From 3b4d2dc17bdbbb3c927b00482ff1f9701c2ca396 Mon Sep 17 00:00:00 2001
From: c42f <chris42f@gmail.com>
Date: Tue, 22 Nov 2022 17:00:53 +1000
Subject: [PATCH] Only warn when float literals underflow to zero

jl_strtod_c can return "underflow" even for valid cases such as `5e-324`
where the source string is parsed to an exact Float64 representation.

So we can't rely on jl_strtod_c to detect "invalid" underflow. Rather,
only warn when underflowing to zero which is probably a programming
mistake.
---
 src/parse_stream.jl | 14 ++++++++++----
 test/diagnostics.jl |  8 ++++++--
 2 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/src/parse_stream.jl b/src/parse_stream.jl
index e073d90f..01ae8ee9 100644
--- a/src/parse_stream.jl
+++ b/src/parse_stream.jl
@@ -865,10 +865,16 @@ function validate_literal_tokens(stream::ParseStream)
             # parse_int_literal
             # parse_uint_literal
         elseif k == K"Float" || k == K"Float32"
+            underflow0 = false
             if k == K"Float"
-                _, code = parse_float_literal(Float64, text, fbyte, nbyte)
+                x, code = parse_float_literal(Float64, text, fbyte, nbyte)
+                # jl_strtod_c can return "underflow" even for valid cases such
+                # as `5e-324` where the source is an exact representation of
+                # `x`. So only warn when underflowing to zero.
+                underflow0 = code == :underflow && x == 0
             else
-                _, code = parse_float_literal(Float32, text, fbyte, nbyte)
+                x, code = parse_float_literal(Float32, text, fbyte, nbyte)
+                underflow0 = code == :underflow && x == 0
             end
             if code == :ok
                 # pass
@@ -876,9 +882,9 @@ function validate_literal_tokens(stream::ParseStream)
                 emit_diagnostic(stream, fbyte, lbyte,
                                 error="overflow in floating point literal")
                 had_error = true
-            elseif code == :underflow
+            elseif underflow0
                 emit_diagnostic(stream, fbyte, lbyte,
-                                warning="underflow in floating point literal")
+                                warning="underflow to zero in floating point literal")
             end
         elseif k == K"Char"
             @assert fbyte < nbyte # Already handled in the parser
diff --git a/test/diagnostics.jl b/test/diagnostics.jl
index 9e3c8000..3324b2b0 100644
--- a/test/diagnostics.jl
+++ b/test/diagnostics.jl
@@ -16,9 +16,13 @@ end
     @test diagnostic("x = 10.0f1000;") ==
         Diagnostic(5, 13, :error, "overflow in floating point literal")
     @test diagnostic("x = 10.0e-1000;") ==
-        Diagnostic(5, 14, :warning, "underflow in floating point literal")
+        Diagnostic(5, 14, :warning, "underflow to zero in floating point literal")
     @test diagnostic("x = 10.0f-1000;") ==
-        Diagnostic(5, 14, :warning, "underflow in floating point literal")
+        Diagnostic(5, 14, :warning, "underflow to zero in floating point literal")
+    # Underflow boundary
+    @test diagnostic("5e-324", allow_multiple=true) == []
+    @test diagnostic("2e-324") ==
+        Diagnostic(1, 6, :warning, "underflow to zero in floating point literal")
 
     # Char
     @test diagnostic("x = ''") ==