldSucceed
+
+    // (The extended multi-dollar form '=$$"""..."""' is covered by the SyntaxTree baseline
+    // SynExprInterpolatedStringAdjacentEqualsExtendedMultiDollar.fs.)
+
     // Operator lexing is unchanged: a '$' anywhere in an operator is still reserved (FS0035).
     // The only thing the fix changes is '=' directly before an interpolated-string opener;
     // everything below still lexes as an operator exactly as before.

tests/service/data/SyntaxTree/String/SynExprInterpolatedStringAdjacentEqualsExtendedMultiDollar.fs

@@ -0,0 +1 @@
+let x =$$"""abc"""

tests/service/data/SyntaxTree/String/SynExprInterpolatedStringAdjacentEqualsExtendedMultiDollar.fs.bsl

@@ -0,0 +1,27 @@
+ImplFile
+  (ParsedImplFileInput
+     ("/root/String/SynExprInterpolatedStringAdjacentEqualsExtendedMultiDollar.fs",
+      false,
+      QualifiedNameOfFile
+        SynExprInterpolatedStringAdjacentEqualsExtendedMultiDollar, [],
+      [SynModuleOrNamespace
+         ([SynExprInterpolatedStringAdjacentEqualsExtendedMultiDollar], false,
+          AnonModule,
+          [Let
+             (false,
+              [SynBinding
+                 (None, Normal, false, false, [],
+                  PreXmlDoc ((1,0), FSharp.Compiler.Xml.XmlDocCollector),
+                  SynValData
+                    (None, SynValInfo ([], SynArgInfo ([], false, None)), None),
+                  Named (SynIdent (x, None), false, None, (1,4--1,5)), None,
+                  InterpolatedString
+                    ([String ("abc", (1,7--1,18))], TripleQuote, (1,7--1,18)),
+                  (1,4--1,5), Yes (1,0--1,18), { LeadingKeyword = Let (1,0--1,3)
+                                                 InlineKeyword = None
+                                                 EqualsRange = Some (1,6--1,7) })],
+              (1,0--1,18), { InKeyword = None })], PreXmlDocEmpty, [], None,
+          (1,0--2,0), { LeadingKeyword = None })], (true, true),
+      { ConditionalDirectives = []
+        WarnDirectives = []
+        CodeComments = [] }, set []))

tests/service/data/SyntaxTree/String/SynExprInterpolatedStringAdjacentEqualsVerbatim.fs

@@ -0,0 +1 @@
+let x =$@"abc"

tests/service/data/SyntaxTree/String/SynExprInterpolatedStringAdjacentEqualsVerbatim.fs.bsl

@@ -0,0 +1,24 @@
+ImplFile
+  (ParsedImplFileInput
+     ("/root/String/SynExprInterpolatedStringAdjacentEqualsVerbatim.fs", false,
+      QualifiedNameOfFile SynExprInterpolatedStringAdjacentEqualsVerbatim, [],
+      [SynModuleOrNamespace
+         ([SynExprInterpolatedStringAdjacentEqualsVerbatim], false, AnonModule,
+          [Let
+             (false,
+              [SynBinding
+                 (None, Normal, false, false, [],
+                  PreXmlDoc ((1,0), FSharp.Compiler.Xml.XmlDocCollector),
+                  SynValData
+                    (None, SynValInfo ([], SynArgInfo ([], false, None)), None),
+                  Named (SynIdent (x, None), false, None, (1,4--1,5)), None,
+                  InterpolatedString
+                    ([String ("abc", (1,7--1,14))], Verbatim, (1,7--1,14)),
+                  (1,4--1,5), Yes (1,0--1,14), { LeadingKeyword = Let (1,0--1,3)
+                                                 InlineKeyword = None
+                                                 EqualsRange = Some (1,6--1,7) })],
+              (1,0--1,14), { InKeyword = None })], PreXmlDocEmpty, [], None,
+          (1,0--2,0), { LeadingKeyword = None })], (true, true),
+      { ConditionalDirectives = []
+        WarnDirectives = []
+        CodeComments = [] }, set []))

tests/service/data/SyntaxTree/String/SynExprInterpolatedStringAdjacentEqualsVerbatimAtDollar.fs

@@ -0,0 +1 @@
+let x =@$"abc"

tests/service/data/SyntaxTree/String/SynExprInterpolatedStringAdjacentEqualsVerbatimAtDollar.fs.bsl

@@ -0,0 +1,27 @@
+ImplFile
+  (ParsedImplFileInput
+     ("/root/String/SynExprInterpolatedStringAdjacentEqualsVerbatimAtDollar.fs",
+      false,
+      QualifiedNameOfFile
+        SynExprInterpolatedStringAdjacentEqualsVerbatimAtDollar, [],
+      [SynModuleOrNamespace
+         ([SynExprInterpolatedStringAdjacentEqualsVerbatimAtDollar], false,
+          AnonModule,
+          [Let
+             (false,
+              [SynBinding
+                 (None, Normal, false, false, [],
+                  PreXmlDoc ((1,0), FSharp.Compiler.Xml.XmlDocCollector),
+                  SynValData
+                    (None, SynValInfo ([], SynArgInfo ([], false, None)), None),
+                  Named (SynIdent (x, None), false, None, (1,4--1,5)), None,
+                  InterpolatedString
+                    ([String ("abc", (1,7--1,14))], Verbatim, (1,7--1,14)),
+                  (1,4--1,5), Yes (1,0--1,14), { LeadingKeyword = Let (1,0--1,3)
+                                                 InlineKeyword = None
+                                                 EqualsRange = Some (1,6--1,7) })],
+              (1,0--1,14), { InKeyword = None })], PreXmlDocEmpty, [], None,
+          (1,0--2,0), { LeadingKeyword = None })], (true, true),
+      { ConditionalDirectives = []
+        WarnDirectives = []
+        CodeComments = [] }, set []))