diff --git a/gcc/fortran/simplify.c b/gcc/fortran/simplify.c
index c27b47aa98f..016ec259518 100644
--- a/gcc/fortran/simplify.c
+++ b/gcc/fortran/simplify.c
@@ -4512,6 +4512,60 @@ gfc_simplify_leadz (gfc_expr *e)
 }


+/* Check for constant length of a substring.  */
+
+static bool
+substring_has_constant_len (gfc_expr *e)
+{
+  ptrdiff_t istart, iend;
+  size_t length;
+  bool equal_length = false;
+
+  if (e->ts.type != BT_CHARACTER
+      || !(e->ref && e->ref->type == REF_SUBSTRING)
+      || !e->ref->u.ss.start
+      || e->ref->u.ss.start->expr_type != EXPR_CONSTANT
+      || !e->ref->u.ss.end
+      || e->ref->u.ss.end->expr_type != EXPR_CONSTANT
+      || !e->ref->u.ss.length
+      || !e->ref->u.ss.length->length
+      || e->ref->u.ss.length->length->expr_type != EXPR_CONSTANT)
+    return false;
+
+  /* Basic checks on substring starting and ending indices.  */
+  if (!gfc_resolve_substring (e->ref, &equal_length))
+    return false;
+
+  istart = gfc_mpz_get_hwi (e->ref->u.ss.start->value.integer);
+  iend = gfc_mpz_get_hwi (e->ref->u.ss.end->value.integer);
+  length = gfc_mpz_get_hwi (e->ref->u.ss.length->length->value.integer);
+
+  if (istart <= iend)
+    {
+      if (istart < 1)
+	{
+	  gfc_error ("Substring start index (%ld) at %L below 1",
+		     (long) istart, &e->ref->u.ss.start->where);
+	  return false;
+	}
+      if (iend > (ssize_t) length)
+	{
+	  gfc_error ("Substring end index (%ld) at %L exceeds string "
+		     "length", (long) iend, &e->ref->u.ss.end->where);
+	  return false;
+	}
+      length = iend - istart + 1;
+    }
+  else
+    length = 0;
+
+  /* Fix substring length.  */
+  e->value.character.length = length;
+
+  return true;
+}
+
+
 gfc_expr *
 gfc_simplify_len (gfc_expr *e, gfc_expr *kind)
 {
@@ -4547,6 +4601,13 @@ gfc_simplify_len (gfc_expr *e, gfc_expr *kind)
        of the unlimited polymorphic entity.  To get the _len component the last
        _data ref needs to be stripped and a ref to the _len component added.  */
     return gfc_get_len_component (e->symtree->n.sym->assoc->target, k);
+  else if (substring_has_constant_len (e))
+    {
+      result = gfc_get_constant_expr (BT_INTEGER, k, &e->where);
+      mpz_set_si (result->value.integer,
+		  e->value.character.length);
+      return range_check (result, "LEN");
+    }
   else
     return NULL;
 }
diff --git a/gcc/testsuite/gfortran.dg/pr100950.f90 b/gcc/testsuite/gfortran.dg/pr100950.f90
new file mode 100644
index 00000000000..f06db45b0b4
--- /dev/null
+++ b/gcc/testsuite/gfortran.dg/pr100950.f90
@@ -0,0 +1,18 @@
+! { dg-do run }
+! PR fortran/100950 - ICE in output_constructor_regular_field, at varasm.c:5514
+
+program p
+  character(8), parameter :: u = "123"
+  character(8)            :: x = "", s
+  character(2)            :: w(2) = [character(len(x(3:4))) :: 'a','b' ]
+  character(*), parameter :: y(*) = [character(len(u(3:4))) :: 'a','b' ]
+  character(*), parameter :: z(*) = [character(len(x(3:4))) :: 'a','b' ]
+  if (len (y) /= 2) stop 1
+  if (len (z) /= 2) stop 2
+  if (any (w /= y)) stop 3
+  if (len ([character(len(u(3:4))) :: 'a','b' ]) /= 2)  stop 4
+  if (len ([character(len(x(3:4))) :: 'a','b' ]) /= 2)  stop 5
+  if (any ([character(len(x(3:4))) :: 'a','b' ]  /= y)) stop 6
+  write(s,*) [character(len(x(3:4))) :: 'a','b' ]
+  if (s /= " a b    ") stop 7
+end