Fix current_exceptions when called with backtrace=false (#757).

src/Compat.jl

@@ -1101,17 +1101,37 @@ if VERSION < v"1.7.0-DEV.1106"
 
     if VERSION >= v"1.1"
         function current_exceptions(task=current_task(); backtrace=true)
-            stack = Base.catch_stack(task, include_bt=backtrace)
-            ExceptionStack(Any[(exception=x[1],backtrace=x[2]) for x in stack])
+            old_stack = Base.catch_stack(task, include_bt=backtrace)
+            # If include_bt=true, Base.catch_stack yields a Vector of two-tuples,
+            # where the first element of each tuple is an exception and the second
+            # element is the corresponding backtrace. If instead include_bt=false,
+            # Base.catch_stack yields a Vector of exceptions.
+            #
+            # Independent of its backtrace keyword argument, Base.current_exceptions
+            # yields an ExceptionStack that wraps a Vector of two-element
+            # NamedTuples, where the first element of each named tuple is an exception
+            # and the second element is either a correpsonding backtrace or `nothing`.
+            #
+            # The following constructs the ExceptionStack-wrapped Vector appropriately.
+            new_stack = backtrace ?
+                Any[(exception=exc_and_bt[1], backtrace=exc_and_bt[2]) for exc_and_bt in old_stack] :
+                Any[(exception=exc_only,      backtrace=nothing) for exc_only in old_stack]
+            return ExceptionStack(new_stack)
         end
     else
         # There's no exception stack in 1.0, but we can fall back to returning
         # the (single) current exception and backtrace instead.
         @eval function current_exceptions(task=current_task(); backtrace=true)
             bt = catch_backtrace()
-            # `exc = Expr(:the_exception)` is the lowering for `catch exc`
-            exc = isempty(bt) ? nothing : $(Expr(:the_exception))
-            ExceptionStack(isempty(bt) ? Any[] : Any[(exception=exc, backtrace=bt)])
+            stack = if isempty(bt)
+                Any[]
+            else
+                # Note that `exc = Expr(:the_exception)` is the lowering for `catch exc`,
+                # and please see the comment in the implementation for >v1.1 regarding
+                # the `backtrace ? bt : nothing`.
+                Any[(exception=$(Expr(:the_exception)), backtrace = backtrace ? bt : nothing)]
+            end
+            return ExceptionStack(stack)
         end
         @eval function the_stack()
            $(Expr(:the_exception)), catch_backtrace()

test/runtests.jl

@@ -1112,36 +1112,77 @@ end
 
 # https://github.com/JuliaLang/julia/pull/29901
 @testset "current_exceptions" begin
-    # Display of errors which cause more than one entry on the exception stack
-    excs = try
-        try
-            __not_a_binding__
+    # Helper method to retrieve an ExceptionStack that should contain two exceptions,
+    # each of which accompanied by a backtrace or `nothing` according to `with_backtraces`.
+    function _retrieve_exception_stack(;with_backtraces::Bool)
+        exception_stack = try
+            try
+                # Generate the first exception:
+                __not_a_binding__
+            catch
+                # Catch the first exception, and generate a second exception
+                # during what would be handling of the first exception:
+                1 ÷ 0
+            end
         catch
-            1 ÷ 0  # Generate error while handling error
+            # Retrieve an ExceptionStack with both exceptions,
+            # and bind `exception_stack` (at the top of this block) thereto:
+            current_exceptions(;backtrace=with_backtraces)
         end
-    catch
-        current_exceptions()
+        return exception_stack
     end
 
+    excs_with_bts = _retrieve_exception_stack(with_backtraces = true)
+    excs_sans_bts = _retrieve_exception_stack(with_backtraces = false)
+
+    # Check that the ExceptionStack with backtraces contains backtraces:
+    BACKTRACE_TYPE = Vector{Union{Ptr{Nothing}, Base.InterpreterIP}}
+    @test all(exc_with_bt[2] isa BACKTRACE_TYPE for exc_with_bt in excs_with_bts)
+
+    # Check that the ExceptionStack without backtraces contains `nothing`s:
+    @test all(exc_sans_bt[2] isa Nothing for exc_sans_bt in excs_sans_bts)
+
     if VERSION >= v"1.1"
-        @test typeof.(first.(excs)) == [UndefVarError, DivideError]
+        # Check that the ExceptionStacks contain the expected exception types:
+        @test typeof.(first.(excs_with_bts)) == [UndefVarError, DivideError]
+        @test typeof.(first.(excs_sans_bts)) == [UndefVarError, DivideError]
 
+        # Check that the ExceptionStack with backtraces `show`s correctly:
         @test occursin(r"""
         2-element ExceptionStack:
         DivideError: integer division error
         Stacktrace:.*
 
         caused by: UndefVarError: __not_a_binding__ not defined
         Stacktrace:.*
-        """s, sprint(show, excs))
+        """s, sprint(show, excs_with_bts))
+
+        # Check that the ExceptionStack without backtraces `show`s correctly:
+        @test occursin(r"""
+        2-element ExceptionStack:
+        DivideError: integer division error
+
+        caused by: UndefVarError: __not_a_binding__ not defined"""s,
+        sprint(show, excs_sans_bts))
     else
-        # Due to runtime limitations, julia-1.0 only retains the last exception
-        @test typeof.(first.(excs)) == [DivideError]
+        # Due to runtime limitations, julia-1.0 only retains the last exception.
+
+        # Check that the ExceptionStacks contain the expected last exception type:
+        @test typeof.(first.(excs_with_bts)) == [DivideError]
+        @test typeof.(first.(excs_sans_bts)) == [DivideError]
+
+        # Check that the ExceptionStack with backtraces `show`s correctly:
         @test occursin(r"""
         1-element ExceptionStack:
         DivideError: integer division error
         Stacktrace:.*
-        """, sprint(show, excs))
+        """, sprint(show, excs_with_bts))
+
+        # Check that the ExceptionStack without backtraces `show`s correctly:
+        @test occursin(r"""
+        1-element ExceptionStack:
+        DivideError: integer division error""",
+        sprint(show, excs_sans_bts))
     end
 end